a/c problem fixed

[FinSub97]

I posted this a year ago to Chevytalk forum and I guess some TDP readers could be interested in it too. I am not quite sure if this is exactly the right forum, since it is not specifically diesel related issue, but any way...


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Hi there!
I hope this info is valuable to someone perplexed by defect a/c.

If you have following symptoms in your truck
1) A/C button won't work
2) Recirc won't work
3) Mode of operation knob has no effect (air comes only to floor for example)
4) Rear defog button works normally
5) Temp and fan speed knobs works normally
then you might have a similar problem I had. The fix is both easy and inexpensive. Culprit is bad transistor on your control head. By changing it (cost: 6 cents, time: 15 mins) everything works as usual. Here in Finland GM dealer asks 275 Eur (352 USD) for the head, so there is my motivation to fix it by myself.

To take the head off, remove dash and gently slide the head out. There are clips on both sides of the head, you need a screwdriver to tweak them slightly inwards. Disconnect all cables.

The transistor is MPSA56 (bipolar pnp) and you can locate the faulty one easily. It is the one under the bracket just next to mode of operation knob axis (NOT the easiest one at the edge of the board). Install the new one to the other side of the board, so you don't have to fit it into that extremely confined space. Make sure to connect the legs to the right holes. Emitter, base and collector are marked on the faulty transistor.

I can send more descriptive instructions by email.

Jari
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I got numerous replies from people around the world and it seems it is quite common trouble with GM trucks. Anyway, I hope that info is useful to some of you.

[Johnny B.]

Jari, could you send me detailed instructions I have a similar problem, thanks!

jeb5626@yahoo.com

[DickWells]

Jari: Would you cc. me too? E-mail is dwells@pshift.com. Or, you might like to put the details right here in the Forum.
I have exactly the same symptoms as you describe. Found it out yesterday, here in Brownsville, TX. I determined to exercise my A/C after not using it for quite a few days and that's when I found that the A/C switch button light wasen't glowing and I didn't feel the compressor kick in. ALL of your symptoms were there.
I went after the thing today with what I had here 2300 road miles from home and didn't get very far. I have the Helm elect/emissions/diagnosis manual, a digital OHM meter and test light, but the manual dosen't even tell me where the solid state compresser control is. Tells where the AUX controls are, though.
So, I was going to write in a question letter of my own when I came onto your note. Any details would be greatly appreciated.
Thanks, Dick Wells.

[a5150nut]

OK, spring is here and guess what.....no a/c.
I have the head unit out on the table. There is a pc board on the back of the switch panel and on on the back of that where the cable ataches. on the larger board there are two transistors just right of center towards the switch panel. Then there is one midle, just past center and three on the left close to the rear. No visible numbers on any of them.

Next?????? :eek:

Or do I need to be looking on the smaller pc board?

[DickWells]

Hi A5150nut:
Jari has good info, but I couldn't fix mine as easily as he did. So, don't be like me. I worked for over a week on mine, before I went to the Chevy store and paid $300 for a new control head.
WHAT I shoulda, coulda done was ask for a newer unit at a junk yard. The one I got from GM is much different from the old, but slips right in. So, I believe that anything up through 99 will fit. The new one is smaller, but the leads are longer. I could have fixed my AC for $85 if I had known about the interchangeability. Not as good as the $2 transistors that I put in before, but better than $300.
Lots of luck. Stick with the DP, and you'll get it fixed.
Dick Wells