The ABCs of AHC - How to Measure, Flush, and Adjust all in one place (7 Viewers)

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So I would just like to say that I am incredibly grateful for the work that you guys have done putting this guide together. I try to pay it back by documenting most of the work that I do in video and uploading it to YouTube.

I finally got all of the tools that I needed to do a full AHC service on my vehicle and using this guide it went pretty damn smoothly considering I basically had no idea what I was doing.

The only issue I ran into was when I bought this vehicle a year ago I spent a week grinding off every single speck of rust from the undercarriage and then I put two coats of POR15 on it. Well unbeknownst to me I coated The ride height sensors in a layer of POR15 making them almost impossible to adjust without breaking them. I still was able to get them relatively close.

One point of clarification, the ride height sensors are to be measured with the engine off and ignition on. Is that correct? Because the ride height sensors change when you cut the engine on.

Anyhow, after following the guide step by step, this was my final result.

Screenshot_20220827-140418.png



Screenshot_20220827-140625.png

The first image is The final result with the engine running after transitioning from low to neutral.

The second image was from when I adjusted the ride height sensors with the engine off and ignition on.

It should be noted that I have installed King springs in the rear, and I measured them but they are the same height.

All in all I'm very happy with the results and it is very true that you should not let perfection be the enemy of Good when it comes to this maintenance.
 
TreatSmash said:
So I would just like to say that I am incredibly grateful for the work that you guys have done putting this guide together. I try to pay it back by documenting most of the work that I do in video and uploading it to YouTube.

I finally got all of the tools that I needed to do a full AHC service on my vehicle and using this guide it went pretty damn smoothly considering I basically had no idea what I was doing.

The only issue I ran into was when I bought this vehicle a year ago I spent a week grinding off every single speck of rust from the undercarriage and then I put two coats of POR15 on it. Well unbeknownst to me I coated The ride height sensors in a layer of POR15 making them almost impossible to adjust without breaking them. I still was able to get them relatively close.

One point of clarification, the ride height sensors are to be measured with the engine off and ignition on. Is that correct? Because the ride height sensors change when you cut the engine on.

Anyhow, after following the guide step by step, this was my final result.

View attachment 3098617


View attachment 3098618
The first image is The final result with the engine running after transitioning from low to neutral.

The second image was from when I adjusted the ride height sensors with the engine off and ignition on.

It should be noted that I have installed King springs in the rear, and I measured them but they are the same height.

All in all I'm very happy with the results and it is very true that you should not let perfection be the enemy of Good when it comes to this maintenance.
Click to expand...

Great progress in getting a scanner working and getting on top of the AHC/TEMS systems!!

With no intention of raining on the party, here are some points to ponder:

  • Check that all physical tape measurements of heights and all Height Sensor readings and adjustments were made on level ground, such as a garage floor, not in the street or on a driveway? Do not want any fall built into the measurements – from side to side of the vehicle, nor front to rear of the vehicle,

  • Check that before Height Control Sensor adjustments were made, Front was ‘cross-levelled’ at “N” height setting with torsion bar adjusters per FSM with engine OFF (not running) to equalise Front heights as measured by tape measure and also to equalise torsion bar loads? Do not want any vehicle ‘lean’ built into the adjustments and causing fake measurements and confusion. [At the start, it does not matter what the measurements may be, just want them equalised at “N” height setting, set the ‘ride heights’ later with the Height Control Sensor adjusters, not the torsion bar adjusters],

  • Check Front and Rear hub-to-fender ‘ride heights’ with tape measure at “N” height setting? If necessary adjust by raising Front and Rear of vehicle by “Active Test” or with jacks etc to the desired ‘ride heights’ all round, usually hub-to-fender Front 19.75 inches (500 mm) and Rear 20.50 inches (520 mm). Then with Ignition ON, Engine OFF (not running), adjust Height Control Sensors as close as possible to zero using sliding bolt (and/or the very fiddly heim adjusters at Front). Do not want engine ON (running) at this time because do not want AHC system to try and compensate for adjustments while they are being made – futile and also dangerously unsafe.

  • Check that actual physical hub-to-fender tape measurements also were made at each wheel to match each set of readings of Height Control Sensors? The Sensor readings cannot be interpreted properly without the physical tape measurements at each wheel.

Some comments on the reported numbers ….

Set of readings: Ignition ON, Engine OFF (not running), after adjusting Height Control Sensors as close as possible to zero:

- 0.0 mm Front Left Height Control Sensor
- 1.2 mm Front Right Height Control Sensor
- 4.8 mm Rear Height Control Sensor

The above numbers are reasonable – all within the expected range of 5 mm of one another. Maybe the Rear Sensor could be raised slightly to take it nearer to zero. What were the hub-to-fender tape measurements at this time?

