AHC on its last legs? (2 Viewers)

[nutter]

So I've had my LX470 for about 8 years now, and the AHC has worked great up until now, with little maintenance. However, in the last year it has decided it doesn't want to go into high mode anymore, and even going from low to normal is a gamble. Finally, a few days ago, the AHC OFF light came on when I tried to move from L to N. I did a flush with new fluid hoping that would help. Two of the globes had aerated fluid which ran clear after bleeding a few ounces. The accumulator, however, only bled a few mls. I even tried bleeding it while switching from L to N, but nothing came out. After a few attempts, I was able to get the car into N, and it drives fine in N, but obviously something is not right.

I read the codes and I'm getting:

• C1751 Continuous current to compressor
• C1762 Abnormal oil pressure for pump

Pressures after going from L to N are:

• Front: 6.6
• Rear: 0.0 (faulty reading or sensor?)
• Accumulator: 7.1

I've read pages and pages of troubleshooting threads about AHC, and my best guess is that the pump is bad, the accumulator might be bad, and perhaps the rear pressure sensor is bad?

I told myself that once the AHC failed I would replace it with a conventional suspension, which I'm close to pulling the trigger on. I'm carrying quite a bit of additional weight (bumpers, drawers, RTT). My front Tbars are cranked all the way and I have King springs in the rear.

My question is, is there any EASY fix that I may be overlooking before I just replace the AHC? I don't want to spend a lot of time and money on troubleshooting, but if there is something easy and/or obvious, I'd like to give it a shot.

 

[OrionZodiac]

You can try replacing the pump PN: 48901-60010

 

[GTV]

Something definitely up with that rear sensor. Also, have you reindexed your torsion bars?

 

[IndroCruise]

So I've had my LX470 for about 8 years now, and the AHC has worked great up until now, with little maintenance. However, in the last year it has decided it doesn't want to go into high mode anymore, and even going from low to normal is a gamble. Finally, a few days ago, the AHC OFF light came on when I tried to move from L to N. I did a flush with new fluid hoping that would help. Two of the globes had aerated fluid which ran clear after bleeding a few ounces. The accumulator, however, only bled a few mls. I even tried bleeding it while switching from L to N, but nothing came out. After a few attempts, I was able to get the car into N, and it drives fine in N, but obviously something is not right.

I read the codes and I'm getting:

• C1751 Continuous current to compressor
• C1762 Abnormal oil pressure for pump

Pressures after going from L to N are:

• Front: 6.6
• Rear: 0.0 (faulty reading or sensor?)
• Accumulator: 7.1

I've read pages and pages of troubleshooting threads about AHC, and my best guess is that the pump is bad, the accumulator might be bad, and perhaps the rear pressure sensor is bad?

I told myself that once the AHC failed I would replace it with a conventional suspension, which I'm close to pulling the trigger on. I'm carrying quite a bit of additional weight (bumpers, drawers, RTT). My front Tbars are cranked all the way and I have King springs in the rear.

My question is, is there any EASY fix that I may be overlooking before I just replace the AHC? I don't want to spend a lot of time and money on troubleshooting, but if there is something easy and/or obvious, I'd like to give it a shot.

It boils down to how you want to spend your money -- maintain the AHC/TEMS or if you don’t want to get involved with that, spend more replacing AHC/TEMS with a conventional suspension.

C1751 and C1762 often arrive together and are saying that there is insufficient fluid pressure being developed in the AHC system. The readings (Front AHC: 6.6 MPa; Rear AHC: 0.0 Mpa; Height Control Accumulator: 7.1 Mpa) and difficulties with LO and N height are saying the AHC Pump is being stopped by ECU and that ECU is putting the system in one or other "fail safe function" as described in the FSM extracts below. This is happening before the Front AHC pressure builds to a credible value for a heavy vehicle and before the Height Control Accumulator builds to anywhere near the expected 10.0 Mpa to 10.5 Mpa. A fully pressurised Height Control Accumulator should release about 300 millilitres on bleeding. If no fluid or very little fluid bleeds out, it is because there is no fluid or very little fluid in there.

If the 'globes' are 10+ years old (on your 2001 vehicle?), then they are way past their best. Slowly but surely nitrogen from one or more 'globes' will have passed through the membrane and will have infected the entire AHC system with AHC Fluid polluted with compressible gas. No amount of bleeding of fluid will fix this -- until the 'globes' are replaced. A bleed near each 'globe' giving a clear stream after only a few millilitres provides no reassurance wnen old 'globes' are involved.

