AHC troubles, what’s my next step? (1 Viewer)

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Well, at this point I’ve gone though just about everything on the AHC system trying to chase down an issue that I can’t see. A couple weeks ago I had a rear axle overload issue, and it dropped into low (basically on the bump stops) and started flashing the AHC off light.

On TechStream I am not able to read any of the AHC related DTC’s, getting an error code every time I attempt to read the trouble codes.

At this point I have added non-AHC springs with airbags in the rear to help with the heavy weight, and now it is driveable, but not handling well, and also not able to go into High or even Neutral.

So in an attempt to get the AHC functioning again without being able to see the codes, I’ve replaced the AHC ECU with a known good unit, the height control switch (on the center stack), all related mini fuses, swapped and tested relays, got a new main AHC relay (also tested), known good rear height sensor, made sure sensors were all reading within .5inches of one another, taken most of the weight off of the system with jacks, checked main valve body connectors on the drivers frame rail, and last but not least tested the pump to make sure it functions.

The only thing in all of that that did give me cause for suspicion was when I was testing the pump, it turned on with JUST power to the positive post on the plug, the negative post wasn’t hooked up to anything. Is that cause to deem the pump dead?

I’m getting pretty annoyed at this point, but we’ve got a trip we were suppose to leave on about an hour ago, so gotta get this thing done. Thank you all for your help!
 
A couple other things, I’m showing a low voltage for both IG Power Source Voltage, and +B Power Source Voltage.

12.5v for IG, 1.6v for +B.

If I attempt to run the active tests in TechStream, it bumps the +B voltage up to around 10.5-11v for a second, then it drops back down.

Alternator was installed last year and puts out 14+ Volts at idle, also battery was new at the same time.

I also have attempted several variations of jumping pins on DLC1 (passenger side fender) and DLC3 (obd2), can’t get anything to happen other than the DLC1 pin jump causes the Off light to blink every .25 seconds, regardless of weather or not you hit the down height control switch 5 times in under 3 seconds.
 
The “+B” to the ECU comes from the AHC Main Relay (which is different from the AHC relay which powers the pump). Have you looked at voltages and operation of this relay?
 
The “+B” to the ECU comes from the AHC Main Relay (which is different from the AHC relay which powers the pump). Have you looked at voltages and operation of this relay?
Sure did, and even replaced it with a new relay, and it does turn on when the ignition is turned to the run position.

It sounds like it turns on and back off when you turn the car on.
 
Sure did, and even replaced it with a new relay, and it does turn on when the ignition is turned to the run position.

It sounds like it turns on and back off when you turn the car on.

So I read this thread while looking for something else. That B+ voltage certainly is way, way low at 1.6 volts and the IG voltage does not look good at 12.5 volts either.

There is something wrong here but the cause is not clear to me. It has to be sorted out to get anywhere. Maybe other IH8MUD Members can help here??

These voltages are VERY different to my last results (multiple tests): B+ 12.9 to 13.2 volts and IG 13.8 to 14.0 volts, in my case 2006 LC100 with new Toyota (DENSO) alternator Part No. 27060-17230 about 2.5 years old and correct for my 1HD-FTE turbodiesel engine; batteries 1.5 years old (two in parallel is standard for 1HD-FTE turbodiesel engine). Some electrics might be a little different with 2UZ-FE gasoline engine but not THAT different – and the AHC system is the same across the models except for some differences in RHD and LHD ECU’s (different Part Numbers) and different AHC Pump location and underbody pipework.

Every time I see a Techstream screenshot of an electrically healthy AHC system with 2UZ-FE engine, the B+ voltage and IG voltage are very similar to mine, as above.

I have used the attached hands-on “Active Test” procedure (long name in FSM: “Height Control Operation Test”) obtained from a post long ago by @PADDO, (using an unbent paperclip as the bridging wire rather than the mythical Special Service Tool (SST) 09843-18020 -- which is just a piece of bridging wire). It has worked many times and every time. It is more convenient to use the DLC1 “DIAGNOSTIC” socket in the engine bay above the Right Hand Front wheel arch rather than the slightly different pin-out in the DLC3 socket under the dashboard and as described in some more recent FSM versions.

Each time I have been in a discussion where the “Active Test” has not worked, the cause has been either (1) occasional incorrect procedure -- sometimes that is just a gentle reminder to close all doors and tailgate, or (2) often AHC ‘system’ has not allowed power to get through to the motor driving the AHC Pump, even if the pump motor operation has been tested and been found healthy by direct connection to a 12 volt source.

In the case of (2), the search then begins for the cause or fault in the AHC ‘system’ and/or a physical fault which is results in power supply not arriving at the motor driving the AHC Pump while under the control of the ECU (not by direct connection of the motor to a 12 volt supply).

