Ahc problems. (1 Viewer)

Joined
Jan 28, 2021
Messages
6
Location
singapore
Hi, i have a lx470 from 1999 and recently the ahc is not working. It is showing the off light.

i have started replacing the following parts.

all accumulators.
Complete pump assembly incl all switches
All 4 shock looking things (the ones that raise or lower the car.
Relay for ahc.
all 3 height sensors.


So i kinda changed everything.

the error codes i keep getting and can not delete because they are instant back are:

C1743 main relay, but its new. I even changed it again just to check.

C1761 ahc ecu malfunction.

Note that all this started as following. Ahc has always worked. But 1 day some month ago when i stopped the car i was hearing the ahc pump buzzing.

The pump started buzzing every time when i stopped the car and pulled the key out of the ignition. Key in ignition pump did not make noise.
Key out, pump started running and kept running.
I thought. Ok the pump is on its end so lets buy all complete new.
New pump installed, and it did the same, same noise also.
So after that i installed all new. And never got the ahc working again.

car has 0 rust. All fuses are fine. And ahc ecu looks brand new both sides.
Door signal works on all doors. Same as ahc control switch. Checked all on active data with obd while running the car. Height sensors give data while driving. But 0 pressure readings.
I can manually in obd activate all relays and they click. But Errors stays.



8717804A-57DE-4605-8B81-5A2A05580CF9.jpeg
41493EE1-D6A4-4C01-8455-C3AF8043AFF4.jpeg
08DF53D7-D8AA-440B-B6CC-CE148F169AC2.jpeg
1B52F8EC-D422-436E-9C14-42DDA1E01E55.jpeg
 
Last edited:
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
Hi, i have a lx470 from 1999 and recently the ahc is not working. It is showing the off light.

i have started replacing the following parts.

all accumulators.
Complete pump assembly incl all switches
All 4 shock looking things (the ones that raise or lower the car.
Relay for ahc.
all 3 height sensors.


So i kinda changed everything.

the error codes i keep getting and can not delete because they are instant back are:

C1743 main relay, but its new. I even changed it again just to check.

C1761 ahc ecu malfunction.

Note that all this started as following. Ahc has always worked. But 1 day some month ago when i stopped the car i was hearing the ahc pump buzzing.

The pump started buzzing every time when i stopped the car and pulled the key out of the ignition. Key in ignition pump did not make noise.
Key out, pump started running and kept running.
I thought. Ok the pump is on its end so lets buy all complete new.
New pump installed, and it did the same, same noise also.
So after that i installed all new. And never got the ahc working again.

car has 0 rust. All fuses are fine. And ahc ecu looks brand new both sides.
Door signal works on all doors. Same as ahc control switch. Checked all on active data with obd while running the car. Height sensors give data while driving. But 0 pressure readings.
I can manually in obd activate all relays and they click. But Errors stays.



View attachment 2583817View attachment 2583818View attachment 2583819View attachment 2583820

Easy answers do not seem obvious. May I suggest that you work through the Diagnostic section of the Factory Service Manual.

Start at this link:
Then use the Index on the left hand side of the opening page and follow these tabs:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL & SKYHOOK TEMS

It is worthwhile to read the whole section even though it is quite long. Then proceed methodically with possible causes that match your observed symptoms and the various tests which are described in the relevant sub-sections. Obviously, you should look at the sub-sections which relate to the Diagnostic Trouble Codes (DTC's) appearing on your scanner but may I also suggest looking at other similar sub-sections. Sometimes the codes can be a little misleading, especially on non-Techstream scanners.

At first sight, your DTC codes 1761 and 1743 taken together do suggest problems with the Electronic Control Unit (ECU) itself. This is a very unusual situation, so it is worth doing further checks.

For example, as you have already located the Electronic Control Unit, it may be sensible to follow the test procedure described in the "Power Source Circuit" sub-section.

The initial purpose is to make sure that the problem actually is in the ECU and is not caused by a bad connector somewhere or some damage to the wiring harness or some other problem. Such things are quite possible in a vehicle which is ~22 years old but it can take a lot of effort to find the cause. Naturally, you do not want to replace the expensive ECU only to find that the problem is somewhere else.

The checks and tests in the DIAGNOSTIC section are not difficult but you will need a multi-meter capable of measuring 12 volts DC as well as able to measure resistance in automotive circuits and also able test continuity of these circuits. Such multi-meters seem likely to be available at places in Singapore such as automotive stores, some electrical shops and some hardware shops and also can be purchased from on-line stores.

May I assume that in replacing components mentioned in your Post #1 in this thread that you also replaced AHC Fluid with genuine Toyota/Lexus AHC Fluid? it is very important not to use any other kind of fluid in the AHC system, otherwise internal damage will occur. However, I assume that because the "active test" did not operate the AHC Pump, it was not possible to bleed the system properly?

I notice that the Height Control Sensor readings are all large negative values, so I assume that the suspension is stuck in the "LO" position or maybe the vehicle is resting on the bump stops?

Hopefully you can make some progress with this very frustrating situation. Please post your experiences in this thread on IH8MUD.

Factory Service Manual for AHC and TEMS.jpg
 
Joined
Jan 28, 2021
Messages
6
Location
singapore
Easy answers do not seem obvious. May I suggest that you work through the Diagnostic section of the Factory Service Manual.

Start at this link:
Then use the Index on the left hand side of the opening page and follow these tabs:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL & SKYHOOK TEMS

It is worthwhile to read the whole section even though it is quite long. Then proceed methodically with possible causes that match your observed symptoms and the various tests which are described in the relevant sub-sections. Obviously, you should look at the sub-sections which relate to the Diagnostic Trouble Codes (DTC's) appearing on your scanner but may I also suggest looking at other similar sub-sections. Sometimes the codes can be a little misleading, especially on non-Techstream scanners.

At first sight, your DTC codes 1761 and 1743 taken together do suggest problems with the Electronic Control Unit (ECU) itself. This is a very unusual situation, so it is worth doing further checks.

For example, as you have already located the Electronic Control Unit, it may be sensible to follow the test procedure described in the "Power Source Circuit" sub-section.

The initial purpose is to make sure that the problem actually is in the ECU and is not caused by a bad connector somewhere or some damage to the wiring harness or some other problem. Such things are quite possible in a vehicle which is ~22 years old but it can take a lot of effort to find the cause. Naturally, you do not want to replace the expensive ECU only to find that the problem is somewhere else.

The checks and tests in the DIAGNOSTIC section are not difficult but you will need a multi-meter capable of measuring 12 volts DC as well as able to measure resistance in automotive circuits and also able test continuity of these circuits. Such multi-meters seem likely to be available at places in Singapore such as automotive stores, some electrical shops and some hardware shops and also can be purchased from on-line stores.

May I assume that in replacing components mentioned in your Post #1 in this thread that you also replaced AHC Fluid with genuine Toyota/Lexus AHC Fluid? it is very important not to use any other kind of fluid in the AHC system, otherwise internal damage will occur. However, I assume that because the "active test" did not operate the AHC Pump, it was not possible to bleed the system properly?

I notice that the Height Control Sensor readings are all large negative values, so I assume that the suspension is stuck in the "LO" position or maybe the vehicle is resting on the bump stops?

Hopefully you can make some progress with this very frustrating situation. Please post your experiences in this thread on IH8MUD.

