A couple days ago the Air Suspension (AS) compressors on my ’05 H2 did not turn off when I shut the engine off. I’ve seen a few posts on alternate websites, but I have never found a solution to any situation exactly like the one I am experiencing. With the truck not running, both compressors continue to run, and the exhaust valves can be heard opening and relieving pressure and then closing, reopening, closing…you get the idea. Notably, the compressors also run when the vehicle speed exceeds 40 MPH, with the engine stopped, engine running, doors open, doors shut, you name it – they just don’t deenergize – Ever!

My thought is that 12-volt power is being continuously applied to the compressors via the 60 amp VSES/ECAS Fuse located in the Underhood Fuse Block via the “Electronically Controlled Air Suspension Relay” located on the frame rail and that this relay is for some reason, stuck in the “Closed” position (connecting Pin 87 to Pin 30).

THIS WOULD BE THE EASY FIX! Alternatively, any of a myriad of signals can be causing the ECAS Relay to remain shut/energized. There are no DTCs.

Looking for any other thoughts, suggestions that anyone might have. Thanks!

 

The main big power relay is on the firewall down up under the brake booster. Almost sounds like that one is stuck. That or the ASM module by the compressor might be toasted.

 

Thanks for the heads up on the power relay - I haven't looked for it yet, but I was thinking since my last post that it just doesn't seem possible that one of the standard "relays" that are used all over every type of vehicle would be able to handle a 60 amp circuit. I could be wrong, but regardless, my thought is that it will be easy enough to test the power relay. I'll post whatever I discover

 

check your ride height sensors maybe something is amiss there. with a tech 2 it would be a lot easier to troubleshoot as you can read the live data. it can also be an issue with the pressure transducer on the compressor valve body not detecting full pressure causing the pressure relief valve to unload because of over pressurization.

 

Hmmmm...Agree, all those items are what I include in the "myriad of signals" in my original post. My luck is that it will be one of these other items - history has proven to me over and over again, that it's not the quick and easy fix!

Related to this, can you tell me the technical details of the/a Tech 2? I'm at a point that I could really use one. I've been getting by for about 7 or 8 years with a Blue Driver, but it would be nice to be able to actually conduct the specified diagnostics using a fully capable scan tool. Thanks!

 

Pretty much as @MixManSC and @bronxteck commented.

The compressors are wired in parallel, so always run together, also there's only a single exhaust valve (plus left+right air-bag inlet valves)

There are 2 connectors connecting the AS Tray, the 16-Way goes mainly to the EC Module, the 2-Way goes to the compressors.

Unplugging the 16-Way (battery disconnect 1st) will force the AS Relay to de-energize, so you'll know if it's the relay stuck or if it's being driven by the ECM.

Also check the Ground G402 eyelet tag on the tray, it's very prone to corrosion,

Please let us know how you get on..

 

Did some checks with my limited use Blue Driver. I did find that DTC C0711 was set. I’ve learned that the air suspension pressure sensor sends a voltage signal to the air suspension module that corresponds to a pressure within the range of 30-180 psi. The air suspension module also uses this signal to verify that the proper amount of pressure is achieved in the air dryer within 23 seconds prior to opening of the inlet valves for inflation of the air springs. It’s also important to know that this DTC will be active when the vehicle is moving, but then it will go to history when the vehicle is not moving. Several days ago, when I checked for codes, nothing came up and I’m assuming it was because the vehicle was not moving. Without any further analysis at that time, I cleared all codes by default. However, shortly after this I started and drove the truck a couple hundred feet and it is likely that the code set again during that short drive.

Here's my analysis of the DTC and possible cause of the DTC and the reason for the compressor to run continuously even with the engine not running. It would be helpful for anyone who would like, to take a look at this analysis and let me know if you agree or disagree or even provide an alternate analysis.

DTC C0711 is set when:

· When there is an intermittent or permanent loss of continuity between the compressor and the compressor supply voltage during an upwards leveling event.

o Eliminated. This would cause an intermittent stopping and starting of the compressor. The problem in this case is that the compressor never stops.

· When there is an air leak in the system.

o Eliminated. The system stays completely pressurized when the battery is disconnected, and the compressor doesn’t run. Even overnight, the vehicle height does not change.

