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

[Obioban]

I'm replacing the bushings on the AHC.... dampers? Shocks? Struts? Whatever the term is for the long tube behind the control arms (as in, in other cars they would be struts, but don't know if they are for AHC).

Anyway, to do this I had to remove the struts from the truck, which meant the AHC line was disconnected and some fluid came out of the shocks (and line).

After reassembly, do I need to do anything beyond the normal AHC bleed process? Will that bleed the struts AND the accumulators? Because undoubtedly they're going to have some air in them, but not sure how bleeding would work when only one line goes to them and it's downstream of the bleed valve...

 

[uHu]

I'm replacing the bushings on the AHC.... dampers? Shocks? ....
After reassembly, do I need to do anything beyond the normal AHC bleed process? Will that bleed the struts AND the accumulators? Because undoubtedly they're going to have some air in them, but not sure how bleeding would work when only one line goes to them and it's downstream of the bleed valve...

You should be fine with a normal bleed process. You'll need at least two full rounds with bleeding to get all air and gunk out.
It is probably good to change the O-rings inside the connectors at the shock tops since you already have them off, but they might just as well be good for another 10 years.

PS:. it's normally possible to change the lower shock bushings without removing the shocks.

 

[Gartenmeister]

PS:. it's normally possible to change the lower shock bushings without removing the shocks.

Any chance you have a link to instructions on how to do this?

 

[PabloCruise]

I'm replacing the bushings on the AHC.... dampers? Shocks? Struts? Whatever the term is for the long tube behind the control arms (as in, in other cars they would be struts, but don't know if they are for AHC).

Anyway, to do this I had to remove the struts from the truck, which meant the AHC line was disconnected and some fluid came out of the shocks (and line).

After reassembly, do I need to do anything beyond the normal AHC bleed process? Will that bleed the struts AND the accumulators? Because undoubtedly they're going to have some air in them, but not sure how bleeding would work when only one line goes to them and it's downstream of the bleed valve...

Did you just do the fronts?
I have heard that the rears are very difficult to remove up top.

 

[uHu]

Any chance you have a link to instructions on how to do this?

Might be a thread in here, somewhere, but I have just unbolted the bottom end and pulled the shock off the bolt. There is enough give in the rubber up top if the shock is fully extended. Then used a big C-clamp to push the bushing out, and new one in. That said, I did have to cut the bushing out once, carefully with a sawzall, when it wouldn't budge.

 

[GuazisMotos]

Messed with everything a little this morning. Moving the height sensor sliders had the opposite effect that I expected so it was a learning experience. Where I've landed and where I think it's going to stay for now:

FR Height Control Sensor: -0.2 inch
FL Height Control Sensor: -0.1 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.4 MPa
Rear Pressure Sensor: 6.4 MPa
Accumulator Press Sensor: 10.5 MPa

Thanks for the help @fussychicken and @IndroCruise

Edit: one last question, do I sit in the car when taking the measurements for pressures? It made a .3-.4 MPa difference.

Edit edit: looks like I shouldn't be in the car when measuring.

Last edit I swear:

With me out of the vehicle (after making some more adjustments:

FR Height Control Sensor: -0.2 inch
FL Height Control Sensor: -0.1 inch
RR Height Control Sensor: -0.2 inch
Front Pressure Sensor: 6.7 MPa
Rear Pressure Sensor: 6.5 MPa
Accumulator Press Sensor: 10.5 MPa

Quick question. After you set your height sensors, did the values at N deviate when you cycled from N to H to N for example? For me I'd make the adjustments to be near zero, cycle from N to H to N and the heights would be a completely different value than before.

 

[YotaWagon]

Quick question. After you set your height sensors, did the values at N deviate when you cycled from N to H to N for example? For me I'd make the adjustments to be near zero, cycle from N to H to N and the heights would be a completely different value than before.

I don't remember brother, it was two years ago.

I'm curious about my pressures though so I'll check soon and report back. Are you saying that you get different values every time you cycle through the heights?

 

[geo5]

Quick question. After you set your height sensors, did the values at N deviate when you cycled from N to H to N for example? For me I'd make the adjustments to be near zero, cycle from N to H to N and the heights would be a completely different value than before.

