This is a video for an HVAC electronics repair around the GM climate control unit module when there is a problem with the zone / mode selection being inoperative (air only exits from floor vents) and the AC button also inoperative. There may be other symptoms as well.

The problem can usually be traced to a failed switched 12V PNP transistor on the flexible circuit board. You can either use the same one ACDelco did, or an alternate compatible PNP, I chose to do the former. Sometimes this transistor will look fine, sometimes it will be visibly damaged. You should also check the adjacent resistors while you're in here.

If you have the below list of symptoms and conditions, AND you have a genuine GM / ACDelco climate control module installed, AND your truck is a mid to late 90s old body style (OBS) only - then this repair may be your solution and save you some money:

Mode actuator motor assembly is known OK
AND Air Conditioning button inoperative
AND Recirculation button inoperative
AND Zone / mode selection knob inoperative (air only exits from the floor vents)
AND Fan speed button still OK
AND Temperature selection knob stillOK
Rear defroster (if equipped RPO C49) still OK

The problem is this scenario almost always the switched 12VDC transistor located on the flexible circuit board next to the zone / mode selection knob switch. This PNP switching transistor is typically numbered 2857 or N724, or MPSA56 (cross ref NTE 159), depending on the year the module was manufactured. Both types of 1996 and up control units have a total of three transistors present on the board, but only one of the type involved in the +12V power problem. The repair of the problematic transistor is the same regardless of the board type. You can optionally replace the other two (but only one easily), but they are not typically at fault for these symptoms. Again, look for any other signs of burnt components like adjacent resistors and replace those also, if necessary.

You should replace the N724 / 2857 with a MPSA56 transistor (the version ACDelco most recently used). I did find that a MPS2907A will work as a substitute in a pinch as well but MPSA56 is the preferred choice.

Similarly you could optionally replace the 4692 / 4687 / 4693 / N723 / N728 with a 2N4403 (the version ACDelco last used). I tried a couple of PNP substitutions here for the mode actuator variable power transistor on the edge of the PCB, but they failed to cycle the mode actuator thru all voltage positions (defrost position would not be reached fully) so I ended up sticking with the 2N4403. Again, you don't typically need to change these particular transistors as they are unrelated to the fault symptoms.

This video repair procedure only applies to 1996 97 98 and up factory equipped with the following two genuine GM / ACDelco climate control module units; part 9378805 / 15-72547 (WITH rear defroster option RPO C49) and 9378815 / 15-72548 (w/o rear defroster option) found on Chevrolet, GMC, Holden, and Cadillac trucks.

The 1995 and earlier have a different circuit board layout, the transistor numbers and concept should still apply but the locations will be different.

If the transistor keeps blowing, then it's because of too much current passing thru due to a short to ground. The transistor involved is only rated at 500 mA. Follow the Light Green wire out from the control unit harness to the cycling switch on the side of the AC accumulator (and then beyond) and diagnose separately. You MUST correct this short circuit if present, if you try to bridge the transistor connection without doing so, you WILL allow the short circuit to permanently damage your vehicles VCM.

The eBay and Amazon search links below are paid links, for which I may be compensated and earn a commission, if you choose to buy the parts and tools shown in this video thru them:

Replacement transistors: https://ebay.us/ABssSc

And if I've saved you some $$ here, consider some coins for the coffee tip-jar! https://ko-fi.com/drshock

Chapters:
0:00 - Problem overview
0:39 - Removing the control module
3:33 - Repair overview
3:59 - Control module disassembly
5:40 - Replacing transistors
16:55 - Reinstall and test

#drshock #carac #autoac #obstruck #obstrucks

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