LX570 AHC “Basics” - Added as sticky thread for AHC Issues (6 Viewers)

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Stupid AHC question of the day:
The ride comfort selector is an electronic baffling system (or sorts), that switches between different sized chambers, correct? Does that particular chamber remain selected once you shut off the rig?

Use case: when camping in RTT and switching to Sport mode before you shut off the rig - would this give you more stability when moving around up there vs having it in Comfort mode when you shut off?
In it's simplest form, the actuator is essentially two different valves, one soft and one hard damping, with a stepped spool valve that shifts how much of each valve is used for the hydraulic path. So soft is spool valve shifted to only use the soft valve, hard is the spool valve shifted to only allow fluid through the hard dampening valve, and medium is the spool valve to allow the fluid to split between the two valves. There are 16 steps total between full hard and full soft.

So the problem with your theory is the damping steps are constantly changing. Changing the AVS switch changes the "scheme" of the spool value settings, but they are almost always changing depending on speed and whether you are turning and possibly various other sensors. I've found that they usually fall to 8 (the middle) when you are in park. I believe this is the case regardless of how the AVS switch is set.
 
Bought 2x 2.5L Metal Cans of AHC fluid locally. I performed the flush last weekend using the write up by @TLCLVR found here: LX570 AHC “Basics” - Added as sticky thread for AHC Issues - https://forum.ih8mud.com/threads/lx570-ahc-basics-added-as-sticky-thread-for-ahc-issues.1053154/page-9#post-12706789

The rig: '09 LX with ~150k miles. I've owned it for 1 year. No dealer indication of AHC flush that I can recall. Florida/Texas truck, no rust. Stock wheels/tires.

Flush Notes
  • The write-up calls for bleeding in the order of: RF-LF-RR-LR. I bled in the order of RF-RR-LF-LR.
  • Before starting the flush, I observed my reservoir was between Min and Max in low-mode with the old fluid.
  • I only managed to siphon out ~1L of old fluid from the reservoir before filling it with ~2.5L of new fluid. The filler neck overflowed on the last pump. Oops.
  • I pulled out as 0.4-0.5L from each bleeder in an attempt to get the old fluid out.

Issues
  • After bleeding all 4 corners (did not touch 5th accumulator valve), I started the engine
  • Observed that the car still thought I was in 'N' height mode, despite bleeding each side down to the bump stop and then some
  • The '4-wheel AHC' error popped up in the dash. I shut down, restarted, and quickly hit UP to take the car into 'Hi'-mode. This worked. Car effectively went from Lo to Hi, despite thinking it was already in N mode.
  • Admittedly I puckered up when I saw the AHC error, as I don't have a techstream.
    • Did I accidentally introduce air
  • I then pushed the car from 'Hi' back into 'Lo'-mode to stress test the system. I should not have done this, as it could theoretically push any remaining old fluid back into the reservoir.
    • I should have stayed in Hi-Mode and performed a full bleed to purge the rest of the old fluid.

Bleed #2
  • I pumped another ~1L of fresh fluid into the reservoir
  • I bled ~0.3L from each of the 4 corners. Fluid definitely became more clear, though I don't know if it ever went full red.
  • I noticed the rear fluid had what seemed to be air bubbles in it. Did not observe that from the front fluid.
    • Front Ride quality seems very improved. I'm still observing that my rear feels like it bottoms out on bumps. I'm starting to think PO did a lot of towing and the rear globes may be leaking nitrogen? This is all a guess.

Result
  • no more lurching AHC at stoplights
  • somewhat improved dampening experience
  • AHC manual adjustments are much faster. Auto adjustments (IE dropping at 62mph) are much smoother.

Next Steps
  • I have 4 replacement globes, but as I've never bled brakes or anything, I wanted to start with this small project first. After our upcoming roadtrip, I will attempt to replace all 4 globes on my own and use the remaining ~1.5L to help with flushing any air bubbles out.
 
After bleeding all 4 corners (did not touch 5th accumulator valve), I started the engine
I always start the engine after each bleed.


1) park in Lo mode
2) extract old fluid.
3) Fill res with fluid, then raise to Normal
4) it's not in the FSM, but I bleed the 5th accumulator first.
5) start rig and let ahc pump fill missing fluid.
6) turn off and bleed driver front accumulator Until suspension stops (I use 12oz topo chico bottles and stop around 10oz)
7) start rig and let ahc pump fill missing fluid. You'll see suspension raise and level out.
8) repeat 5-6 with other corners.

Still unclear if FSM likes the Driver front, driver rear, then passenger front, and pass rear last. or FR, FL, RR, RL
 
I always start the engine after each bleed.


1) park in Lo mode
2) extract old fluid.
3) Fill res with fluid, then raise to Normal
4) it's not in the FSM, but I bleed the 5th accumulator first.
5) start rig and let ahc pump fill missing fluid.
6) turn off and bleed driver front accumulator Until suspension stops (I use 12oz topo chico bottles and stop around 10oz)
7) start rig and let ahc pump fill missing fluid. You'll see suspension raise and level out.
8) repeat 5-6 with other corners.