There is an acceptable range of height sensor reading variation, +/- 0.2” I believe, according to the handy dandy AHC cheat sheet.
What kind of readings are you seeing?

 

[Vthelexus]

Got to do mine too.

 

[GuazisMotos]

There is an acceptable range of height sensor reading variation, +/- 0.2” I believe, according to the handy dandy AHC cheat sheet.
What kind of readings are you seeing?

I don't remember brother, it was two years ago.

I'm curious about my pressures though so I'll check soon and report back. Are you saying that you get different values every time you cycle through the heights?

My bad Yota, I didn't realize how far back this thread went before I asked lol.

To answer both of your questions, yes I do get different values after I cycle heights. I'll get the front sensors to 0mm +- .2mm at N. I'll cycle from N to H then back to N and the readings at N would now be like -2mm for the left and -5mm for the right. From what I understand, the sensors should be back to near zero when height is set to N or do I have that idea wrong?

I haven't been able to mess with the rear sensor yet since the nut has rusted and is currently rounded.

 

[jaewastooshort]

can we please "sticky" this thread?

 

[jaewastooshort]

possibly a weird question for anyone intimately familiar with the AHC system...

is there any way to use a scanning tool (techstream or 3rd party scanner that will interact) to fully bleed the pressure from the AHC system to let the suspension drop as far as possible? not sure if there's some kind of service mode like this, but I'm specifically wanting to bring it down past even "low" as far as possible to try and get it into a garage, and just no idea if that's possible without just physically cracking the bleeders to relieve all pressure from the system.

 

[Snow Wagon]

Hello jaewastooshort,
How far below "Low" do you need to go?
I don't know of a way to do it with AHC, other than bleeding.
And if you bleed the AHC, you'll need to roll it into the garage without starting it because starting it will put it back to "Low".
If you only need an inch or two below "Low", just reduce your tire air pressure to 10 PSI.
This should give you an inch or two.
Good luck.
Tom

 

[jaewastooshort]

Hello jaewastooshort,
How far below "Low" do you need to go?
I don't know of a way to do it with AHC, other than bleeding.
And if you bleed the AHC, you'll need to roll it into the garage without starting it because starting it will put it back to "Low".
If you only need an inch or two below "Low", just reduce your tire air pressure to 10 PSI.
This should give you an inch or two.
Good luck.
Tom

I don't know yet, I won't be able to measure until I get it back. Given the repair that needs to be done on the front pillar (looks like I may need to repair into the roof structure) I'll probably have to pull the roof rack anyway, which should get me another few inches as well, and airing down the tires might be enough to make it work. What definitely isn't going to help me is having the AHC shock spacers installed... but I also have no height sensors hooked up right now as the new UCA's don't have brackets until I weld them on, so I have no idea what the AHC computer is going to try and do once I start it. Kinda thinking I just pull all AHC related fuses, bleed the hydraulic pressure, and see what happens.

I was asking because I seem to recall that there were some kind of "secret" modes that could be accessed to put it into more-than-high or less-than-low mode. ETA: found the reference to the "extra high" mode, and google AI tells me

"Extra" Low Mode (Active Test Mode):
While the factory switch only offers Low, Normal, and High, the system can be forced into a much lower "extra low" state—often referred to as "slammed"—using a specialized "Active Test" mode via diagnostic tools or a homemade override switch that disconnects the speed sensors. This is not a standard driving mode and is typically used for maintenance or by owners aiming for a specialized appearance.

guess I'll just break out my scanner tool when I get it back.

 

[uHu]

I was asking because I seem to recall that there were some kind of "secret" modes that could be accessed to put it into more-than-high or less-than-low mode.

There is a mode called Active Test, where you can move front or rear up and down with a button, independent of the height sensors. Active Test mode is activated either in TechStream or a manual method, both described in the FSM. That said, my 2004 I never could get into Active Test, while the 2000 had no problem with it.
But, if you do not have the front sensors attached, the ahc ecu might not want to cooperate.

Otoh, you can look at the bump stops and see if you can get any lower. Depending on weight, adjustments and spring tension/age, you might be on the bumps stops in low, or you might not quite reach them. If you bleed and then pull the plug to the pump motor, you should be safe for moving a few yards.

