Factory AC suddenly stopped working

[mikemileski]

I have a '72 GMC 4x4 with factory air. Occasionally, when I would have the levers in the cold and air conditioning position, I would turn on the fan switch and the compressor wouldn't engage. I would flip the switch to off and then back on and the system would start working. Today after work, the compressor wouldn't engage no matter what I did. When I got home, I jumper wired the compressor and it turned on just fine and blew cold air. Is the culprit my switch? I'm assuming it is but wanted some input here before I go and change it. Thanks.

Mike Mileski
Tucson, AZ

[ole dollor]

i think you hit it on the head....sounds like your a/c dash switch went bad....while you are in there i would make sure the blower motor switch is in good shape also. they are not fun to change....

[ChevLoRay]

If I had to guess....I'd go with the switch. After all, it's 40 years old. At least the components are available to rebuild your controller, or you could replace it with a used one and now know if the components are okay, or not.

[mikemileski]

I'm not quite understanding. Is it the three speed fan switch I should be replacing, the one turns the fan off and has a low, med and high position? Or is there some other switch in the controller?

Mike Mileski
Tucson, AZ

[ole dollor]

mike if i remember right you have a fan speed and a compressor switch behind your lever panel, should be on the heater a/c temp lever.....i has been a long time since i have had a factory panel out....or maybe on the cable at the a/c heater box....should be one or the other....if i'm wrong some one else should chime in, my 69 c.s.t. had factory a/c and i had to repair it....but that only been about 28 years ago....

[mikemileski]

I also forgot to mention that I did check the AC fuse and it was okay.

Mike Mileski
Tucson, AZ

[ChevLoRay]

Does the fan run, but it just doesn't cool?

If the fan isn't running on High, there's a resistor on the air box inside the cab that controls that feature.

But, if the fan runs, then it's probably is the switch for the compressor that is on the slide lever. I believe that was the problem on mine, too. When I got it, there was a switch hanging below the dash that had to be engaged to make the compressor run. Otherwise, I used my control panel in a normal fashion. It just wouldn't send power to the clutch to engage it. It's an easy thing to wire it and make it work.

[mikemileski]

Yes, my fan blows fine on all three speeds. The problem is that the compressor won't engage but it will work fine if I jumper wire it, so I know that it still functions. I wasn't aware that there was another switch associated with the control panel besides the fan switch, other than the vacuum port one. I guess I'll undo the two screws on the underside of the dash that secures the control panel and check it out.

Mike Mileski
Tucson, AZ

[brianthelion02]

Having the same issues with my 71. I just replaced the whole a/c control panel and only the blower switch is there. I pulled the short plenum in the ac duct work out and there is the compressor switch. However I still have no power at that switch. Hopefully that switch is you're culprit!
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[Coley]

Ok, the AC compressor clutch is not actually 'electrically' activated with the main a/c control panel.
There is a separate clutch compressor (plunger operated) switch that turns the clutch on. It is activated or 'enabled' by the linkages that leave the a/c control panel.
This switch can be found up in behind the main (in cab) a/c box as you look upwards. It is just to the right of the main panel and up underneath the dash.
So, get a small flashlight (and your be doing some wriggling 'backwork' here) and look up in behind in this area. You will see the (2) wires rising up into this small 'plunger' switch.
Basically when you swing the a/c lever to the left on the main panel you actuate a simple metal plate in there that presses that button in, thus actuating your compressor. Visually when you look up this will make more sense and you can see the linkage/line and switch up under there.
If you want to test it...you can start your truck and without even touching the main controls, push that plunger button down (little bit of finger/wriggling in there to do this). You should hear the compressor clutch immediately kick in.
Give it a look, good luck and a try and keep us posted.
Coley

[brianthelion02]

And with a little emery cloth and contact cleaner my compressor works now,on to find the high speed blower relay.....
Posted via Mobile Device

[ole dollor]

coley: that sounds right....thanks for chiming in....i knew it was there some where, and was hard to get to....my 69 the fan switch and compressor switch was bad.....and its to dang hot to be out driving around without a good cold A/C....

[brianthelion02]

DId ya get it figured out Mike?

[mikemileski]

I hope to crawl in there this afternoon and will report afterwards. Thanks for all the info everyone.

Mike Mileski
Tucson, AZ

[mikemileski]

Well, turns out it is the little switch that's mounted on the backside of the ducting under the dash (it's held on with two little screws). I disassembled it and its kind of chewed up inside so I'm ordering a new one. In the meantime, I jumper wired the two wire connector going to it and I'm back in business. Thanks again for all the input from Forum members. This place is great.

Mike Mileski
Tucson, AZ