Two of the critical checks are the “AHC-B” fuse and the health of the “AHC Main Relay” (not to be confused with the “AHC Relay”). @Rebus Knebus has described this previously in this post.

I do have to say that such a low B+ voltage makes me wonder what is going on with the AHC Main Relay, or its connector, or its harness back to the ECU and the connectors there -- meaning is the relay actually receiving the voltage it needs to operate properly?

Anyway, if you are now convinced that all is good with relevant fuses and AHC Main Relay per your post #4 in this thread – then what’s next?

In post #4, you mention:

“It sounds like it [the AHC Main Relay?] turns on and back off when you turn the car on”.

Does this happen also when you do the “Active Test”?

If this is what is happening, it may mean that power actually does arrive at the AHC Pump motor – but does not stay"ON" for long.

That situation might suggest that the AHC Pump is blocked at its small internal filter/screens. This may be caused by old dirty fluid and debris picked up in the system, OR, it may be a legacy of problematic AHC Fluid in 1 litre black bottles in the US market in 2021 – reported in the IH8MUD “100 series Forum” and the “200 series Forum”.

In that case, Techstream might reveal DTC 1751 and DTC 1762 if the AHC Pump is starved for fluid and is drawing air around the seals. You would expect to hear an unusually noisy pump in that situation, rather than the normal subdued noise.

Or Techstream may reveal C1743 if a downstream blockage causes over-pressure inside AHC Pump. The Pump is a positive displacement gear pump, so the pressure build-up and protective automatic ‘switch-off’ would be very quick.

Either way, the fix would mean pulling the AHC Pump and dis-assembling and cleaning it, OR, simply replacing the AHC Pump and relevant O-rings and other seals for a parts cost of around USD200 from IMPEX or Partsouq delivered in USA.

It appears that Techstream has indicated 9 DTC’s but has not revealed what they are – so if it is helpful, the relevant descriptions for the abovementioned C1743, C1751 and C1762 (and indeed all of the AHC DTC's) can be found at
LC100 Workshop Manual - https://lc100e.github.io/
Then tabs in left hand panel on the opening page:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS
This reference is good for AHC systems on both LC100 and LX470

If there are 9 DTC’s, there could be other causes of problems.

Two "simple" but different frustrations are described here by @Mauser and separately here by @suprarx7nut.
 

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So I read this thread while looking for something else. That B+ voltage certainly is way, way low at 1.6 volts and the IG voltage does not look good at 12.5 volts either.

There is something wrong here but the cause is not clear to me. It has to be sorted out to get anywhere. Maybe other IH8MUD Members can help here??

These voltages are VERY different to my last results (multiple tests): B+ 12.9 to 13.2 volts and IG 13.8 to 14.0 volts, in my case 2006 LC100 with new Toyota (DENSO) alternator Part No. 27060-17230 about 2.5 years old and correct for my 1HD-FTE turbodiesel engine; batteries 1.5 years old (two in parallel is standard for 1HD-FTE turbodiesel engine). Some electrics might be a little different with 2UZ-FE gasoline engine but not THAT different – and the AHC system is the same across the models except for some differences in RHD and LHD ECU’s (different Part Numbers) and different AHC Pump location and underbody pipework.

Every time I see a Techstream screenshot of an electrically healthy AHC system with 2UZ-FE engine, the B+ voltage and IG voltage are very similar to mine, as above.

I have used the attached hands-on “Active Test” procedure (long name in FSM: “Height Control Operation Test”) obtained from a post long ago by @PADDO, (using an unbent paperclip as the bridging wire rather than the mythical Special Service Tool (SST) 09843-18020 -- which is just a piece of bridging wire). It has worked many times and every time. It is more convenient to use the DLC1 “DIAGNOSTIC” socket in the engine bay above the Right Hand Front wheel arch rather than the slightly different pin-out in the DLC3 socket under the dashboard and as described in some more recent FSM versions.

Each time I have been in a discussion where the “Active Test” has not worked, the cause has been either (1) occasional incorrect procedure -- sometimes that is just a gentle reminder to close all doors and tailgate, or (2) often AHC ‘system’ has not allowed power to get through to the motor driving the AHC Pump, even if the pump motor operation has been tested and been found healthy by direct connection to a 12 volt source.

In the case of (2), the search then begins for the cause or fault in the AHC ‘system’ and/or a physical fault which is results in power supply not arriving at the motor driving the AHC Pump while under the control of the ECU (not by direct connection of the motor to a 12 volt supply).

Two of the critical checks are the “AHC-B” fuse and the health of the “AHC Main Relay” (not to be confused with the “AHC Relay”). @Rebus Knebus has described this previously in this post.