View attachment 2584465
Thanks, i will start reading. Yes i have a multi meter and all other tools to check electrics. I did manage to bleed the car. And it ofcoarse has original toyota fluid. And since all is changed fluid in the complete system is as clean as it can be.

yes the car is flat down and stuck in the low position. The car after install of the new parts had the pump running and we bleed all 5 points multiple times. When that was finished we drthe car in normal position and within 1 mile it sank to low and the light started flashing ahc off.

never seen the ahc off in all years before.

but what should the 3 height sensor values be when the car is in low position?
 
Joined
Jan 28, 2021
Messages
6
Location
singapore
If i know the height sensor value for the car when its in low position i can try to set that correct.
For now car is still parked and i can not use it.
 
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
If i know the height sensor value for the car when its in low position i can try to set that correct.
For now car is still parked and i can not use it.

It is most likely that this vehicle has several other problems but in this post let’s start with an answer to your question.

In a later post, some suggestions can be added on possible ways to address the real problems with your AHC system.

Firstly, the “height sensor value for the car when its in low position” cannot be “set” in a variable or adjustable way on Techstream or other scanner, as you may have hoped. The Height Control Sensor used in LC100/LX470 is a simple potentiometer (or voltage divider) of a kind invented by Thomas Edison way back in 1872. There is nothing “programmable” or “adjustable” inside the Height Control Sensors. Many external and internal pictures of these Sensors can be found by searching IH8MUD or Google.

Each Sensor sends a voltage signal to the AHC Electronic Control Unit (ECU) according to the position of the tiny brushes inside the Sensor. The ECU uses the voltage signals from the sensors (one at Rear, average of two sensors at Front) to set the Rear and Front heights of the vehicle to match the selection of “LO” or “N” or “HI” at the AHC switch on the centre console.

In normal operation, the ECU causes the AHC pump to operate if fluid and pressure is required to raise the vehicle. Alternatively, the ECU causes the Levelling Valves in the Control Valve Assembly to open if fluid and pressure must be released (back to the AHC tank) if it is required to lower the vehicle. In either case, the ECU actively seeks to match the Sensor voltage with the voltage set in the ECU for the selected height setting, “LO” or “N” or “HI”.

Secondly, the actual voltage settings for “LO”, “N” and “HI” heights are set permanently in the AHC Electronic Control Unit (ECU) firmware. These settings are the different, fixed, specific voltage signals which the ECU recognises as “LO” or “N” or “HI” heights respectively. These settings within the ECU are neither adjustable nor reprogrammable on LC100/LX470 vehicles.

The voltage signals corresponding to “LO”, “N” and “HI” heights can be interpreted from the diagram of the test procedure shown below. Note that “N” height occurs at ~2.25 volts at mid-swing of the sensor arm. At “N” height, if the system and components are healthy, then the Height Sensor readout on Techstream or other scanner should read close to zero inches or zero millimetres (depending whether metric or US units have been set). (Based on https://lc100e.github.io/manual/ then tabs Repair Manual > DIAGNOSTICS > Active Height Control Suspension & Skyhook TEMS > C1711 to arrive at Page DI-227).

LC100 Sensor Voltages.jpg



The information in the extract pasted below from the Factory Service Manual (FSM) explains the physical height changes of the vehicle for heights, up and down from the “N” height setting, as intended by Toyota/Lexus. These physical heights are measured with a tape-measure, nothing to do with the Techstream or other scanner readings. [Source: https://lc100e.github.io/manual/ then tabs Repair Manual > Suspension and Axle > Active Height Control System (Independent Front Suspension) > ON-VEHICLE INSPECTION to arrive at Page SA-305].

LC100 Height Change.jpg



The FSM also gives a complicated method to check and set physical heights. This method rarely is used. See https://lc100e.github.io/manual/ then tabs Repair Manual > Suspension and Axle > Active Height Control System (Independent Front Suspension) > ADJUSTMENT to arrive at Page SA-315 for the actual FSM specifications.

Instead, IH8MUD members mostly use good approximations of the FSM-specfied ‘operating heights’ with the vehicle set at “N” height, and then check and if necessary adjust measurements from hub centre up to the fender lip –

Front (both sides): 19.75 inches or 500 millimetres,
Rear (both sides): 20.50 inches or 520 millimetres.


These approximations have been found to be reliable for most practical purposes on stock vehicles.

So you can use the above information to estimate where the vehicle should sit physically at “LO” height setting by measuring the hub-to-fender distances with a tape measure, then compare with the ‘standard’ IH8MUD operating height measurements mentioned above, after adjustment for the expected height change.

Therefore at “LO”, the Front Hub-to-Fender tape measurement could be expected to be in range 445 millimetres to 460 millimetres (500mm less the height drop), and, at “LO” the Rear Hub-to-Fender tape measurement could be expected to be in range 475 millimetres to 490 millimetres (520mm less the height drop) if the AHC system is healthy.

Perhaps this indirectly answers your question?

It must be said that ALL standard measurements and adjustments and tests concerning the AHC suspension are specified in the FSM to be undertaken at “N” height.


Measurements at other heights usually do not help in diagnosis.

The height of the vehicle can be varied by physically moving the Height Control Sensor adjusters. The sliding adjusters (or the threaded heim bolt on front only), can be moved upward to increase actual height at any setting (sometimes called a “sensor lift”), or downward to reduce the height at any setting.

Visual Inspection  - Front Height Control Sensors.JPG


Such adjustments do not change any electrical settings and do not change the way the ECU responds. The ECU has no knowledge of the position of the Sensor on the slider bracket. It just means that the actual vehicle height matching the same voltage signals for “LO” or “N” and “HI” will be different after the change. The engine and AHC must be operating to allow the new heights to be adopted. However, increasing vehicle height also causes an increase in AHC pressures.

Obviously, if the AHC system cannot operate, then this is a futile exercise. Other problems must be solved first.

This post already is too long. Another post on the possible problems preventing the operation of your AHC system will follow later.

Meanwhile, if not already seen, it is worthwhile to read the General Description of the AHC/TEMS system (about 20 pages with many helpful diagrams) at https://lc100e.github.io/manual/ then tabs New Car Features (first item in the list) >CHASSIS > Suspension > Active Height Control Suspension & Skyhook TEMS.

Late edit: FSM references added and typos corrected.
 
Last edited:

ramangain

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When you bleed the system and reprime everything to the AHC N position (which you may need to do again, it is OK to recycle fresh fluids to keep costs in check), what are the ride height values? When you raise from L to N and monitor pressure readings, where do they stabilize?

Do the above without driving the rig. Let it sit for a week then recheck. We need to ensure the system holds pressures and heights before trying to drive it again in a known good working order state.

PS - You're in good hands with @IndroCruise and @suprarx7nut available to help in this forum.
 
Last edited:
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
It is most likely that this vehicle has several other problems but in this post let’s start with an answer to your question.

In a later post, some suggestions can be added on possible ways to address the real problems with your AHC system.

Firstly, the “height sensor value for the car when its in low position” cannot be “set” in a variable or adjustable way on Techstream or other scanner, as you may have hoped. The Height Control Sensor used in LC100/LX470 is a simple potentiometer (or voltage divider) of a kind invented by Thomas Edison way back in 1872. There is nothing “programmable” or “adjustable” inside the Height Control Sensors. Many external and internal pictures of these Sensors can be found by searching IH8MUD or Google.

