Hoping someone can give some insight to a recent issue regarding AC. It appears my digital HVAC controls are not sending the proper 12v to the cycle pressure switch on the accumulator. Previously I was charging my system and jumped the two wires for the cycle press. switch on the accumulator to engage the comp. clutch, which it did but only momentarily. Ever since, I can't get it to do it anymore. This switch on the accumulator is now reading less than 3v, on one side, and less than 1v on the other. If I force 12v to this wire from the HVAC harness, I then get the proper 12v to that switch, however, the AC high pressure switch causes it to blow a fuse unless I disconnect it from the compressor.

My question is, by me originally jumping those two wires to get the compressor to kick on, and now it won't and the volts are way low, what happened?

HVAC harness. Wire circled is supposed to have 12v, it does not.

 

I'm not 100% certain , but it sounds to me that , if the high pressure switch is shorted to ground , which it sounds as though it is , and you jumped the cycling switch putting the control head's output into a dead short to ground , that you have fried the output device (transistor) in the control head .

 

I know it sounds like a mess, it really is lol. Not sure what I did since even years prior, never had a problem with normal compressor function. Maybe the digital control head was too sensitive for what I was doing, which was suggested by many to kick the compressor on, and is also in my service manual. But going by the book, it says if I'm not getting the proper 12v from the green wire in the pic, then the control head needs replacing.

 

Normally , had your high pressure switch not been shorted to ground , what you did by jumping the cycling switch to get the compressor to run wouldn't have fried the head . But since the switch is shorted , it drew mad power through the head and the switching circuit suffered . Now , I'll freely admit to never having one of those heads apart myself , so I don't know just how replaceable any of it's components are , but , were this mine and since I tinker with electronics , I'd give a try to getting in there and replacing whatever burnt due to the overload . May be only a small section of the printed circuit's foil has burned open . If you know any electronics tinkerers (dare I say "computer nerd") among your friends , I'd have one of em take a look at it before you lay out cash for a new one ...

 

That makes sense. It's funny you mention that about electronics as I'm "nerd" lol. Been messing with computers for over a decade but never went deep into diodes and transistors, etc. I have my unit apart and noticed a circuit was broken on the board but I think that's when I had it open and connected, plugged up to the car, and my antenna lead touch the board. Oops. Got that patched up though but this of course isn't my original issue. Testing the transistor going by BCE, seems there is a short and volts are only 0.2. Better explained in the pics below.

B-C=0
B-E=.270
C-E=.270