The AHC doesn't even give you 2 inches down from N, so I don't know if this will give you enough anyway.

 

[98LXbeast]

Another satisfied customer !

 

[JM63]

Hi,

I’m looking for some advice from experienced AHC owners/tuners regarding ride quality after refreshing the suspension on my 2005 HDJ100 (4.2 TD).

The truck is still basically fully stock:

• factory AHC
• factory suspension geometry
• no lift
• no front/ rear bumper/tire carrier/etc.

The only added weight is:

• a front winch
• underbody protection/skid plates
• approximately 220–330 lbs (100–150 kg) of gear/luggage inside the vehicle

Before the work, the truck still had the original 20-year-old AHC setup:

• original rear springs
• original globes/accumulators
• no faults
• ride comfort was actually very good

Old measurements/settings:
AHC pressures:

• Front: 8.1 MPa
• Rear: 8.8 MPa

Ride height (hub center to fender):

• LF: 18.9" (48 cm)
• RF: 19.5" (49.5 cm)
• LR: 19.1" (48.5 cm)
• RR: 19.2" (48.8 cm)

Recently I replaced:

• all 4 globes/accumulators with new ones
• both rear springs with new OEM Toyota springs

After that, the system was flushed and calibrated by a mechanic

Current behavior:

• long smooth undulations are still OK (but stiffer)
• short sharp bumps became much harsher/stiffer
• overall comfort is noticeably worse than before
• current suspension setting “1” now feels approximately like setting “3” felt before the work

Current pressures fluctuate a bit depending on when I check them, but generally:

• Front: around 8.4–8.8 MPa
• Rear: around 8.3–9.1 MPa

Current ride heights:

• LF: 18.9" (48 cm)
• RF: 18.9" (48 cm)
• LR: 20.1" (51 cm)
• RR: 19.7" (50 cm)

So my question is:
Would you:

1. lower the rear closer to the previous height (~19.1"/48.5 cm) and see if pressures/comfort improve,

OR

1. raise the front slightly with the torsion bars to chase the "perfect" factory rake/book ride height?

My concern with raising the front is that additional torsion bar preload could make small-bump compliance even worse and also add even more pressure to the rear as the weigh balance would change, so rear ahc should carry even more.

 

[GTV]

Current pressures fluctuate a bit depending on when I check them, but generally:

• Front: around 8.4–8.8 MPa
• Rear: around 8.3–9.1 MPa

So my question is:
Would you:

1. lower the rear closer to the previous height (~19.1"/48.5 cm) and see if pressures/comfort improve,

OR

1. raise the front slightly with the torsion bars to chase the "perfect" factory rake/book ride height?

My concern with raising the front is that additional torsion bar preload could make small-bump compliance even worse and also add even more pressure to the rear as the weigh balance would change, so rear ahc should carry even more.

As per the very first post on this thread, you want the front pressures to be 6.4-7.4 and the rear to be 5.6-6.7. Your current pressures are way too high and are the cause of your poor ride quality. To reduce front pressure you can either lower the suspension by adjusting the sensors or you can adjust the torsion bars. Your rear numbers are high enough to indicate that you need a heavier duty coil, a coil spacer isn't going to lower it enough. I would start with the front since it's free and also takes some of the pressure off of the rear. Once the front is dialed in you can make an educated choice on which rear coils you need. The torsion bars are not used to raise an AHC vehicle, they are for correcting pressures and leveling side to side. The sensors are used for height adjustment.

 

[IndroCruise]

Hi,

I’m looking for some advice from experienced AHC owners/tuners regarding ride quality after refreshing the suspension on my 2005 HDJ100 (4.2 TD).

The truck is still basically fully stock:

• factory AHC
• factory suspension geometry
• no lift
• no front/ rear bumper/tire carrier/etc.