Each Sensor sends a voltage signal to the AHC Electronic Control Unit (ECU) according to the position of the tiny brushes inside the Sensor. The ECU uses the voltage signals from the sensors (one at Rear, average of two sensors at Front) to set the Rear and Front heights of the vehicle to match the selection of “LO” or “N” or “HI” at the AHC switch on the centre console.

In normal operation, the ECU causes the AHC pump to operate if fluid and pressure is required to raise the vehicle. Alternatively, the ECU causes the Levelling Valves in the Control Valve Assembly to open if fluid and pressure must be released (back to the AHC tank) if it is required to lower the vehicle. In either case, the ECU actively seeks to match the Sensor voltage with the voltage set in the ECU for the selected height setting, “LO” or “N” or “HI”.

Secondly, the actual voltage settings for “LO”, “N” and “HI” heights are set permanently in the AHC Electronic Control Unit (ECU) firmware. These settings are the different, fixed, specific voltage signals which the ECU recognises as “LO” or “N” or “HI” heights respectively. These settings within the ECU are neither adjustable nor reprogrammable on LC100/LX470 vehicles.

The voltage signals corresponding to “LO”, “N” and “HI” heights can be interpreted from the diagram of the test procedure shown below. Note that “N” height occurs at ~2.25 volts at mid-swing of the sensor arm. At “N” height, if the system and components are healthy, then the Height Sensor readout on Techstream or other scanner should read close to zero inches or zero millimetres (depending whether metric or US units have been set). (Based on https://lc100e.github.io/manual/ then tabs Repair Manual > DIAGNOSTICS > Active Height Control Suspension & Skyhook TEMS > C1711 to arrive at Page DI-227).

View attachment 2612539


The information in the extract pasted below from the Factory Service Manual (FSM) explains the physical height changes of the vehicle for heights, up and down from the “N” height setting, as intended by Toyota/Lexus. These physical heights are measured with a tape-measure, nothing to do with the Techstream or other scanner readings. [Source: https://lc100e.github.io/manual/ then tabs Repair Manual > Suspension and Axle > Active Height Control System (Independent Front Suspension) > ON-VEHICLE INSPECTION to arrive at Page SA-305].

View attachment 2611419


The FSM also gives a complicated method to check and set physical heights. This method rarely is used. See https://lc100e.github.io/manual/ then tabs Repair Manual > Suspension and Axle > Active Height Control System (Independent Front Suspension) > ADJUSTMENT to arrive at Page SA-315 for the actual FSM specifications.

Instead, IH8MUD members mostly use good approximations of the FSM-specfied ‘operating heights’ with the vehicle set at “N” height, and then check and if necessary adjust measurements from hub centre up to the fender lip –

Front (both sides): 19.75 inches or 500 millimetres,
Rear (both sides): 20.50 inches or 520 millimetres.


These approximations have been found to be reliable for most practical purposes on stock vehicles.

So you can use the above information to estimate where the vehicle should sit physically at “LO” height setting by measuring the hub-to-fender distances with a tape measure, then compare with the ‘standard’ IH8MUD operating height measurements mentioned above, after adjustment for the expected height change.

Therefore at “LO”, the Front Hub-to-Fender tape measurement could be expected to be in range 445 millimetres to 460 millimetres (500mm less the height drop), and, at “LO” the Rear Hub-to-Fender tape measurement could be expected to be in range 475 millimetres to 490 millimetres (520mm less the height drop) if the AHC system is healthy.

Perhaps this indirectly answers your question?

It must be said that ALL standard measurements and adjustments and tests concerning the AHC suspension are specified in the FSM to be undertaken at “N” height.


Measurements at other heights usually do not help in diagnosis.

The height of the vehicle can be varied by physically moving the Height Control Sensor adjusters. The sliding adjusters (or the threaded heim bolt on front only), can be moved upward to increase actual height at any setting (sometimes called a “sensor lift”), or downward to reduce the height at any setting.

View attachment 2611421

Such adjustments do not change any electrical settings and do not change the way the ECU responds. The ECU has no knowledge of the position of the Sensor on the slider bracket. It just means that the actual vehicle height matching the same voltage signals for “LO” or “N” and “HI” will be different after the change. The engine and AHC must be operating to allow the new heights to be adopted. However, increasing vehicle height also causes an increase in AHC pressures.

Obviously, if the AHC system cannot operate, then this is a futile exercise. Other problems must be solved first.

This post already is too long. Another post on the possible problems preventing the operation of your AHC system will follow later.

Meanwhile, if not already seen, it is worthwhile to read the General Description of the AHC/TEMS system (about 20 pages with many helpful diagrams) at https://lc100e.github.io/manual/ then tabs New Car Features (first item in the list) >CHASSIS > Suspension > Active Height Control Suspension & Skyhook TEMS.

Late edit: FSM references added and typos corrected.

Hi @old470,

You have invested heavily in renovating the AHC system on your 1999 LX470 and hopefully you have overcome the previous frustrations in getting the system to work?

If not, here are some further notes in case they are of assistance to you or to some other IH8MUD Member with similar problems.

Can you please give an update about your experiences? There is always something that can be learned from IH8MUD Members’ stories.

Further to Post #5 in this thread, here are my recommendations on what to do next. Some notes on “Background Issues and Symptoms” have been added at the end of these recommendations for further information.

My suggestion is to proceed in a sequence in a methodical way based on observations and measurements, rather than jump from one guess to the next.

In the notes below, please be very aware that the absence of a Diagnostic Trouble Code (DTC) does not mean that there is no fault. The DTC’s only provide a guide and may or may not be revealed by Techstream or other scanner, especially when there is more than one related fault. As far as possible, it is best to check and test in a basic way that gives positive confirmation that a component or circuit is healthy.

A repeated DTC which cannot be cleared does indicate a fault.

Step 1: Cross levelling and Sensor Check

First, check ‘cross level’ of the front of the vehicle by measuring with a tape-measure the distance from the centre of the hub to the lip of the fender vertically above, at each front wheel.

Theoretically and as stated in the Factory Service Manual (FSM), ’cross levelling’ should be done when the vehicle is at “N” height.

In your case, you cannot raise to “N” height, so it is worthwhile to look at this even in the “LO” height position.

Just remember that this is a non-standard approach and it is important to ‘cross level’ properly at “N” height when the AHC system is working again. (There can be slightly different side-to-side effects at the extremes of “LO” and “HI” compared to “N” height due wear of springs and torsion bars or worn or out-of-adjustment mechanical components, especially on older vehicles).

When ‘cross-levelling’ the actual tape-measurement numbers at Front Right and Front Left hub-to-centre are not important but they should be equal, Right and Left, within 10mm (0.39 inches).

The purpose of front ‘cross levelling’ has two parts:
  • to ensure that both torsion bars are carrying equal loads, and,
  • to avoid a built-in ‘lean’. This in turn causes confused readings ‘to be sent to the ECU by the Height Control Sensors (even with new Sensors as in this case), and then in turn this may cause the ECU to switch to ‘fail safe mode’ which may prevent AHC operation.
If the static (engine “OFF”) front hub-to-fender heights not close to equal, adjust using the torsion bar adjusters and NOT the Height Control Sensor adjusters to equalise the front side-to-side heights (at least within 10 mm difference, less difference is better) with the engine “OFF”.

