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2004 gmc envoy_sle_xl
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Discussion Starter · #1 ·
I have an 04 Envoy XL. I have a problem with the rear a/c. When I control the rear a/c from the front, things work fine. I get cool air in the back. I switch the front switch to "R" and the rear control is set to cool, the air temp then warms up. Switch back to control from the front and the temp is cool again. I'm thinking the temp actuator in the rear. Any other ideas? How can I test it?
 

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The test of the temp actuator is to see if it works when controlled by EITHER front or rear module. If it changes temp, then it works. When your front switch is NOT set to "R", the rear temp is controlled by your front passenger temp control, by a serial data message to the rear control module. So if it works at all, a lot of the system is OK.

When you set the front switch to "R", and then the rear control module fails to set the temp I'd be suspicious of the rear control module in the console. Does it control the airflow mode as expected? Floor versus ceiling?

Can you find a buddy to trade control modules with for an experiment?
 

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Discussion Starter · #3 ·
The rear control does change the air flow. The rear fan speed control also works fine. It acts like the rear temp control does not trigger a/c mode. I did not check the operation of non-a/c mode. Is rear temp control triggering any temp change? I'll have to check that this evening.

I don't have anyone from whom I can borrow a rear control module. Can I check for a signal at either the temp selector or back at the aux air unit?
 

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The rear control can't force the FRONT module to turn on the AC compressor. Just make sure the front system is in a mode that by itself is calling for the AC compressor to be on.

From the TB manual:
 

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2004 gmc envoy_sle_xl
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Discussion Starter · #5 ·
OK, so the front controls turning on the compressor. That makes sense.

The front control a/c is on and works fine. The rear a/c works fine as long as it is managed by the front control. When the rear control manages things it works fine (fan speed and output location) except the air is warm. What would cause the cooling to stop when the front control tells the rear control to manage things?

Roadie, you suggested something in the rear control. As I learn more how this works, I tend to agree with you. Other than swapping, how can I test it? I haven't looked at it yet. Can just the temp selector portion be replaced (since fan and location appear to work fine)? Or is the complete rear control module replaced?
 

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The front control a/c is on and works fine. The rear a/c works fine as long as it is managed by the front control. When the rear control manages things it works fine (fan speed and output location) except the air is warm. What would cause the cooling to stop when the front control tells the rear control to manage things?
One more detailed question. Are you saying that the front air temp is fine, and the receiver/drier is cold and the AC compressor clutch is engaged, and ONLY the rear air temp gets hot? Or does transferring the control to the rear cause the FRONT air temp to go hot as well and the compressor shuts off when the front is calling for cold air?
Other than swapping, how can I test it?
The module is a black box, and you can only buy it complete. Sometimes they're cheap on Ebay. The only way to comprehensively test it because so many functions are on the serial data bus among the modules, is with a high end scan tool like the GM Tech II tool. That can talk directly to the rear control HVAC module and exercise each of its functions.
 
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