I do have to say that such a low B+ voltage makes me wonder what is going on with the AHC Main Relay, or its connector, or its harness back to the ECU and the connectors there -- meaning is the relay actually receiving the voltage it needs to operate properly?

Anyway, if you are now convinced that all is good with relevant fuses and AHC Main Relay per your post #4 in this thread – then what’s next?

In post #4, you mention:

“It sounds like it [the AHC Main Relay?] turns on and back off when you turn the car on”.

Does this happen also when you do the “Active Test”?

If this is what is happening, it may mean that power actually does arrive at the AHC Pump motor – but does not stay"ON" for long.

That situation might suggest that the AHC Pump is blocked at its small internal filter/screens. This may be caused by old dirty fluid and debris picked up in the system, OR, it may be a legacy of problematic AHC Fluid in 1 litre black bottles in the US market in 2021 – reported in the IH8MUD “100 series Forum” and the “200 series Forum”.

In that case, Techstream might reveal DTC 1751 and DTC 1762 if the AHC Pump is starved for fluid and is drawing air around the seals. You would expect to hear an unusually noisy pump in that situation, rather than the normal subdued noise.

Or Techstream may reveal C1743 if a downstream blockage causes over-pressure inside AHC Pump. The Pump is a positive displacement gear pump, so the pressure build-up and protective automatic ‘switch-off’ would be very quick.

Either way, the fix would mean pulling the AHC Pump and dis-assembling and cleaning it, OR, simply replacing the AHC Pump and relevant O-rings and other seals for a parts cost of around USD200 from IMPEX or Partsouq delivered in USA.

It appears that Techstream has indicated 9 DTC’s but has not revealed what they are – so if it is helpful, the relevant descriptions for the abovementioned C1743, C1751 and C1762 (and indeed all of the AHC DTC's) can be found at
LC100 Workshop Manual - https://lc100e.github.io/
Then tabs in left hand panel on the opening page:
Repair Manual > DIAGNOSTICS > ACTIVE HEIGHT CONTROL SUSPENSION & SKYHOOK TEMS
This reference is good for AHC systems on both LC100 and LX470

If there are 9 DTC’s, there could be other causes of problems.

Two "simple" but different frustrations are described here by @Mauser and separately here by @suprarx7nut.
@IndroCruise Thank you so much for your detailed reply! Somehow I failed to notice that I had received another reply on this thread, so I apologize for taking so long to revisit this.

At this point I have continued to attempt fixes on this issue, but still have not landed on a solution. Up until last month, I was driving it in "low" mode, and really not that unhappy with how the suspension felt, other than sitting lower than I would like. But front shock bushing failure necessitated removing the front AHC shocks to install bushings, which in turn also required that I open up the AHC system and deal with this issue. I also upgraded front torsion bars to stock Land Cruiser bars to help with all of my additional weight, as well as changed out ball joints and UCA's to NitroGear units. I drained both front shocks after removing them, and proceeded to bleed all of the AHC fluid out of the globes, and suction the fluid out of the AHC fluid reservoir. I then removed the AHC pump to disassemble and clean. After cleaning out the entire pump assembly, I put everything back together and filled the reservoir with a brand new 2.5L can of AHC fluid.

I still was not able to run the DLC1 or the TechStream "Active Test" it would not ever respond other than a single click before displaying an error for the TechStream Relay tests.

I was able to use the 'OFF" light in conjunction with the DLC1 port to go though the DTC codes by the flashing method, and was able to finally figure out what the AHC ECU is having an issue with, which gave me the following codes:
13 - Open or Short rear height sensor
18 - Open or Short fluid pressure sensor circuit
31 - Open or Short front gate value solenoid circuit
32 - Open or Short front leveling valve solenoid circuit
33 - Open or Short rear gate valve solenoid circuit
34 - Open or Short rear leveling valve solenoid circuit
36 - Open or Short circuit in accumulator solenoid circuit
43 - AHC Main Relay circuit malfunction
63 - Fluid Pressure Malfunction

Unless you totaled your truck rolling down a mountain, I seriously doubt you would experience that many failures all at once, which makes me think there is an issue with wiring causing the ECU to go into a "failsafe" mode of some kind. The only thread I was able to find about this subject had similar issues with their truck was where @suprarx7nut responded to check the Bl1 connector, but was not able to find any faults with the wiring back there, or the connector itself.

I was able to confirm that the AHC pump functions, and that it draws in fluid by putting 12v to the pump from the battery, it also sounds markedly quieter after cleaning it out. I swapped the AHC Main relay back and forth a couple of times and was able to confirm that the B+ voltage stays the same regardless of which AHC ECU or AHC main relay is used.

I am going to run back though the fuses and relays, then I think I am down to chasing wiring unfortunately. Thanks again for your help and wealth of knowledge!
 

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