The only added weight is:

• a front winch
• underbody protection/skid plates
• approximately 220–330 lbs (100–150 kg) of gear/luggage inside the vehicle

Before the work, the truck still had the original 20-year-old AHC setup:

• original rear springs
• original globes/accumulators
• no faults
• ride comfort was actually very good

Old measurements/settings:
AHC pressures:

• Front: 8.1 MPa
• Rear: 8.8 MPa

Ride height (hub center to fender):

• LF: 18.9" (48 cm)
• RF: 19.5" (49.5 cm)
• LR: 19.1" (48.5 cm)
• RR: 19.2" (48.8 cm)

Recently I replaced:

• all 4 globes/accumulators with new ones
• both rear springs with new OEM Toyota springs

After that, the system was flushed and calibrated by a mechanic

Current behavior:

• long smooth undulations are still OK (but stiffer)
• short sharp bumps became much harsher/stiffer
• overall comfort is noticeably worse than before
• current suspension setting “1” now feels approximately like setting “3” felt before the work

Current pressures fluctuate a bit depending on when I check them, but generally:

• Front: around 8.4–8.8 MPa
• Rear: around 8.3–9.1 MPa

Current ride heights:

• LF: 18.9" (48 cm)
• RF: 18.9" (48 cm)
• LR: 20.1" (51 cm)
• RR: 19.7" (50 cm)

So my question is:
Would you:

1. lower the rear closer to the previous height (~19.1"/48.5 cm) and see if pressures/comfort improve,

OR

1. raise the front slightly with the torsion bars to chase the "perfect" factory rake/book ride height?

My concern with raising the front is that additional torsion bar preload could make small-bump compliance even worse and also add even more pressure to the rear as the weigh balance would change, so rear ahc should carry even more.

Initial comments:

• Double-check for air still in the AHC system following replacement of AHC 'globes' -- this is a common cause of poor damping and poor ride quality,
• Confirm that replacement AHC 'globes' are new genuine Toyota/Lexus parts correct for LC100/Lexus470,
• Front cross-level is good,
• Front hub-to-fender distance is low --use Height Control Sensor Adjusters evenly on both sides (NOT torsion bar adjusters) to vary ride height -- should be about 500mm hub-to-fender,
• Front AHC pressure is VERY high -- reduce to Factory Service Manual (FSM)-specified range, using torsion bar adjusters evenly on both sides. (Current high AHC pressures compromise damping and ensures poor ride quality),
• Rear hub-to-fender height is low -- should be about 520mm hub-to-fender -- use the single Rear Height Control Sensor adjuster to correct Rear Hub-to-fender height,
• Correct height settings are important for correct AHC pressure settings and also help front-to-rear balance of the vehicle,
• Rear AHC pressure is way high with new springs -- consider preloaded springs with spacers, or consider different springs or airbags within springs for heavy loads,
• Advise actual Part Numbers of replacement springs,
• Review all mechanical suspension components, searching for any cause of mechanical interference, and also worn bushings everywhere Front and Rear -- including shock absorbers, control arms, sway bars -- degraded bushings will cause poor ride quality over minor bumps.
• Read more AHC-related posts on this Forum, especially this ABC of AHC thread, from the beginning.

 

[JM63]

As far as I’ve been reading the forum and understanding how the AHC system works, it seems to me that raising the vehicle back to the factory ride height values would only increase the pressures and make the ride comfort even worse. The vehicle weighs only slightly more than stock, and since the AHC system together with the factory springs/torsion bars is designed for payloads up to 800 kg (about 1,760 lbs), it seems to me that switching to stiffer springs shouldn’t be necessary to achieve a good result.

I’m ruling out any mechanical causes of poor ride comfort, such as bushings, because before replacing the globes and rear springs, the ride quality was much better. The globes came from Japan in original packaging, looked to be in very good condition, and my mechanic confirmed that as well — if they had looked suspicious, he wouldn’t have installed them.

The torsion bars are already close to the maximum range of adjustment/rotation, and from what I understand about how they work, the more preload/tension they have, the worse they will behave over small sharp bumps, right?

That’s why I’m wondering whether, instead of chasing the exact factory ride heights, it might be better to start by lowering the rear a bit first (which in theory should reduce the pressures). Those were roughly the ride heights my AHC had before the globe replacement, and everything actually worked pretty well then. Only if that doesn’t improve the comfort, it seems to me that it would make sense to try setting the factory ride heights and continue troubleshooting from there.

Unless I’m misunderstanding how the AHC system works, and raising the vehicle through adjustment could somehow reduce the pressures.