Air Compressor Self Test Question

[Costal_Cat]

Hi all,

2012 Denali XL
128,000 miles
Purchased at around 88,000

I had a short post this weekend about replacing my air shocks on my 2012 Denali XL. I replaced the Air Compressor in July, and this was the next step.


When I changed the compressor, I noticed that when I turned the truck on it do the self-test.
When I finished the new shocks this continued and then this morning I noticed that it was not doing the self-test. So I checked

• all the fuses, none has blown
• removed the relay and jumped the connectors compressor turns on

So I figured maybe the relay was shot and grabbed a new one which is only $5 so no big deal. Add the new relay same thing no self-test. So I figure let me hitch up the boat, I needed to move it anyway, and see if the compressor kicks in. Sure enough, it does just like it should, change in the position sensor with the weight of the boat and the compressor kicks on and shocks inflate. I stand on the hitch with the boat attached and same thing compressor kicks on shocks inflate. I get off the hitch, and I hear the shocks deflate due to the position sensor change like they should. I take the boat off, and I hear the shocks deflate again as they should about three times I hear the Exhaust Valve then a little air comes out and then again until level. I grab the kids have them sit in the back, I get in the back, and the compressor kicks on and levels out, we get out, and the air is released....


Shut the truck off and then restart and no self-test that I can hear anyway....

Questions

• Should I be concerned or is this actually normal with good shocks on the back?
• Is the ESCM smart enough to know that air is not needed so it doesn't actually kick on the compressor during the self-test?
• If this is not normal what else should I check?

Oh, no codes from my ODB reader.....

TIA

 

[Costal_Cat]

Well, I think I answered my question.

Per this notice https://static.nhtsa.gov/odi/tsbs/2013/SB-10052488-6277.pdf

CONDITION #4: Some customers may comment that the automatic level control compressor is no longer performing a self-test (compressor activating for a few seconds) at startup.

CONDITION #4: The automatic level system normally will not or very infrequently cycle the compressor at key up. The Electronic Suspension Control module checks the automatic level system pressure at start up. If the pressure is below 10 psi (69 kpa) then the system will operate the compressor to add air to the shocks as a replenishment cycle. This typically would only happen if the vehicle sat for a long period of time or there was a very small leak.


So the answer appears to be that the ESCM is smart enough to know if the pressure is below 10 psi and operate the compressor. So back to my post on the shocks, I took off here

https://www.tahoeyukonforum.com/threads/replacing-shocks-today-and-need-advice.107934/

Even though the shocks looked good, there was a little leak causing the compressor to kick on during the self-test because I had enough of a leak to not maintain 10 psi at startup...

BTW relay 12177235 can be had for $5 with Duralast #20179 this is for a fan but an OMRON relay. Guessing mine is fine, and now I have a backup

Cheers... Hope this is helpful to everyone

 

[Doubeleive]

from what I understand the compressor should release air and perform a self test every time the key is cycled.
the only reason I know of or can think of that it would not activate every-time is if the air release valve on the pump was not functioning properly.
I replaced my pump a few months back because it would only run infrequently and I could not hear it release any air, after replacing the pump it now runs every time I cycle the key and I can hear it release air just before it runs the self-test.

 

[Costal_Cat]

from what I understand the compressor should release air and perform a self test every time the key is cycled.
the only reason I know of or can think of that it would not activate every-time is if the air release valve on the pump was not functioning properly.
I replaced my pump a few months back because it would only run infrequently and I could not hear it release any air, after replacing the pump it now runs every time I cycle the key and I can hear it release air just before it runs the self-test.

This is what I thought as well @Doubeleive but it seems to contradict the TSBS link above. Given all my testing I can't figure out what I'm missing or why it doesn't power up at startup except to think that what the bulletin states is the correct sequence. If what you and I think is correct then what am I missing and what should I test as I am stumped on what to do next if there is a problem......

 

[Doubeleive]

This is what I thought as well @Doubeleive but it seems to contradict the TSBS link above. Given all my testing I can't figure out what I'm missing or why it doesn't power up at startup except to think that what the bulletin states is the correct sequence. If what you and I think is correct then what am I missing and what should I test as I am stumped on what to do next if there is a problem......

ya what is weird is that the tsb above does not really give a solution for problem #4 except to contradict what it should be doing every time
i.e.
Each time the compressor is activated, the ESCM commands ON the exhaust valve for 1.5 seconds to release air from the compression chamber in the compressor’s cylinder head ensuring low motor current draw upon compressor activation. The ESCM also limits the amount of compressor run time to 255 seconds to protect the compressor components from thermal damage.
The system also performs a self test on each ignition ON cycle in which the ESCM commands the ELC relay ON, activating the compressor for 4 seconds then monitors the position sensor’s signal voltage inputs to verify the compressor is functioning and the system is holding air pressure.

 

[Doubeleive]

I want to add that I also purchased my 2012 at 82k and had the same problem, I replaced both the rear shocks because they both had a slow leak and I replaced the pump and now it all works properly, it self test's every time releases air and then pumps them up.

 

[Costal_Cat]

ya what is weird is that the tsb above does not really give a solution for problem #4 except to contradict what it should be doing every time
i.e.
Each time the compressor is activated, the ESCM commands ON the exhaust valve for 1.5 seconds to release air from the compression chamber in the compressor’s cylinder head ensuring low motor current draw upon compressor activation. The ESCM also limits the amount of compressor run time to 255 seconds to protect the compressor components from thermal damage.
The system also performs a self test on each ignition ON cycle in which the ESCM commands the ELC relay ON, activating the compressor for 4 seconds then monitors the position sensor’s signal voltage inputs to verify the compressor is functioning and the system is holding air pressure.

Yep, we read from the same page that is exactly why I was think that as well.... I wish I had access to a tech 2 scanner so I could dig deeper.... I do have a buddy with a 2014 Denali I'm going to call and ask him to check. He has under 50k on it so presuming he has not encountered any issues yet....

 

[Costal_Cat]

ya what is weird is that the tsb above does not really give a solution for problem #4 except to contradict what it should be doing every time
i.e.
Each time the compressor is activated, the ESCM commands ON the exhaust valve for 1.5 seconds to release air from the compression chamber in the compressor’s cylinder head ensuring low motor current draw upon compressor activation. The ESCM also limits the amount of compressor run time to 255 seconds to protect the compressor components from thermal damage.
The system also performs a self test on each ignition ON cycle in which the ESCM commands the ELC relay ON, activating the compressor for 4 seconds then monitors the position sensor’s signal voltage inputs to verify the compressor is functioning and the system is holding air pressure.

Yep, we read from the same page that is exactly why I was think that as well.... I wish I had access to a tech 2 scanner so I could dig deeper.... I do have a buddy with a 2014 Denali I'm going to call and ask him to check. He has under 50k on it so presuming he has not encountered any issues yet....

 

[97audia4]

Yep, we read from the same page that is exactly why I was think that as well.... I wish I had access to a tech 2 scanner so I could dig deeper.... I do have a buddy with a 2014 Denali I'm going to call and ask him to check. He has under 50k on it so presuming he has not encountered any issues yet....

I have owned mine since 98k and never once heard it kick on. I have also towed 4500 pounds with zero zag and perfect level. Both shocks hold air and no issue with ride quality .

 

[swathdiver]

I wish I had access to a tech 2 scanner so I could dig deeper...

Where do you live?

My Pontiacs both have a load leveling suspension with the air compressor. They almost always come on with the key for 4 seconds and you can hear the valve open briefly (1.5 sec) too. Why sometimes not? I don't know, never bothered to look into it.