49141-60010 (front) -- two required, Impex price USD102.23 each plus delivery
49151-60010 (rear) -- two required, Impex price USD102.23 each plus delivery

Meanwhile, the AHC Pump struggles to build pressure. A common cause, especially in older vehicles where AHC Fluid has not been changed routinely, is blocked strainers within the AHC Pump sub-assembly. This sub-assembly can be removed, disassembled and the strainers cleaned -- there are multiple posts describing this -- but there is little point in reinstalling an old Pump when the reliability of a new AHC Pump sub-assembly (Part Number 48901-60010) currently costs USD95.55 plus delivery from Impex.

The other 'wear items' on the AHC system are the Height Control Sensors. These Sensors commonly are faulty without showing a DTC code and then give rise to strange vehicle behaviours.

All of the above replacements are EASY and inexpensive compared to a replacement with a conventional suspension.

Systematic testing with Techstream is vital -- it is important to detect whether other faults are present.

Recent posts do show increasing reports of wiring harness issues. These are findable and fixable but they are a PITA.

 

[suprarx7nut]

So I've had my LX470 for about 8 years now, and the AHC has worked great up until now, with little maintenance. However, in the last year it has decided it doesn't want to go into high mode anymore, and even going from low to normal is a gamble. Finally, a few days ago, the AHC OFF light came on when I tried to move from L to N. I did a flush with new fluid hoping that would help. Two of the globes had aerated fluid which ran clear after bleeding a few ounces. The accumulator, however, only bled a few mls. I even tried bleeding it while switching from L to N, but nothing came out. After a few attempts, I was able to get the car into N, and it drives fine in N, but obviously something is not right.

I read the codes and I'm getting:

• C1751 Continuous current to compressor
• C1762 Abnormal oil pressure for pump

Pressures after going from L to N are:

• Front: 6.6
• Rear: 0.0 (faulty reading or sensor?)
• Accumulator: 7.1

I've read pages and pages of troubleshooting threads about AHC, and my best guess is that the pump is bad, the accumulator might be bad, and perhaps the rear pressure sensor is bad?

I told myself that once the AHC failed I would replace it with a conventional suspension, which I'm close to pulling the trigger on. I'm carrying quite a bit of additional weight (bumpers, drawers, RTT). My front Tbars are cranked all the way and I have King springs in the rear.

My question is, is there any EASY fix that I may be overlooking before I just replace the AHC? I don't want to spend a lot of time and money on troubleshooting, but if there is something easy and/or obvious, I'd like to give it a shot.

You can try replacing the pump PN: 48901-60010

Something definitely up with that rear sensor. Also, have you reindexed your torsion bars?

Hang on everyone.


@nutter You said two globes had aerated fluid. Those are dying/dead globes. Replace those 2 (or more appropriately, all of them - $600 shipped, give or take from Impex). The nitrogen from the globes makes it impossible for the pump to do it's job and will give you those codes. The pump is fine. The gas is an issue. The fix is a new globe (or globes) and a fluid bleed. Easy Peasey, as my daughter says.

@GTV Why the sensor? it's reading 0.0 for pressure because the gas in the fluid is preventing it from raising. I bet the sensor is ok.

 

[GTV]

@GTV Why the sensor? it's reading 0.0 for pressure because the gas in the fluid is preventing it from raising. I bet the sensor is ok.

Knee jerk reaction from seeing 0.0. To be fair, it's easy and a lot cheaper than globes, even if that isn't the solution then you've got an extra sensor on hand which isn't a bad thing.

 

[suprarx7nut]

Knee jerk reaction from seeing 0.0. To be fair, it's easy and a lot cheaper than globes, even if that isn't the solution then you've got an extra sensor on hand which isn't a bad thing.

Well that's all true.

 

[BullElk]

If no fluid or very little fluid bleeds out, it is because there is no fluid or very little fluid in there.

If there is very little to no fluid bleeding it can very well be there is no pump pressure pushing it out. I had this very thing happen. I replaced O rings in pump, primed pump with direct power to motor from battery, got pressure to build, pressure to hold and pressure to bleed. Reservoir was properly filled before and after.

 

[nutter]

Thank you, all, for the in-depth advice. It sounds like folks are suggesting I replace all 4 globes, and probably the pump. I might need to replace the rear sensor as well. I assume after changing all that I'd also need to flush to remove air bubbles?

 

[suprarx7nut]

Thank you, all, for the in-depth advice. It sounds like folks are suggesting I replace all 4 globes, and probably the pump. I might need to replace the rear sensor as well. I assume after changing all that I'd also need to flush to remove air bubbles?

Personally, I would absolutely not replace the pump.

Just change the globes and bleed/flush it.

To clarify, I say this because those error codes mean there's gas in the system. In almost all cases they do not mean that the pump is faulty in any way.