Still unclear if FSM likes the Driver front, driver rear, then passenger front, and pass rear last. or FR, FL, RR, RL
I haven't tested this in practice, but I wonder if you could avoid bleeding the 5th accumulator by going to high after the L and exchange fluid step. It seems like if you have a reservoir full of fresh fluid, when you go from L -> N -> H, that would most likely be enough fluid movement to push the old fluid out of the accumulator, and from the hydraulic line between the reservoir and the height control valve assembly into the system forward of the height control valve. Then the height valves close once truck reaches H, and then the pump would fill the accumulator with fresh fluid, making it unnecessary to purge that bleeder.

Just a thought, it would be interesting to see if it worked out that way. Also, i wonder if doing the bleeding process might not be easier using the AHC utility. Forcing one corner to lift and bleeding it down, then lifting it again until you have clean fluid. It also seems like there was an option to lift front or rear with only the accumulator and not the pump. That may be another strategy for evacuating the accumulator without using the bleeder.
 
Review on my situation:

- RR damper actuator bad
- Line nut rusted/rounded making removal and reuse difficult or impossible

The shop's solution is to replace the entire line, which will require dropping the gas tank. All in all, I'm just happy to have a solution at this point.

QUESTION:

Is there anything at all that I should do or have done while the tank is dropped. Anything to replace, or inspect? Vehicle has 135K, more rust that I originally thought when I bought it, making some repairs difficult. I plan to keep this thing as long as possible.
 
New to the thread, new 200 series owner (2010 LX). My truck has 140k miles on it, and has had the fluid flushed at the dealer in 2014 at 87k miles. The seller also indicated he did it himself once more a few months ago. I'm getting a check AHC light and getting code C1755 Rear Gate Valve on Techstream. I haven't noticed any leaks but plan to do a more thorough inspection. Any other advice on what to check in order to diagnose the issue? Thanks.
 
C1755

When either of the following is detected:
  • With the rear gate valve not activated, an open signal of the rear gate valve is detected for 1 second or more.
  • With the rear gate valve activated, a short signal of the rear gate valve is detected 8 times successively.
  • Harness or connector
  • No. 1 height control valve
  • Suspension control ECU
  • Each front suspension control valve (Each front spring rate switching valve)
 
The rear gate valve is part of the height control valve. It allows the ECU to balance the pressure between the left and right side on the rear axle. The FSM probably has a system for narrowing down the issue to wiring or the height control module. I suspect the module isn’t serviceable, so ideally it’s a cabling issue.
 
C1755

When either of the following is detected:
  • With the rear gate valve not activated, an open signal of the rear gate valve is detected for 1 second or more.
  • With the rear gate valve activated, a short signal of the rear gate valve is detected 8 times successively.
  • Harness or connector
  • No. 1 height control valve
  • Suspension control ECU
  • Each front suspension control valve (Each front spring rate switching valve)

If I had to guess, it's probably just a loose/dirty harness connector.
 
Thanks all for the input, I'll follow up when I have a chance to inspect the wiring harnesses and hydraulic tubes.
Certainly not a bad idea to inspect the hydraulic lines, especially between the pump and the height control valve. The line that runs over the muffler is the usually suspect if you had a leaky part. The code registered doesn't indicate any leaks though. The valve being referenced is part of main height control valve module. This is located on the inside of the frame rail on the LH side (USA Drivers) under the rear door. The suspension ECU is mounted up in the interior behind the quarter panel just behind the rear door on that side.

The FSM will include directions on where to test and what values you should see when you test with a multimeter at the height control valve electrical connector and at the suspension ECU.
 
Lately, I feel like the ride quality in my LX has been degrading. The car feels almost jittery when going over uneven roads, and it doesn't seem to dampen when going over bumps and potholes, but feels rather firm.

It's a 2011 LX570 with 161k miles. The maintenance records show the AHC fluid was changed as part of the 120k service by the dealer. Other than a somewhat rough ride, I have had no warning lights on the dash and the vehicle seems to raise and lower just fine.

I thought it might be the alignment so I took it in last week and had it aligned, and the tires rotated and balanced. It didn't make any noticeable difference in the ride. I then took it into my indy shop to have them look it over, and they found the front driver shock to be leaking. Unfortunately the system they have isn't able to connect to the AHC module, so they don't have the ability to replace it.

At this point, I don't know if I have any other option besides taking it to the dealer. I've read through this thread and have seen some mention that some fluid leaking from the shock is normal. Is that true? If it isn't the shock causing it, I guess the fluid or accumulators would be next to look at. Trying to get my bearings straight before taking it to the dealer as I've read some of the horror stories about their knowledge of the AHC system.
 
At 10 years old there is probably less Nitrogen in the accumulators (it leaks across the membrane over time, like a ballon goes flat over a couple days after you blow it up), and replacing these will likely address your ride issue re: dampening.
The shocks are passive, and no computer is required to update anything post replacement (or before). Same with accumulators.
Shock leaking is a reason to replace them, but a new shock won't fix the damping issue you see.

Although they have a long lifespan (10 years+), accumulators are a wear component and eventually need to be replaced.
 
