rear AC blower won't turn off

[dlnelsonroca]

My rear AC blower has been sporadically working lately, but as of yesterday won't turn off at all, even with the motor off and key removed. Today I disconnected both the front seat (near the shifter) and first rear seat controls for the rear AC. It didn't change anything. I finally removed the fuse under the first rear seat to get it to stop blowing.
Less than a year ago I replaced the front blower motor control module because the front speeds weren't working right. That was fixed. Three years ago I replaced the rear blower motor control module, which caused the rear AC to start working again. That was no fun with such a tight space to try to loosen and tighten nuts in! Since then there has always been, however, an approximate three minute delay for the rear AC blower to turn on. Now it won't quit working, though the delay is still there.
I live several hours from the nearest Chevy dealer, although there is an electrical mechanic with his own small shop fifteen minutes away.
Any good ideas? Thanks a lot!

 

[DAlastDON]

Going to use process of elimination here. You have one of two potential components that have failed. The Blower Motor Control Processor-Aux or the HVAC Control Modual-Aux. Pulling one fuse is how i would rule each one out. You already pulled one fuse to make it stop. Which fuse was it under the rear seat that you pulled? #13 or #36?

The 30 amp #13 is the Blower Motor-Aux +12v. The 10 amp #36 is the HVAC CM-Aux.

#13 will stop it no matter what because you are cutting the 12V from the BM-Aux. But #36 looks like it would stop it only if the HVAC CM-Aux was the fault. If you pull #36 and it keeps spinning then it is the BMCP-Aux.

This is how the HVAC CM-Aux controls the BMCP-Aux.

Blower Motor Control Processor

The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control module provides a low side pulse width modulated signal to the blower motor control processor over the blower motor speed control circuit. As the requested blower speed increases, the HVAC control module increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control module decreases the amount of time that the signal is modulated to ground.

The three minute delay smells of the recalibration when the Keep Alive Memory is lost in the HVAC CM-Aux. Makes me think the HVCA CM-Aux is the problem even before pulling the fuse.

Auxiliary HVAC Control Module - VIN 6

The auxiliary HVAC control module is a class 2 device that interfaces between the rear seat occupants and the auxiliary HVAC system to maintain auxiliary air temperature and auxiliary air distribution settings. The battery positive voltage circuit provides power that the control module uses for KAM. If the battery positive voltage circuit loses power, all auxiliary HVAC DTCs and settings will be erased from KAM. The auxiliary HVAC control module will perform a recalibration of the electric actuators when commanded with a scan tool or if KAM is lost. This will ensure the actuators are moving with in the calibrated range. During this calbration process the auxiliary blower motor will be disabled for up to 3 minutes.

 

[dlnelsonroca]

Thanks for the great and helpful information! The fuse I pulled was the #13. I've driven the car today with the fuse back in, and the rear blower won't come on at all.
It has had this behavior before, sometimes working, sometimes not, or sporadically while driving. And also sometimes not blowing at all or later blowing constantly, even with the key removed, and both rear blower controls on both ends of the center console disconnected from the wiring.
So I can't tell today whether the #36 fuse will stop the motor or not because it's not blowing at all.
Assuming that the problem is most likely the HVCA CM-Aux because of the delay, do you know of anywhere that I could purchase another? I looked briefly online but saw nothing. There are virtually no junkyards within hours of where I am.
Thanks again!

 

[DAlastDON]

You have to get one for an trailblazer ext or envoy xl. The face plates look exactly the same as the non ext/xl. I would pull yours and get the part number off of it then search for the part number.

 

[The_Roadie]

Bad transistor in item #5.

 

[dlnelsonroca]

Thanks to both of you for your insights! So, a small update. This morning the rear fan came on again like before. I had to remove the #13 fuse to get it to stop. The #36 fuse didn't do anything when I removed it. So, is the problem exclusively the blower motor control processor aux, or is it just the blower speed control module, or both? I replaced the blower speed control module a few years ago, but the three minute delay was still present even then.
And would you have a picture or location for the blower motor control processor aux?
Thanks again!