Set of readings: Ignition ON, Engine ON (running), after raise from “LO” to “N”:

- 7.4 mm Front Left Height Control Sensor
- 1.0 mm Front Right Height Control Sensor
-10.4 mm Rear Height Control Sensor

This set of numbers is strange – noticeable difference Left Front to Right Front, large differences Front to Rear. What were the hub-to-fender tape measurements at this time? Have they changed?

The overall negative numbers suggests that the “LO” to “N” raise is not quite completed. The vehicle is expected to raise to the close-to-zero settings of the Height Control Sensor adjusters as previously set at “N” height – that is exactly what the Active Height Control (AHC) system is designed to do.

There are a number of possibilities for this. If the bullet points at the start of this post all are checked and OK, then the next suspicions are the 16 years old Height Control Sensors, even though this is a relatively young, low mileage vehicle. For the reasons explained in earlier posts in this thread and elsewhere, absence of a Height Control Sensor DTC does not provide assurance that the Sensors are healthy.

If ride comfort is good, it probably is too early to worry too much – but it is worth monitoring over time, especially if ride comfort deteriorates and Height Sensor-related AHC ‘fail safe function’ is suspected.
 
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Good thread. What OBDII scanner can I get to read all this information that doesn't involve my phone?
 
Tentsontop said:
Good thread. What OBDII scanner can I get to read all this information that doesn't involve my phone?
Click to expand...

I use the OBDLink MX+ but you have to use it with your phone. It's rather inexpensive compared to the standalone devices. Most people here use techstream, but I don't have a laptop so that's a no-go for me.
 
Yeah. I'd rather have a stand alone reader. I don't do anything on my phone if I can avoid doing so.
 
Tentsontop said:
Yeah. I'd rather have a stand alone reader. I don't do anything on my phone if I can avoid doing so.
Click to expand...

Some choices appear below in this cut-and-paste from elsewhere:

It IS important to see the AHC-related Diagnostic Trouble Codes (DTC’s) stored on the Suspension Electronic Control Unit (ECU) – because this helps give pointers to what is happening – and just as important, the absence of DTC’s may (but not always) help eliminate possibilities. When added to physical observations and measurements around the vehicle, ECU data from a scanner helps reduce the guesswork and allows a more systematic diagnostic approach.

None of this is difficult.

This means obtaining and using a scanner to ‘read’ the Suspension Electronic Control Unit (ECU), including the following (among other things):
  • Front AHC pressure (MPa),
  • Rear AHC pressure (MPa),
  • Height Control Accumulator pressure (MPa),
  • Left Front Height Control Sensor (inches or millimetres),
  • Right Front Height Control Sensor (inches or millimetres),
  • Rear Height Control Sensor (inches or millimetres),
  • Diagnostic Trouble Codes (DTC’s) with actual codes identified.
A scanner by itself can tell nothing about the torsion bar adjustments, nor whether the vehicle is cross-level at the Front, nor condition of rear springs, nor whether the ‘globes’ are in good or bad condition. Vehicle observation and hands-on physical measurements also are required.

The Suspension ECU resides under the dashboard. It is the ‘brains’ of the Active Height Control (AHC) system. Just as importantly, it is the ‘ brains’ of the Toyota Electronic Modulated Suspension (TEMS) system – which is responsible for ‘ ride comfort’ by varying damping response almost instantaneously in real time in up to 16 steps according to road conditions, with other inputs including speed, turns, braking, and a range of other factors.

This ECU interacts with multiple other ECU’s on the vehicle and takes input from them. Some examples:
  • inputs are received from the door switches and rear hatch (if any of these are open, AHC/TEMS does not work);
  • inputs are received from the brake light switch, and if the foot remains on the brake pedal for an extended period, AHC/TEMS does not work;
  • inputs from wheel speed sensors are used by both the Suspension ECU and the ABS ECU;
  • etc, etc, etc.
Scanner connection to the Suspension ECU is via Diagnostic Link Connector #3 (DLC3) which also is under the dashboard near the brake pedal.

Choices of scanners include the following:



  • for service by a Toyota/Lexus Dealer: ask workshop to run the tests AND provide print-outs (not just verbal advice), using in-house Techstream scanner or Toyota/Lexus Intelligent Tester II by DENSO – a fee will be charged.

In addition and before doing anything else, it is important to record the actual hub-to-fender distances at each of the four wheels using a tape-measure.