Then read the Height Control Sensor adjusters again on your scanner. The readings won’t approach zero until the vehicle can be raised to “N”, so don’t worry about this yet, but reducing the differences in the readings also is very important – see “Background Issues and Symptoms” notes far below.

If the vehicle has been ‘cross levelled’ as well as can be done, but the Height Control Sensors are still showing such widely different readings, then it is possible that the Electronic Control Unit (ECU) has put the AHC/TEMS system into ‘fail safe mode’ -- as explained in this post and in the FSM reference given in my previous post. In a ‘fail safe mode’, the AHC/TEMS system will not work properly and may not work at all. There are other possibilities and these also are mentioned in the “Background Issues and Symptoms” notes below.

As a starting point, persistent widely different Height Control Sensor readings lead to suspicion about the condition of the Height Control Sensors or the various connectors and/or reliable continuity of the harness wiring to the ECU.

For this vehicle, even though the Height Control Sensors are new, it is still worthwhile to remove them and check them physically and electrically. This is set out in the FSM and is summarised above in Post #5 and later in this Post.

At the same time, suggest check electrical continuity of all connectors and relevant harnesses. (Note: Harness damage is unusual but this is a 22 year old vehicle, so physically inspect the underbody carefully for harness damage near connectors, and especially at places where any harness is visible and may have been struck by stones or road debris — although both are uncommon in Singapore. Follow up any suspicions by testing electrical continuity).

At this time, also suggest check the voltage at the connectors and at the relevant terminals (SHB and SHG) at the ECU as described in the same section of the FSM as the other Height Control Sensor checks (C1711 at Page DI-226).

Again, absence of a DTC does not tell you that the system is healthy.

Special Note: There is no point in trying to correct a cross-levelling problem with different Right and Left Height Control Sensor adjustments. If the vehicle steering is pointed straight ahead, or if the vehicle is stationary, then the Gate Valves in the Control Valve Assembly are open and the front AHC pressures are the same on Right and Left sides. The Right and Left Front AHC pressures are not made different side-to-side by adjusting the position of the Right and Left Sensors and so the side-to-side heights cannot be changed by adjusting the position of the Front Sensors. Instead, the AHC system will change the Front height to match the average of the Right and Left Sensor voltages. As discussed above in post #5, the ECU will seek the voltage signal at the Sensors which match the console switch selection of "LO", "N" or "HI" by causing active raising or lowering the vehicle until a match is achieved (subject to the specified load and speed limits). This is why the system is called Active Height Control. If the voltage signals from the two Front sensors are widely different for a sufficient period of time (not just over a bump), then the ECU will cause a 'fail safe mode' to be adopted until the difference is resolved. The AHC system on LC100/LX470 is a two-channel system -- Front and Rear -- not a four-channel system raising and lowering all four corners separately. Attempts at ‘cross levelling’ with Height Control Sensors will result only in confusion and frustration and possibly cause the ECU to adopt a 'fail safe mode' which must then be corrected.

Step 2: Increase load on front torsion bars

Turn both Torsion Bar adjusters clockwise (when looking up from underneath the vehicle) by say 5 to 8 turns or even more. Purpose: The vehicle is stuck in the “LO” height position, so AHC pressures cannot be measured by raising from “LO” to “N”. Therefore it is necessary to make a guess to increase Torsion Bar load to make sure that the AHC system is not overloaded. When the AHC system is working again, AHC pressures can be measured correctly and then at least front pressures can be adjusted easily and correctly using the Torsion Bar adjusters.

Step 3: Test AHC Pump operation direct from 12 volt battery (not via ‘Active Test’):

To avoid doubts about the condition of the ECU or the effect of some conditions in ‘fail safe mode’, remove the power connector from the electric motor which drives the AHC Pump, then use leads to connect the motor terminals correctly to the vehicle battery (or some other 12 volt battery). Be very careful to avoid arcing by not to touching the leads together and wear eye protection in case of accidents. For correct polarity, refer carefully to the diagrams in the FSM at this reference:

https://lc100e.github.io/manual/
Then follow these tabs:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS > C1762 at Page DI-257 – Fluid Pressure Abnormality

Purpose: To check using the above test that the AHC Motor and Pump actually are capable of running when voltage is supplied.

Step 4: Further tests

Then, even if no DTC’s are visible, also suggest doing the following tests using the procedure described “IN CASE OF NOT USING HAND HELD TESTER”. Given the symptoms in your case, it is important to test in this way to avoid any confusions with problems in other circuits, or with the ECU problems, or with the Techstream or other scanner reading a faulty ECU and which then gives faulty readings.

As part of the diagnosis, note in each case the specific, different, ‘fail safe mode’ described in these sections of the FSM and compare these with symptoms observed on the vehicle. There is more than one ‘fail safe mode’.

Purpose: To check the health of the following circuits and components, as listed below:

C1718 at Page DI-229 – Fluid Pressure Sensor Circuit

C1719 at Page DI-233 – Fluid Temperature Sensor Circuit

C1731 at Page DI-239 – Control Valve Solenoid Circuit and Accumulator Solenoid Circuit
(Actually, it should be possible to hear and feel the operation of these valves – so these tests may not be necessary)

C1741 at Page DI-243 – AHC Motor Relay Circuit

C1743 at Page DI-248 – AHC Main Relay Circuit

C1751 at Page DI-252 – AHC Pump & Motor Circuit

Power Source Circuit at Page DI-294

Step 4: Check health of AHC Electronic Control Unit (ECU) as described in FSM

C1761 at Page DI-256 – Malfunction in ECU (Memory Error)
This DTC was reported on this vehicle and refused to clear – see Post #1.

If the above Steps 1 to 3 have been completed with positive/favourable results in the checks and tests, then that indicates that those components and circuits in the AHC system are in good health.

Then, regardless of its appearance in the pictures in Post #1, an ongoing DTC C1761 certainly causes suspicion about the ECU – Part No. 89290-60010 (for this Right Hand Drive). In such a case, after eliminating all other possibilities as described in the previous Steps 1 to 3, the ECU should be replaced. (New is expensive --you may wish to consider a secondhand unit, for example, see Singapore-based Megazip website).

Step 5: In case of previous incorrect Sensor adjustments, force different Sensor positions

The following method only is worthwhile after the following conditions have been achieved:
  • cross levelling has been corrected as best possible,
  • condition of Sensors, connectors and harnesses are healthy and reliable,
  • AHC pressures are likely to be at the low end of the range,
  • ECU is healthy
If these conditions have been satisfied and Sensor readings remain widely different and if AHC still is not working, then consider whether there is a possibility that mistakes by some person have been made in the past concerning Height Control Sensor adjustment.

It is possible that in the absence of a good understanding, incorrect adjustment of the Sensors have been left the Sensors in a position in which they are transmitting different and incorrect voltages to the ECU. If so, the ECU may have forced ‘fail safe mode’ which stops AHC operation.

If this is suspected, then the following method may help to rectify differences in Sensor settings and remove the ‘failure mode’ effect, even though the vehicle is stuck at “LO” height:
  • Firstly, for personal safety, secure the vehicle with stands under chassis rails so that it is impossible for downward movement of the vehicle body onto a human body while working under the vehicle. This VERY important to prevent accidents and injury,
  • With Techstream or scanner connected, ignition key “ON” BUT engine “OFF” (important – do not want AHC to ‘wake up’ and raise or lower vehicle body during adjustments), eliminate differences in the Sensor readings at “LO” by moving all Sensors until all scanner readings are approximately the same as the lowest Sensor, shown as -62.4mm in Post #1.
  • Start engine, hoping that AHC will be ready to operate, then press “UP” on the AHC console switch to see whether vehicle now raises from “LO” to “N”.

