AC troubleshooting Help

[cofferson]

Hey everyone. I am trying to work through an issue with the AC on my 2008 Suburban 2500 and need some help gathering info to get me there.

Problem: When AC is turned on in vehicle and temperature is turned to lowest setting, compressor will not engage.

What I've done:
- All fuses and relays good
- Verified pressures are good with manifold gauges
- Both pressure switches were replaced by previous owner
- Verified clutch is capable of engaging by jumping relay. When i jump relay, air blows cold so compressor functions correctly and mechanical system is normal. Blend door is clearly operating.
- No stored codes that would prevent AC operation.
- I did a reset procedure I saw on here from awhile back of turning all HVAC off, disconnecting battery for 10 min, touching battery cables, reconnecting and starting vehicle, wait 5 min. No AC still.

At this point I'm convinced it has to be bad data from a sensor but I do not have all of the info to know which sensors will stop the computer from allowing AC operation. Previous owner (father in law) told me when it stopped working, the AC button would flash but this no longer happens.

 

[mikez71]

evap temp sensor maybe?

Need a scanner to see sensor values...

 

[cofferson]

evap temp sensor maybe?

Need a scanner to see sensor values...

That's what I thought until I saw an old thread saying they didn't have those sensors until something like 2010 or 2012. I did not see a sensor on top of my HVAC box. As far as I am aware, there is the cabin temp sensor about my head, the ambient temp sensor behind the bumper, the two pressure switches. And possibly a solar load sensor? Other than that I'm not aware of any more but I could be wrong.

 

[mikez71]

You're right!

Maybe the wire from ECM to relay interrupted somewhere..

HVAC - Automatic — 2008 Chevrolet Suburban 4WD V8-6.0L Service Manual | Operation CHARM

Detailed repair manual for the 2008 Chevrolet Suburban 4WD V8-6.0L.

charm.li

 

[cofferson]

You're right!

Maybe the wire from ECM to relay interrupted somewhere..

HVAC - Automatic — 2008 Chevrolet Suburban 4WD V8-6.0L Service Manual | Operation CHARM

Detailed repair manual for the 2008 Chevrolet Suburban 4WD V8-6.0L.

charm.li

View attachment 454781

Thank you! These schematics will be helpful

 

[cofferson]

So a little update. I got a hold of a scan tool that can read all gm HVAC data. Everything is acceptable and the ECM gives permission to activate the AC. Only problem is that it doesn't actually ground the clutch relay like it should. The ECM sends data back confirming that the AC is on when I command it on from the scan tool. AC still never actually engages though. Does anyone have any idea how I can verify if it's a physical connection between the relay and the ECM or if it's the ECM itself? I'm not sure if there is anything to be seen under the fuse box because I'm finding it impossible to separate to look under the relay.

 

[Foggy]

So a little update. I got a hold of a scan tool that can read all gm HVAC data. Everything is acceptable and the ECM gives permission to activate the AC. Only problem is that it doesn't actually ground the clutch relay like it should. The ECM sends data back confirming that the AC is on when I command it on from the scan tool. AC still never actually engages though. Does anyone have any idea how I can verify if it's a physical connection between the relay and the ECM or if it's the ECM itself? I'm not sure if there is anything to be seen under the fuse box because I'm finding it impossible to separate to look under the relay.

Do you think that maybe you could just be low on Freon???
I know that's simplistic, but the clutch will not engage if freon level is low

 

[cofferson]

Do you think that maybe you could just be low on Freon???
I know that's simplistic, but the clutch will not engage if freon level is low

No, all requirements below are met and the ECM per a scan tool allows permission to activate the clutch but the clutch will not actually engage because the ECM never grounds the relay. I can just the relay and get cold AC.

The following conditions must be met in order for the ECM to turn on the compressor clutch:

*Battery voltage is between 9-18 volts
*Engine coolant temperature (ECT) is less than 123°C (253°F)
*Engine speed is greater than 600 RPM
*Engine speed is less than 4,760 RPM
*A/C high side pressure is between 2929-269 kPa (39-425 psi)
*Throttle position is less than 100 percent
*Evaporator temperature is greater than 0°C (32°F)
*ECM does not detect excessive torque load
*ECM does not detect insufficient idle quality
*The ambient temperature is above
1°C (34°F)