For AHC/TEMS work, the following are essential:
  • workplace safety -- chassis stands in place to ensure vehicle body can never descend onto human body,
  • level ground – like a garage floor, not in a street or roadway which will have some ‘fall’ or slope for drainage purposes,
  • no people or loads in or on the vehicle (other than permanent fixtures),
  • a tape measure,
  • a multimeter,
  • an unbent paperclip (used in manual "Active Test" in place of Special Service Tool SST 09843-1820 connector!!),
  • before making any AHC-related adjustments, ‘cross-level’ Front using torsion bar adjusters at “N” height – the actual measured values are unimportant at this stage, just make them equal regardless of suspension behaviour – with all tyres in similar condition and at same tyre pressures,
  • adjust ‘ride height’ later using Height Control Sensor adjusters (not Front torsion bar adjusters),
  • when ‘ride heights are correct at “N” height, set Height Control Sensor adjusters to near as possible to zero (ignition ON, engine OFF),
  • after cross-level and ‘ride heights’ are correct, use scanner to measure adjustments to bring Front and Rear AHC pressures into FSM-specified ranges, after height movements “N” to “LO” to “N” -- means torsion bar adjustments at Front, spacers or spring upgrades at Rear,
  • after cross-level, ‘ride heights’ and AHC pressures are all correct, check overall ‘globe’ condition by observing difference in fluid levels at AHC Tank at “LO” height and “HI” height (AHC pressures by themselves tell nothing about ‘globe’ condition),
  • drive vehicle, after suspension has settled, fine-tune all adjustments and measurements as necessary.
 
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That will work. Thank you!
 
I have a question;

The 'cross-level' seems to be a strange thing for me. Does the AHC, and the sensors, not ensure the front of the vehicle is level? Would the system no simply increase pressure to compensate for less pre-load on the TB's, thus making the measure part redundant?

I have similar front pressures L+R, and the same measured height, yet my TB are tensioned differently.
 
harshjay said:
The 'cross-level' seems to be a strange thing for me. Does the AHC, and the sensors, not ensure the front of the vehicle is level?
Click to expand...
No. In normal operation the right and left sides are connected so the pressures will be equal. The ECU can isolate them when cornering to reduce body roll, but can’t independently adjust R/L pressure. See the pdf docs in the second post in this thread for more info.
 
harshjay said:
I have a question;

The 'cross-level' seems to be a strange thing for me. Does the AHC, and the sensors, not ensure the front of the vehicle is level? Would the system no simply increase pressure to compensate for less pre-load on the TB's, thus making the measure part redundant?

I have similar front pressures L+R, and the same measured height, yet my TB are tensioned differently.
Click to expand...

@Rebus Knebus already has answered the question succinctly.

Here is a long note written for someone else long ago and which may be of use to others following this thread and who are new to AHC ….

The Toyota AHC/TEMS system was designed around 30 years ago in the early 1990’s -- and appeared on LX470 (and LC100 where fitted) from 1998. It is a simple two-channel system – Front and Rear. It is not a four-channel system. It cannot and does not raise or lower each wheel independently. This is why on Techstream or other scanner only Front and Rear AHC pressures can be seen, along with the separate measurement of the Height Control Accumulator pressure.

The Control Valve Assembly contains only two Levelling Valves – Front and Rear – and so the system is capable only of raising, lowering or self-levelling the Front and Rear, not each wheel. These valves are ‘normally closed’ and are opened by the Suspension Electronic Control Unit (ECU) only when the height of the vehicle is being changed by the driver at the centre console switch, or, the when the ECU causes the AHC system to self-level the vehicle at Front and Rear (not side-to-side) in response to signals from the Height Control Sensors. It is these Levelling Valves which hold back the AHC pressure and prevent the vehicle from sinking when the engine and/or the AHC system is OFF.

When the vehicle is stationary, or, in motion with the Front wheels straight ahead, the two Gate Valves located in the Control Valve Assembly – one for Front, and one for Rear – are in their ‘normally open’ position. This means that AHC Front Left and AHC Front Right are hydraulically connected and must be at the same pressure in these situations (unless there is a fault or a blockage). Ditto Rear Left and Rear Right.

It is only when the vehicle turns at speed (I don’t remember what speed) that the Gate Valves are closed by the ECU, making the side on the outside of the turn more stiff than the side on the inside of the turn – and in this way providing more roll resistance.

Because of the interconnection, the Front Left and Front Right pressures can never be measured individually or adjusted separately using readings on Techstream or other scanner, nor even with pressure gauges. Again, ditto Rear Left and Rear Right.