Best wishes for success!!

Can you please reply indicating whether there has been any progress with your vehicle?

Late edit: Text has been improved for better clarity in the "Special Note" in "Step 1" above and the missing heading "Step 4: Further Tests" was inserted.
 
Last edited:
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
Hi @old470,

You have invested heavily in renovating the AHC system on your 1999 LX470 and hopefully you have overcome the previous frustrations in getting the system to work?

If not, here are some further notes in case they are of assistance to you or to some other IH8MUD Member with similar problems.

Can you please give an update about your experiences? There is always something that can be learned from IH8MUD Members’ stories.

Further to Post #5 in this thread, here are my recommendations on what to do next. Some notes on “Background Issues and Symptoms” have been added at the end of these recommendations for further information.

My suggestion is to proceed in a sequence in a methodical way based on observations and measurements, rather than jump from one guess to the next.

In the notes below, please be very aware that the absence of a Diagnostic Trouble Code (DTC) does not mean that there is no fault. The DTC’s only provide a guide and may or may not be revealed by Techstream or other scanner, especially when there is more than one related fault. As far as possible, it is best to check and test in a basic way that gives positive confirmation that a component or circuit is healthy.

A repeated DTC which cannot be cleared does indicate a fault.

Step 1: Cross levelling and Sensor Check

First, check ‘cross level’ of the front of the vehicle by measuring with a tape-measure the distance from the centre of the hub to the lip of the fender vertically above, at each wheel.

Theoretically and as stated in the Factory Service Manual (FSM), ’cross levelling’ should be done when the vehicle is at “N” height.

In your case, you cannot raise to “N” height, so it is worthwhile to look at this even in the “LO” height position.

Just remember that this is a non-standard approach and it is important to ‘cross level’ properly at “N” height when the AHC system is working again. (There can be slightly different side-to-side effects at the extremes of “LO” and “HI” compared to “N” height due wear of springs and torsion bars or worn or out-of-adjustment mechanical components, especially on older vehicles).

When ‘cross-levelling’ the actual tape-measurement numbers at Front Right and Front Left hub-to-centre are not important but they should be equal within 10mm (0.39 inches).

The purpose of front ‘cross levelling’ has two parts:
  • to ensure that both torsion bars are carrying equal loads, and,
  • to avoid a built-in ‘lean’. This in turn causes confused readings ‘to be sent to the ECU by the Height Control Sensors (even with new Sensors as in this case), and then in turn this may cause the ECU to switch to ‘fail safe mode’ which may prevent AHC operation.
If the static (engine “OFF”) front hub-to-fender heights not close to equal, adjust using the torsion bar adjusters and NOT the Height Control Sensor adjusters to equalise the front side-to-side heights (at least within 10 mm difference, less difference is better) with the engine “OFF”.

Then read the Height Control Sensor adjusters again on your scanner. The readings won’t approach zero until the vehicle can be raised to “N”, so don’t worry about this yet, but reducing the differences in the readings also is very important – see “Background Issues and Symptoms” notes far below.

If the vehicle has been ‘cross levelled’ as well as can be done, but the Height Control Sensors are still showing such widely different readings, then it is possible that the Electronic Control Unit (ECU) has put the AHC/TEMS system into ‘fail safe mode’ -- as explained this post and in the FSM reference given in my previous post. In ‘fail safe mode’, the AHC/TEMS system will not work properly and may not work at all. There are other possibilities and these also are mentioned in the “Background Issues and Symptoms” notes below.

As a starting point, persistent widely different Height Control Sensor readings lead to suspicion about the condition of the Height Control Sensors or the various connectors and/or reliable continuity of the harness wiring to the ECU.

For this vehicle, even though the Height Control Sensors are new, it is still worthwhile to remove them and check them physically and electrically. This is set out in the FSM and is summarised above in Post #5 and later in this Post.

At the same time, suggest check electrical continuity of all connectors and relevant harnesses. (Note: Harness damage is unusual but this is a 22 year old vehicle, so physically inspect the underbody carefully for harness damage, especially at places where any harness is visible and may have been struck by stones or road debris — although both are uncommon in Singapore. Follow up any suspicions by testing electrical continuity).

At this time, also suggest check the voltage at the connectors and at the relevant terminals (SHB and SHG) at the ECU as described in the same section of the FSM as the other Height Control Sensor checks (C1711 at Page DI-226).

Again, absence of a DTC does not tell you that the system is healthy.

Special Note 1: There is no point in trying to correct a cross-levelling problem by adjusting the Height Control Sensors. If the vehicle steering is pointed straight ahead, or if the vehicle is stationary, then the Gate Valves in the Control Valve Assembly are open and the front AHC pressures are the same on both sides. The Front AHC pressures are not changed by adjusting the position of the Right and Left Sensors and so the side-to-side heights cannot be changed by adjusting the position of the Sensors. Attempts at ‘cross levelling’ with Height Control Sensors will result only in confusion and frustration.

Step 2: Increase load on front torsion bars

Turn both Torsion Bar adjusters clockwise (when looking up from underneath the vehicle) by say 5 to 8 turns or even more. Purpose: The vehicle is stuck in the “LO” height position, so AHC pressures cannot be measured by raising from “LO” to “N”. Therefore it is necessary to make a guess to increase Torsion Bar load to make sure that the AHC system is not overloaded. When the AHC system is working again, AHC pressures can be measured correctly and then at least front pressures easily can be adjusted correctly using the Torsion Bar adjusters.

Step 3: Test AHC Pump operation direct from 12 volt battery (not via ‘Active Test’) and related tests:

To avoid doubts about the condition of the ECU or the effect of some conditions in ‘fail safe mode’, remove the power connector from the electric motor which drives the AHC Pump, then use leads to connect the motor terminals correctly to the vehicle battery (or some other 12 volt battery). Be very careful to avoid arcing by not to touching the leads together and wear eye protection in case of accidents. For correct polarity, refer carefully to the diagrams in the FSM at this reference:

https://lc100e.github.io/manual/
Then follow these tabs:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS > C1762 at Page DI-257 – Fluid Pressure Abnormality

Purpose: To check in the above test that the AHC Motor and Pump actually are capable of running when voltage is supplied.

Then, even if no DTC’s are visible, also suggest doing the following tests using the procedure described “IN CASE OF NOT USING HAND HELD TESTER”. In your case, it is important test in this way to avoid any confusions with problems in other circuits, or with the ECU problems, or with the Techstream or other scanner reading a faulty ECU and which then gives faulty readings.

As part of the diagnosis, note in each case the specific, different ‘fail safe mode’ described in these sections of the FSM and compare these with symptoms observed on the vehicle. There is more than one ‘fail safe mode’.

