The TrailBlazer, originally introduced as an upscale Blazer, was produced from 2002 to 2008 with a powerful I6 engine and with a monster V8 offered in the Trailblazer SS
Platform: GMT 305, 360, & 370.
I have a 2003 TB Ext 4.2L with a manual dual zone a/c system. The rear a/c system is not working either from the front or rear controls. I have replaced the
rear resistor and the rear hvac control module with no results. I have the wiring schematics and during TS I found that the fan will come on when hanging a DVM
(checking resistance) from ground to the aux blower motor control line on the hvac control module. When checking the voltage I do not see a change in values using either the front or rear control *****. If anyone has any advise or experience with a similiar issue it would be appreciated. By the way, the front a/c works fine.
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A little better detail. When I turn the blower speed control **** (next to my right leg on the center console) the blower does not come on for any of the settings. When I set it to the rear control position, the speed selector **** on the control module on the back on the center console will not turn it on either. I am only having this problem with the rear blower. The front blower is working fine.
On the front auxiliary blower motor switch, there is a wire that comes from the auxiliary HVAC control module in the back. It should have a positive voltage. If so, the problem is probably with the front auxiliary blower motor switch, maybe a bad ground wire. If not, the problem is toward the back, either an open in the wire or auxiliary HVAC control module.
I have no voltage on the front switch. There are two wires that come from the switch to the hvac control module. I checked both with reference to gnd and between each of them. I also made sure I had the 12v battery voltage referenced to gnd on the hvac control module. I ohmed out the switch and it appears to be functioning properly. Here are the resistance readings. These measurements were made at the hvac module between pins B2 and A6.
So the front auxiliary blower motor switch and wiring to the auxiliary HVAC control module seem reasonable, other than I thought open meant off. Anyway, the auxiliary HVAC control module gets the battery voltage (always hot) and wakes up when commanded by the BCM (with key in ACC or ON). Maybe it's not getting the battery voltage or is defective. Since it's a little too expensive to try throwing a new one in there, you probably ought to get it scanned for body (B) trouble codes first.
I agree. Think its time to head for a shop. I am going to go ahead and scan all the pins on the hvac control module with a meter and scope to document what is there. Then do it again if it gets fixed. I will post this info when the repair is complete. It may help someone.
I went in to a local shop this morning and the tech was very helpful. He stopped what he was doing and printed out several schematics of the entire a/c system for me. It gave me a better understanding of how the system worked. I found that the 5v reference is being produced on the HVAC control module and outputted to the actuators and front blower motor speed switch. I am missing the 5v reference voltage coming off the HVAC control module on pin A9. I was
able to trace the 5v on the module to the processor on the board. I am fairly confident now that the control module is bad. I'll post the results after I replace it.
Also, the above switch resistances were taken with the module removed. I checked the switch on a friends TB today with the module installed. I can't try his module in my TB since he has an auto HVAC system. I'll list the switch resistances with the module installed.
I received the new HVAC Control Module and decided to bench check it for the 5v reference voltage. The module powered up fine. I ohmed out all the actuators and position sensors to make sure nothing was shorted. I installed the module and turned the key to the on position (engine off). I could here the actuators initialize. I started the vehicle after about a minute and the blower would not come on. I removed the module and bench checked it again and the 5v was still good. I installed the unit again and and tried the calibration again but this time I waited about 5 minutes. I started the vehicle and the module is now working correctly. I had tried a used module a few weeks ago but I may not have waited log enough for the calibration. The used module might have been bad but I'm guessing the cal may take longer than I thought. I didn't listen for the actuators when I did that. The 5v powers the actuators, position sensors and the front control switch. I hope these posts helps someone in the future.