At 10 years old there is probably less Nitrogen in the accumulators (it leaks across the membrane over time, like a ballon goes flat over a couple days after you blow it up), and replacing these will likely address your ride issue re: dampening.
The shocks are passive, and no computer is required to update anything post replacement (or before). Same with accumulators.
Shock leaking is a reason to replace them, but a new shock won't fix the damping issue you see.

Although they have a long lifespan (10 years+), accumulators are a wear component and eventually need to be replaced.
Thanks grinchy, that is good to know. Would any data from Techstream give any clues as to whether these accumulators are functioining corrrectly, or is the rough ride pretty much the best giveaway? I'm curious if the dealer would even be able to tell that the accumulators are bad during a diagnostic, or if they will just try to point to the shock as the reason.

Based off what I've seen on these forums, I'm probably best buying the new accumulators online rather than going to the dealer ($$$). The accumulators and AHC flush seem easy enough to DIY. I'll probably need to take is somewhere for the shock replacement though.
 
Thanks grinchy, that is good to know. Would any data from Techstream give any clues as to whether these accumulators are functioining corrrectly, or is the rough ride pretty much the best giveaway? I'm curious if the dealer would even be able to tell that the accumulators are bad during a diagnostic, or if they will just try to point to the shock as the reason.

Based off what I've seen on these forums, I'm probably best buying the new accumulators online rather than going to the dealer ($$$). The accumulators and AHC flush seem easy enough to DIY. I'll probably need to take is somewhere for the shock replacement though.
The closest thing to an accumulator check they can do is to watch how much fluid leaves/comes into the reservoir after a height change. The more fluid that moves during a height change, the more nitrogen is getting compressed in your accumulators (more fluid moved = good). In the LX570, it's 5 marks on the reservoir, from L to H. You do not need Techstream to do this check. Honestly though, if you are at 160k on your original globes, you can be pretty sure they probably need replacing, especially based on your observations.

I do recommend getting a code reader/Techstream though. The suspension ECU does throw some codes that don't always reflect on the dashboard.


ahc-marks-count-png.2408546
 
The closest thing to an accumulator check they can do is to watch how much fluid leaves/comes into the reservoir after a height change. The more fluid that moves during a height change, the more nitrogen is getting compressed in your accumulators (more fluid moved = good). In the LX570, it's 5 marks on the reservoir, from L to H. You do not need Techstream to do this check. Honestly though, if you are at 160k on your original globes, you can be pretty sure they probably need replacing, especially based on your observations.

I do recommend getting a code reader/Techstream though. The suspension ECU does throw some codes that don't always reflect on the dashboard.


ahc-marks-count-png.2408546
That sounds good. I do have a code reader and Techstream on my laptop, so I'll try to see if I can find any codes on the system.

But sounds like either way, I'll want to get some new globes. I bought new AHC fluid a couple weeks ago, so I'll plan on doing a full flush after installing the new parts. I haven't checked on the number of ticks yet, but did see that my reservoir is at Min when the vehicle is set to N height. Wondering if the low fluid could also be a cause of the rough ride. Or perhaps some air entered the system.
 
Lately, I feel like the ride quality in my LX has been degrading. The car feels almost jittery when going over uneven roads, and it doesn't seem to dampen when going over bumps and potholes, but feels rather firm.

It's a 2011 LX570 with 161k miles. The maintenance records show the AHC fluid was changed as part of the 120k service by the dealer. Other than a somewhat rough ride, I have had no warning lights on the dash and the vehicle seems to raise and lower just fine.

I thought it might be the alignment so I took it in last week and had it aligned, and the tires rotated and balanced. It didn't make any noticeable difference in the ride. I then took it into my indy shop to have them look it over, and they found the front driver shock to be leaking. Unfortunately the system they have isn't able to connect to the AHC module, so they don't have the ability to replace it.

At this point, I don't know if I have any other option besides taking it to the dealer. I've read through this thread and have seen some mention that some fluid leaking from the shock is normal. Is that true? If it isn't the shock causing it, I guess the fluid or accumulators would be next to look at. Trying to get my bearings straight before taking it to the dealer as I've read some of the horror stories about their knowledge of the AHC system.

Agreed with others that this is a classic symptom of a blown accumulator. There's no expected diagnostic code for this failure mode. Reading gradients can help, but there's quite a few accumulators in the system that even if one fails, it's not immediately obvious.

I would go ahead and assume it needs accumulators. Shocks shouldn't weep and this would be a good time to address that, along with the requisite fluid flush that'll be a part of this repair.
 
Agreed with others that this is a classic symptom of a blown accumulator. There's no expected diagnostic code for this failure mode. Reading gradients can help, but there's quite a few accumulators in the system that even if one fails, it's not immediately obvious.

I would go ahead and assume it needs accumulators. Shocks shouldn't weep and this would be a good time to address that, along with the requisite fluid flush that'll be a part of this repair.
Gotcha, thanks. Are there any other bushings or anything else that would be good to replace while changing the accumulators or shocks? If i'm in those areas, I'd rather take care of anything that is easy to change. And it sounds like some say that the shocks should be replaced in pairs, and others say just the one that is leaking.
 

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