Purpose: To check the health of the following circuits and components, as listed below:

C1718 at Page DI-229 – Fluid Pressure Sensor Circuit

C1719 at Page DI-233 – Fluid Temperature Sensor Circuit

C1731 at Page DI-239 – Control Valve Solenoid Circuit and Accumulator Solenoid Circuit
(Actually, it should be possible to hear and feel the operation of these valves – so these tests may not be necessary)

C1741 at Page DI-243 – AHC Motor Relay Circuit

C1743 at Page DI-248 – AHC Main Relay Circuit

C1751 at Page DI-252 – AHC Pump & Motor Circuit

Power Source Circuit at Page DI-294

Step 4: Check health of AHC Electronic Control Unit as described in FSM

C1761 at Page DI-256 – Malfunction in ECU (Memory Error)
This DTC was reported on this vehicle and refused to clear – see Post #1.

If the above Steps 1 to 3 have been completed with positive results in the checks and tests, then that indicates that those components and circuits in the AHC system are in good health.

Then, regardless of its appearance in the pictures in Post #1, an ongoing DTC C1761 certainly forces suspicion on the ECU – Part No. 89290-60010. In such a case, after eliminating all other possibilities as described in the previous Steps 1 to 3, the ECU should be replaced.

Step 5: In case of previous incorrect Sensor adjustments, force different Sensor positions

The following method only is worthwhile after the following conditions have been achieved:
  • cross levelling has been corrected as best possible,
  • condition of Sensors, connectors and harnesses are healthy and reliable,
  • AHC pressures are likely to be at the low end of the range,
  • ECU is healthy
If these conditions have been satisfied and Sensor readings remain widely different and if AHC still is not working, then consider whether there is a possibility that mistakes by some person have been made in the past concerning Height Control Sensor adjustment.

It is possible that in the absence of a good understanding, incorrect adjustment of the Sensors have been left them in a position in which they are transmitting different and incorrect voltages to the ECU. If so, the ECU may have forced ‘fail safe mode’ which stops AHC operation.

If this is suspected, the following method may help to rectify differences in Sensor settings and remove the ‘failure mode’ effect, even though the vehicle is stuck at “LO” height:
  • Firstly, for personal safety, secure the vehicle with stands under chassis rails so that it is impossible for downward movement of the car body onto a human body while working under the vehicle. This VERY important to prevent accidents and injury,
  • With Techstream or scanner connected, ignition key “ON” BUT engine “OFF” (important – do not want AHC to ‘wake up’ and raise or lower vehicle body during adjustments), eliminate differences in the Sensor readings at “LO” by moving all Sensors until all scanner readings are approximately the same as the lowest Sensor, shown as -62.4mm in Post #1.
  • Start engine, hoping that AHC will be ready to operate, press “UP” switch to see whether vehicle now raises from “LO” to “N”.

Best wishes for success!!

Can you please reply indicating whether there has been any progress with your vehicle?

++++++++++++++++++++

BACKGROUND ISSUES AND SYMPTOMS CONSIDERED IN MAKING THE ABOVE SUGGESTIONS IN POST #7:

Previous Posts in this thread about this 1999 LX470 have mentioned the following:

1. There have been a large investment in lot of replacements. Much of the AHC/TEMS systems could be expected to be in very good condition. New items include:

“all accumulators”
(presumably this means Gas Chambers, sometimes called ‘globes’ or ‘spheres’-- Front Part No. for LH & RH: 49141-60010 and Rear Part No. for LH & RH: 49151-60010, but presumably this does NOT include the long cylindrical Height Control Accumulator Part No. 49130-60010, midway along the Left chassis rail?),
  • “complete pump assembly including all switches” (presumably this means Part No. 48910-60012 which includes AHC Pump, Motor, Frame, Pressure Sensor, Temperature Sensor and Tank?),
  • “all 4 shock looking things (the ones that raise or lower the car” (presumably this means ‘Shock Absorbers’ -- Front Part No. 48510-60081 or 48510-69127 and Rear Part No. 48530-60042 or 48530-69145 ??)
  • “all 3 height sensors” (presumably this means Front Left Part No. 89406-60012 and Front Right Part No. 89405-60012 and Rear Part No. 89407-60010 ??)
  • “relay for ahc” -- Part No. uncertain? Does this mean Relay Part No. 88263-24020 or the actual ECU itself, Part No. 89290-60010?
  • Genuine AHC Fluid Part No. 08886-01805 has been used to bleed the system after replacing the above list of parts (presumable at that time the AHC system was working and enabled bleeding to take place??)
  • Presume bleeding method was as set out in the Factory Service Manual (FSM) and as described in the IH8MUD forum in many places?? The methods are important so as to reduce the risk of entrapped are remaining in the system. As well as forming obvious compressible ‘bubbles’, air can become partially dissolved in the AHC Fluid,
  • Presume also that the following IH8MUD Posts have been read,
Props to PADDO's AHC fluid replacement method! - https://forum.ih8mud.com/threads/props-to-paddos-ahc-fluid-replacement-method.925459/#post-10337483 by @PADDO

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/#post-13116520 by @LndXrsr

AHC Basics for dummies Video re CrossLeveling, Height Sensor adjustment, TB tweaking - https://forum.ih8mud.com/threads/ahc-basics-for-dummies-video-re-crossleveling-height-sensor-adjustment-tb-tweaking.1225842/#post-13383401 by @suprarx7nut

2. The vehicle is stuck in ”LO” and will not raise to “N”, and, Front and Rear AHC pressures and Height Control Accumulator pressure all read zero:

This Singapore-based LX470 is ~22 years old and maybe has travelled many kilometres. It is quite likely that the ‘spring rate’ in the front torsion bars and almost certainly ‘spring rate’ in the rear coil springs have deteriorated significantly. This is inevitable and happens over a long period of time. It means that the torsion bars and springs carry insufficient share of the vehicle weight and the AHC system carries an excessive share of the vehicle weight. Inevitably, AHC pressures rise as the vehicle ages. Eventually, a point is reached in which the AHC pressures are excessive. This may seem sudden but actually the situation has been building up slowly for years. At the excessive pressure point, the vehicle may suddenly sink to “LO” and stay there because the AHC pressures are lowest when the vehicle is at “LO” height. The AHC system suddenly may not operate at all in this condition.

When this is suspected in an older vehicle, it is worthwhile to turn both torsion bar adjusters clockwise (when looking up from underneath the vehicle) by say 5 to 8 turns or even more. The aim is to increase the load taken by the torsion bars and reduce the front AHC pressures. Hopefully, this may be enough to reactivate the AHC system.

It is not so easy to transfer the rear load from the AHC system to the rear coil springs – this usually requires at least spacers to be inserted to preload the springs but if springs have never been changed in 22 years, then new springs are long overdue. Anyway, start at the front.

3. The following Height Control Sensor readings were shown in Post #1 for this 1999 LX470 at “LO” height (with the vehicle stuck in “LO”):

-46.2 mm
Front Right Height Control Sensor

-62.4 mm Front Left Height Control Sensor

-15.4 mm Rear Height Control Sensor

Difference, Actual Right to Left: 16.2 mm

Difference,
Actual Front to Rear: (Right) 30.8mm; (Left) 47.0mm

Difference,
Specification, between sensors per FSM: +/- 5 mm

The actual numbers on this vehicle are all very different from one another and very different from FSM Specification, even though all the Height Control Sensors have been replaced.

It is not stated – but it is very important to know – what are the actual hub-to-fender tape-measurements that correspond to these Height Control Sensor readings??

Anyway, what can cause such large differences with new Sensors?