There is no hydraulic connection between the Front and Rear AHC hydraulic circuits. The ECU determines what happens in the Front and Rear parts of the AHC system.

The Front and Rear AHC pressures can be measured only when the vehicle is stationary and when the FSM procedure is followed: temperature sensor disconnected, height change N > LO > N, wait ~30 seconds to ensure that vehicle has settled and Height Control Accumulator has been recharged by the AHC Pump and the AHC Pump has stopped, then record pressure readings.

There is only one Pressure Sensor in the AHC system – mounted next to the AHC Pump. The ECU records each of the three pressures in a sequence – Front, Rear, and Height Control Accumulator – and displays these on Techstream or other scanner. The measurements are not instantaneous in real time as happens with, say, engine oil pressure. The alternative is to read actual AHC pressures at any instant by installing pressure gauges in place of the bleeder screws.

The AHC/TEMS system has no sensory connection with the torsion bars (or rear springs). The Suspension Electronic Control Unit (ECU) has no way of knowing how these are set – or whether the vehicle is leaning side-to-side – or whether the vehicle is sloping incorrectly between Front and Rear. Only the Front and Rear AHC pressures caused by the torsion bar and spring settings can be detected by the ECU, along with the Height Control Sensor signal at each wheel.

The individual Height Control Sensors (two Front, one Rear) each provide a voltage signal to the ECU. The signal is proportional to the height at the relevant wheel position. Techstream (or other scanner) translates this signal into a reading in inches or millimetres because in theory this can be related to the FSM-specified vehicle height at Front and Rear (as distinct from the IH8MUD hub-to-fender reliable approximation which we all use ).

‘Cross-levelling’ at the Front is the fundamental first step in measuring or adjusting the AHC system.

The checks or adjustment of Front ‘cross-level’ must be done on a level surface (like the concrete floor in a garage or workshop, not in the street), without unequal added weight on either side of the vehicle, with fuel tank(s) full, with no persons or temporary loads on board, with front wheels straight ahead. The FSM specifies ‘engine OFF’ – maybe this does not matter so much because of the Left-Right hydraulic connection – but 'cross levelling' should be done as specified by FSM to avoid confusion and for personal safety and to avoid any unwanted response from the AHC system during torsion bar adjustment.

The purposes of ‘cross-levelling’ are two-fold:
  • to avoid – or rectify – any side-to-side ‘lean’ of the vehicle, noting that the AHC system cannot make side-to-side adjustments by itself,
  • to equalise the share of vehicle load carried by each torsion bar – noting that for a spring, force is proportional to the depressed distance (deflection) of the spring, so if the heights on both sides of the vehicle are approximately equal, then the loads on the torsion bars will be approximately equal (unless there is a mechanical fault in the non-AHC part of the suspension or running gear, or the body or the chassis is distorted).
Some further notes:
  • Unequally loaded torsion bars will result in different vehicle behaviour in Right and Left turns,
  • Different to a conventional suspension, on a AHC-equipped vehicle the torsion bar adjusters are not used -- and cannot be used -- to set the ‘ride height’ of the vehicle,
  • After Front ' cross-levelling' has been done correctly, Front and Rear ‘ride heights’ and vehicle Front to Rear rake are set using the Height Control Sensor adjusters,
  • If ‘cross-levelling’ is not correct at the Front of the vehicle, then it cannot be correct at the Rear of the vehicle (unless there is an issue somewhere in the mechanical parts of the suspension or running gear or in the chassis or the body),
  • If ‘cross-levelling’ is not correct at the Front of the vehicle, then the errors here will make it impossible to correctly set or adjust the Height Control Sensors, which in turn will lead to further problems and confusion,
  • If the ECU receives widely different signals from the Height Control Sensors, the ECU may place the system in 'fail safe function' in which AHC function and the variable damping function by TEMS are prohibited,
  • The number of threads visible at the Right and Left torsion bar adjuster bolts may be different – and they often are – but this is not a reliable indicator of torsion bar loading,
  • Equalisation of Front heights as measured with a tape-measure (not Techstream or other scanner) is the required method to equalise torsion bar loadings.
If not seen previously, the attachments may assist understandings of how the Active Height Control (AHC) system and the Toyota Electronic Modulated Suspension (TEMS) are meant to work when all parts are healthy and correctly adjusted. The function of individual components are described with diagrams. The hydraulic circuits are described in words and diagrams in various different situations. Strongly recommended -- definitely worth a re-read any time when AHC and/or TEMS questions arise.

Hope this helps! Congratulations if you got this far – apologies offered if TL:NR ….
 

