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.