Even with the vehicle stuck at “LO”, this leads to an immediate suspicion that the vehicle has not been ‘cross levelled’ properly.

Or it may mean that one original front torsion bar is more ‘tired’ than the other, giving different side-to-side effects -- unusual, it would be more usual for both torsion bars to be similar, even if worn and ‘tired’,

Or it may mean that one original rear coil spring is more ‘tired’ than the other, giving different side-to-side effects. Example: if one rear spring is mismatched and holds one rear corner higher than the other, then the diagonal opposite front corner will be lower than the other front corner (assuming that the chassis is not twisted). Unusual, it would be more usual for both coil springs to be similar, even if worn and ‘tired’. However, it is another reason why the hub-to-fender tape measurements are important to collect as well as the Techstream or other scanner readings. Physical data is at least as important as data from scanners.

Or it may mean that some other mechanical component(s) in the suspension (between body and road, including wheel bearings, balljoints, bushes) have worn or are unequal in some way, giving different side-to-side effects.

Or is may mean that there is a problem with continuity at the electrical connectors to Sensors, or with continuity between the connectors and the harness leading back to the ECU.

Or such large differences in Height Sensor Control readings may mean that the AHC/TEMS system has switched itself into ‘fail safe mode’ for self-protection against some fault in the system. In this condition, the AHC system may not work fully, or may not work at all.

Or it may mean that the Electronic Control Unit (ECU) has failed.
 
Last edited:

bonestock

Transportation Specialist
SILVER Star
Joined
Feb 26, 2015
Messages
1,032
Location
Lookout Mountain, GA
I too have an LX470 that began having AHC issues. I saved myself a ton of time, effort, and money by deleting the AHC and replacing with all Land Cruiser parts, new OEM Torsion Bars, Rear Coils and Shocks. The AHC definitely rode better by I am very satisfied with the conversion.

Just my $0.02.
 
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
++++++++++++++++++++

BACKGROUND ISSUES AND SYMPTOMS CONSIDERED IN MAKING THE ABOVE SUGGESTIONS IN POST #7:

Previous Posts in this thread about this 1999 LX470 have mentioned the following:

1. There have been a large investment in lot of replacements. Much of the AHC/TEMS systems could be expected to be in very good condition. New items include:

“all accumulators”
(presumably this means Gas Chambers, sometimes called ‘globes’ or ‘spheres’-- Front Part No. for LH & RH: 49141-60010 and Rear Part No. for LH & RH: 49151-60010, but presumably this does NOT include the long cylindrical Height Control Accumulator Part No. 49130-60010, midway along the Left chassis rail?),
  • “complete pump assembly including all switches” (presumably this means Part No. 48910-60012 which includes AHC Pump, Motor, Frame, Pressure Sensor, Temperature Sensor and Tank?),
  • “all 4 shock looking things (the ones that raise or lower the car” (presumably this means ‘Shock Absorbers’ -- Front Part No. 48510-60081 or 48510-69127 and Rear Part No. 48530-60042 or 48530-69145 ??)
  • “all 3 height sensors” (presumably this means Front Left Part No. 89406-60012 and Front Right Part No. 89405-60012 and Rear Part No. 89407-60010 ??)
  • “relay for ahc” -- Part No. uncertain? Does this mean Relay Part No. 88263-24020 or the actual ECU itself, Part No. 89290-60010?
  • Genuine AHC Fluid Part No. 08886-01805 has been used to bleed the system after replacing the above list of parts (presumable at that time the AHC system was working and enabled bleeding to take place??)
  • Presume bleeding method was as set out in the Factory Service Manual (FSM) and as described in the IH8MUD forum in many places?? The methods are important so as to reduce the risk of entrapped are remaining in the system. As well as forming obvious compressible ‘bubbles’, air can become partially dissolved in the AHC Fluid,
  • Presume also that the following IH8MUD Posts have been read,
Props to PADDO's AHC fluid replacement method! - https://forum.ih8mud.com/threads/props-to-paddos-ahc-fluid-replacement-method.925459/#post-10337483 by @PADDO

The ABCs of AHC - How to Measure, Flush, and Adjust all in one place - https://forum.ih8mud.com/threads/the-abcs-of-ahc-how-to-measure-flush-and-adjust-all-in-one-place.1211999/#post-13116520 by @LndXrsr

AHC Basics for dummies Video re CrossLeveling, Height Sensor adjustment, TB tweaking - https://forum.ih8mud.com/threads/ahc-basics-for-dummies-video-re-crossleveling-height-sensor-adjustment-tb-tweaking.1225842/#post-13383401 by @suprarx7nut

2. The vehicle is stuck in ”LO” and will not raise to “N”, and, Front and Rear AHC pressures and Height Control Accumulator pressure all read zero:

This Singapore-based LX470 is ~22 years old and maybe has travelled many kilometres. It is quite likely that the ‘spring rate’ in the front torsion bars and almost certainly ‘spring rate’ in the rear coil springs have deteriorated significantly. This is inevitable and happens over a long period of time. It means that the torsion bars and springs carry insufficient share of the vehicle weight and the AHC system carries an excessive share of the vehicle weight. Inevitably, AHC pressures rise as the vehicle ages. Eventually, a point is reached in which the AHC pressures are excessive. This may seem sudden but actually the situation has been building up slowly for years. At the excessive pressure point, the vehicle may suddenly sink to “LO” and stay there because the AHC pressures are lowest when the vehicle is at “LO” height. The AHC system suddenly may not operate at all in this condition.

When this is suspected in an older vehicle, it is worthwhile to turn both torsion bar adjusters clockwise (when looking up from underneath the vehicle) by say 5 to 8 turns or even more. The aim is to increase the load taken by the torsion bars and reduce the front AHC pressures. Hopefully, this may be enough to reactivate the AHC system.

It is not so easy to transfer the rear load from the AHC system to the rear coil springs – this usually requires at least spacers to be inserted to preload the springs but if springs have never been changed in 22 years, then new springs are long overdue. Anyway, start at the front.

3. The following Height Control Sensor readings were shown in Post #1 for this 1999 LX470 at “LO” height (with the vehicle stuck in “LO”):

-46.2 mm
Front Right Height Control Sensor

-62.4 mm Front Left Height Control Sensor

-15.4 mm Rear Height Control Sensor

Difference, Actual Right to Left: 16.2 mm

Difference,
Actual Front to Rear: (Right) 30.8mm; (Left) 47.0mm

Difference,
Specification, between sensors per FSM: +/- 5 mm

The actual numbers on this vehicle are all very different from one another and very different from FSM Specification, even though all the Height Control Sensors have been replaced.

It is not stated – but it is very important to know – what are the actual hub-to-fender tape-measurements that correspond to these Height Control Sensor readings??

Anyway, what can cause such large differences with new Sensors?

Even with the vehicle stuck at “LO”, this leads to an immediate suspicion that the vehicle has not been ‘cross levelled’ properly.

Or it may mean that one original front torsion bar is more ‘tired’ than the other, giving different side-to-side effects -- unusual, it would be more usual for both torsion bars to be similar, even if worn and ‘tired’,

Or it may mean that one original rear coil spring is more ‘tired’ than the other, giving different side-to-side effects. Example: if one rear spring is mismatched and holds one rear corner higher than the other, then the diagonal opposite front corner will be lower than the other front corner (assuming that the chassis is not twisted). Unusual, it would be more usual for both coil springs to be similar, even if worn and ‘tired’. However, it is another reason why the hub-to-fender tape measurements are important to collect as well as the Techstream or other scanner readings. Physical data is at least as important as data from scanners.