· When vehicle speed is greater than 3 km/h (1.8 mph) and rate of height change is out of limits.

o Eliminated. Vehicle is parked. Engine is not running therefore cannot achieve both greater than 1.8 mph AND rate of height change parameter.

· When the 60 A compressor fuse is open.

o Eliminated. Compressor runs. Plus, I checked it.

· The pneumatic status of the air suspension system is out of limits

o Could be that the air pressure is above 180 psi due to continuous running of air compressors, thus throwing a 0711 DTC.

· When excessive amounts of time have elapsed when inflating the air springs.

o I’m not sure how the timing is measured, but if it uses anything related to the compressor running then this would throw the DTC due to compressor run time is excessive.

· When the compressor relay is defective.

o Is this the culprit??? If the compressor runs continuously because the relay has failed in the "energized" position, the pneumatic status could be out of limits, and it could cause an excessive amount of time to elapse when inflating the springs – both of which will throw the 0711 code.

My action plan at this point is to replace the Electronically Controlled Air Suspension Relay. This relay costs less than $20. This part is a high current, 4 pin relay for Hummers and another 177 GM vehicles which were sold in the U.S. from 2000 to 2012. It's OEM part number is 15319851, and a corresponding ACDelco part number 212-559.

If this doesn’t fix the problem, I will replace the automatic level control solenoid, which includes the air suspension pressure sensor. Sadly, this part is almost $400.00, but is widely used in a variety of GM vehicles from 2003 to 2005. The OEM part number is 88982408.

Also, I looked for the "main big power relay on the firewall down up under the brake booster" as suggested by MixManSC, but there is none there to be found - I mean nothing there at all. I even pulled the complete cover off the fuse box to make sure it wasn't somehow tucked up underneath the cover - nope, not there either. I have found a GM diagram that shows the same relay on the chassis just forward of the left rear tire - I'm off to look there next.

I’ll post what I find and do.

 

The relay was actually easier to get at from the front driver side wheel well for me. It is down very low against the firewall under the hydroboost. This is a pic I found online - you can see where it ties into the harness to go to the back. There is an attachment to hold it up against the firewall just above and to the right from where you see it hanging (cannot see the attachment point in this pic).

 

Well, I went back to look for the relay again. Waaaa-Laaaa, there it was on the firewall, below the master cylinder, right where MixManSC said it would be. Now, I have to correct myself. I stated earlier that I also had a diagram that showed this relay on the chassis near the left rear wheel - I was wrong! This diagram accurately shows it right where MixManSC said it would be - my bad. I apparently was looking at the diagram upside down, backwards, and inside out - that would confuse the best of us! So, I found the relay, degreased it as it was covered with 93K miles of grime - you know the kind. I pulled it out and sure enough, there is continuity across Pin 87 and Pin 30 (voltage supply and compressor control) which indicates the relay is apparently stuck in the "on" (or technically, energized) position causing the compressors to run continuously.

While I was crawling around under the truck tracing the harness from the compressors back to the engine compartment (looking for the relay before I got self smarted and turned the diagram right side up, frontwards and right-side out) I checked the ground connections G402 and G405 - both were in great shape. So, a new relay has been ordered, I'll repost when I get it installed. Hopefully, all will be well again.

 

PA05H2 good work on the Part Numbers, I was not aware of 88982408 (I had it down as GM19333035) if you manage to find one please post, I already told Joseph Reyjim they were no longer available, I'm sure he would be very interested.

FYI the pressure transducer provides a 0 to 5V signal directly proportional to the pressure (30-180psi range maybe) and it's powered from the same +5V ref which goes to the Ride-Height Sensors.

The sensor itself has E320020137 43304 96762E stamped on it, but I've never managed to find it or anything similar that would fit.

You mentioned the "automatic level control solenoid" it's housing could be leaking thus it cannot build-up any chamber pressure.

One puzzling thing, the ECM electronically monitors the IGN3 voltage and via Class2 Serial "Engine Running" certainly the tire inflator needs those conditions. According to the manual, "The levelling function will be disabled when any door is open or when the inflator is being used" in addition the AS performs a self-test with every ignition activation.

Electronically the ECM is capable of driving all the solenoids at any time (just never witnessed it), indeed it would be risky that the AS could run arbitrarily without even a key in the ignition, yet it can. It certainly has a quiescent current drain all the time.

BTW pulling the HVAC/ECAS fuse should also deactivate the AS relay.

Certainly hope the new relay solves it.