Attachments

  • AHC - Cross level per FSM.pdf
    364.6 KB · Views: 36
  • AHC LC100-LX470 - General Description and Diagrams.pdf
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Just trying to post on a relevant thread.

We are going to check out a LX470 this weekend and I am new to the AHC world. All I know is the system tends to fail and most people swap it out with a normal setup

Q : If the system is no longer working, does the vehicle still drive and handle OK?
Q : The salesman said he pressed the button the up and down control, the light flashed a bunch and nothing appeared to be happening. Safe to say the system is not functioning properly?


They want $9k with 220k miles on a 1999 LX470. Spendy, but maybe worth it.
 
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When I bought mine years ago, the AHC did not cycle up and down. Handling was ok. I flushed the system, a one banana job, and the ride felt slightly better. I didn't feel it was a huge difference. Led me to believe that my AHC system just needed a flush and fill. Cycled up and down with no issues after the flush. It had likely never been done in 12 years prior to me buying it. But if it doesn't cycle, it may be problems with other components in the system.

Keep in mind, the system won't cycle, for saftey reasons, if you have the door open, hatch open, e-brake on, and maybe some other things. If working properly, it should cycle while idling in park, all doors shut, flat ground.
 
90WT said:
When I bought mine years ago, the AHC did not cycle up and down. Handling was ok. I flushed the system, a one banana job, and the ride felt slightly better. I didn't feel it was a huge difference. Led me to believe that my AHC system just needed a flush and fill. Cycled up and down with no issues after the flush. It had likely never been done in 12 years prior to me buying it. But if it doesn't cycle, it may be problems with other components in the system.

Keep in mind, the system won't cycle, for saftey reasons, if you have the door open, hatch open, e-brake on, and maybe some other things. If working properly, it should cycle while idling in park, all doors shut, flat ground.
Click to expand...
Great advice thanks
 
If the front AHC pressure is too high, a few cranks of the torsion bars (clockwise) might relieve enough pressure to raise back up. If you use techstream already, you can see the sensor values, if one sensor shows no data or something, maybe buy one of those from the classifieds here. I bought a set of 3 takes offs here on mud and leave them in my LX when i'm far from home, just in case one of them fails.
 
90WT said:
Keep in mind, the system won't cycle, for saftey reasons, if you have the door open, hatch open, e-brake on, and maybe some other things. If working properly, it should cycle while idling in park, all doors shut, flat ground.
Click to expand...
This … ^^^
 
A few weeks ago when wheeling, I noticed that the front left seemed lower then the right front.
Finally last weekend I got my tape measure out and took measurements.
From bottom rim to bottom fender LF was 704mm, RF was 720mm.
Even though the difference is only 16mm, it was noticeable.

Out came my old laptop with XP with Techstream on it and read the heights and pressures:

1666581121049.png



So I cranked the TB and now the heights are pretty close. Checked TS again and it looks way better.
I have non-AHC springs in the rear for the additional load of long range tank, dual spare wheel carrier which accounts for the low pressure in the rear.

1666581161254.png
 
Long-time reader, first-time poster. I owe many of you several beers for the trouble you’ve saved me over the years. I wanted to share some fun numbers for you from my current AHC maintenance/troubleshooting quest.

My 99.5% stock ride (2006 LX470) has been very bouncy lately. I think one YouTuber calls it the pogo-stick effect. Quite accurate. My rig still travels the full 4 inches between L and H, but there is no discernible difference between comfort and sport, and very poor damping.

I’ve used YotaMD’s 100 series AHC Cheat Sheet, and @Getco’s TechStream in 5 minutes (and interface cable) to determine the following abysmal numbers:

Front pressure: 8.9MPa
Rear pressure: 8.2MPa
Gradation change: 4(!)

I tried to purchase the 1L bottles of AHC fluid as suggested here but the distributor I tried wanted $176 hazardous materials fee so I called up my local stealership and was charged $120 for the 2.5L can. While I was there, I asked, on a whim, what the cost of the accumulator globe replacement kits would cost me.

Are you ready?

Front globe kit (49141-60010): $1,186.66 plus tax.
Rear globe kit (49151-60010): $1,271.03 plus tax.

…for a total combined price (parts only+tax) of $5,420.97 o_O

OUCH!

Needless to say, YES, I will complete steps B and C this weekend, but based on what I’m reading here in the forums, with a gradation change of only 4, I’ll likely be buying all 4 globes from impex for ~$700.

Just wanted to share the latest cost quotes here in Silicon Valley, California, USA. :cheers:
 
I just ordered front control arm bushings. About half the price of anywhere in the US.
 
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