Or it may mean that some other mechanical component in the suspension (between body and road) has worn or is unequal in some way, giving different side-to-side effects.

Or is may mean that there is a problem with continuity at the electrical connectors to Sensors, or with continuity between the connectors and the harness leading back to the ECU.

Or such large differences in Height Sensor Control readings may mean that the AHC/TEMS system has switched itself into ‘fail safe mode’ for self-protection against some fault in the system. In this condition, the AHC system may not work fully, or may not work at all.

Or it may mean that the Electronic Control Unit (ECU) has failed.


As mentioned previously in Post #5, it seems possible that this vehicle may have more than one fault in the AHC system.

There is another possibility which I should have mentioned for this 1999 LX470.

Some vehicles manufactured before October 25, 2002 have experienced blockages in the tiny strainers inside the AHC Pump caused by failure of an O-ring inside the Height Control Accumulator, with debris then passing through the system.

The attached Technical Service Bulletin (TSB) CP-3006 dated July 2003 provides the Diagnosis & Repair procedure for the following AHC malfunctions:
  • AHC does not work -- vehicle height does not go up,
  • Vehicle height goes up, but only slowly,
  • Cause of problem: Abnormal wear of main accumulator O−ring caused a clog in the AHC pump strainer,
  • Toyota/Lexus/KYB adopted the following countermeasure: O−ring material has been changed with an increase in anti wear performance as of Nov. 2002 production,
  • This TSB recommends the following ‘fix’ for this problem: Change the height control accumulator and pump sub−assembly as a set.
This a very expensive answer!!

The AHC Pump on this 1999 LX470 was replaced only recently, so if this problem is identified it would be better to replace only the Height Control Accumulator and dis-assemble the AHC Pump and clean the strainers. This is an unusual job but it is not difficult.

Some explanations with pictures concerning the AHC Pump can be found at

Post #67 by @BullElk at AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13401098 for pictures of a clogged pump.

Posts #69 and #70 by @IndroCruise at AHC pump out? - https://forum.ih8mud.com/threads/ahc-pump-out.1226629/page-4#post-13407297 and onwards shows pictures of AHC Pump disassembly and also show the tiny filters (strainers) within the Pump.

Pump disassembly, cleanup of the pump and backwashing the filters (strainers) are all straightforward and is made easy by removing the motor/pump/tank assembly from the vehicle..

However, this possibility does not explain all the AHC trouble symptoms reported for this 1999 LX470. In particular, it does not explain the very large differences in the new Height Control Sensor readings. It seems likely that the AHC system on this 22 years’ old vehicle may have several problems. All of the problems can be fixed with a methodical approach.

Therefore the methodical Steps set out in Post #7 are still recommended.
 

Attachments

  • AHC TSB for Diagnosis & Repair Procedure of AHC Pump Sub-Assembly_CP3006_1_1 (6).pdf
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Last edited:
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
I too have an LX470 that began having AHC issues. I saved myself a ton of time, effort, and money by deleting the AHC and replacing with all Land Cruiser parts, new OEM Torsion Bars, Rear Coils and Shocks. The AHC definitely rode better by I am very satisfied with the conversion.

Just my $0.02.
Agreed -- this can be the most economic and least-time/effort-required option, particularly for older vehicles depending on condition. However, as seen in Post #1 and Post #8, @oldlx470 already has spent a ton of money and time before filing the first post. It appears that nearly the entire AHC/TEMS system already has been replaced -- except only the Height Control Accumulator, Attenuator, ECU, pipework and harnesses, and the AHC Control Switch on the centre console. Singapore location is shown -- which is not large nor arduous from a vehicle environment point of view and so the vehicle is likely to be in otherwise good condition, depending of course on past maintenance. Hopefully, the information provided will enable the completion of the refurbishment and overcome current frustrations without too much more expense and allow benefit from the investment already made.
 
Last edited:
Joined
Jan 28, 2021
Messages
6
Location
singapore
Hi

thanks for all above.
I have not got it working jet.
The car keeps giving error about the ecu as per first post.
This leaves me to believe that the ecu is broken.
I have ordered a new ecu. Which when it arrive i will install, until that time the car is parked. Sad to not being able to use the car for so long.
 
Last edited:
Joined
Jan 28, 2021
Messages
6
Location
singapore
Agreed -- this can be the most economic and least-time/effort-required option, particularly for older vehicles depending on condition. However, as seen in Post #1 and Post #8, @oldlx470 already has spent a ton of money and time before filing the first post. It appears that nearly the entire AHC/TEMS system already has been replaced -- except only the Height Control Accumulator, Attenuator, ECU, pipework and harnesses, and the AHC Control Switch on the centre console. Singapore location is shown -- which is not large nor arduous from a vehicle environment point of view and so the vehicle is likely to be in otherwise good condition, depending of course on past maintenance. Hopefully, the information provided will enable the completion of the refurbishment and overcome current frustrations without too much more expense and allow benefit from the investment already made.
Yes the car is actually complete new. It has 0 rust. Never seen salt. And always stored inside. It really is a brand new car. All ball joints i had replaced 2000km ago. And all rubbers. Same as timing belt, rollers, bearings, new high output alternator and about 160 other things.
We made the choise to keep it forever and had a shop rebuilt everything to new spec. Even upgraded entire aircon to a stronger system and extra cooler. Put an extra oilcooler for the gearbox so it will last longer. Etc etc etc.
 
Joined
Dec 6, 2016
Messages
310
Location
Brisbane, Australia
Yes the car is actually complete new. It has 0 rust. Never seen salt. And always stored inside. It really is a brand new car. All ball joints i had replaced 2000km ago. And all rubbers. Same as timing belt, rollers, bearings, new high output alternator and about 160 other things.
We made the choise to keep it forever and had a shop rebuilt everything to new spec. Even upgraded entire aircon to a stronger system and extra cooler. Put an extra oilcooler for the gearbox so it will last longer. Etc etc etc.

As noted far above in Post #2, your DTC codes 1761 and 1743 taken together do suggest problems with the AHC Electronic Control Unit (ECU) itself.

This is a very unusual situation, so the subsequent suggestions related to further checks and procedures.

Thank you for showing the internal parts of the ECU in Post #1. These are rare pictures.

Anyway, let’s hope the new ECU fixes things and allows this fantastic vehicle to be enjoyed on the road again.

Could you please continue to post the ongoing story so that we all can learn from your experiences?
 
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Joined
Jan 28, 2021
Messages
6
Location
singapore
As noted far above in Post #2, your DTC codes 1761 and 1743 taken together do suggest problems with the AHC Electronic Control Unit (ECU) itself.

This is a very unusual situation, so the subsequent suggestions related to further checks and procedures.

Thank you for showing the internal parts of the ECU in Post #1. These are rare pictures.

Anyway, let’s hope the new ECU fixes things and allows this fantastic vehicle to be enjoyed on the road again.

Could you please continue to post the ongoing story so that we all can learn from your experiences?
The new ecu will arrive end next week i hope. So i will install it right away and report back. We simply can not find another problem so we have to trust the scantool and replace the ecu.
 

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