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Discussion Starter #1
I would like the wiring diagram for a 1994 GMC 3500 a/c system. And the location diagram to go with it.

Thanks

In case there is a tech very familiar with the '94 and earlier system, my problem is:
- The a/c works great, but cuts out after an hour and a half or two hours. The system resets after some time, without doing anything (no battery disconnecting required).
- When the system stops, there is no power to the compressor clutch or the low pressure switch.
- It is possible that once I simply clicked the a/c switch off and on and it restarted, but maybe it was co-incidence or my memory is ....

I am wondering about the relay, where ever it is located.

Thanks
 

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The relay is in the main fuse panel under the hood.
I don't have a wiring diagram for your truck; but the basics are the same.
The sensors that tell the system everything is ok are connected to the dash mounted control unit.
When AC is turned on and the sensors say everything is good, a signal to operate is sent to the relay.
The operated relay contacts complete the circuit for the clutch coil.
The energized clutch coil acts like a solenoid magnet and pulls the clutch plate to the pulley which operates the compressor.

I just fixed a Saturn with the same problem, the AC operated fine when first turned on, shortly after it would stop being cold.

My wife's car, it acted like that for a couple of weeks and when I looked at, usually after supper, it worked fine.

However, only took a few days and it stopped working all together.

The clutch coil was open, a new coil and we're cold as ever.

The first thing you need to find out is the state of the relay. If it stays operated and the contacts are good, the problem is wire, coil or connectors.
If the relay does not stay operated, the problem is in the relay operate circuit, or in a sensor circuit that inputs to the control unit.
 

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Discussion Starter #3
Thanks for your response.

This is a '94 truck (90,000 miles on it) one year before the nice fuse/relay panels were placed on the driver's side inner fender.

I have found two relays mounted behind a plastic panel mounted above the drier. I expect one of those is it, but I don't know which and I the only wiring diagram I have found appears to be for earlier truck and does not match. I have a '95 service manual, but it does not match

I have also found a diagnostic chart, also, I am sure for a slightly different truck. The first step is to disconnect the compressor (C221), turn on the ignition and test for power. Mine has one terminal with power and the chart suggests on open on the second wire. But, if I plug it in and fire it up everything works properly, so I suspect the chart is not for my system. I have a '95 Service Manual, but the wiring and switches are the newer '95 and up.

It seems to me I remember someone mentioning that the dash switches on the '88 to '94 trucks were a common problem.
 

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Ok, an easy way to find a relay is to lay a finger, or two, on the relay case while someone turns the system on and off.
If the system is operating correctly you will feel the relay operate under your finger(s).
 

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One relay is for your fuel pump (s/b on the left looking at them from the front), the other is for your A/C. Here's a bunch of manuals including your 94.

Another thing, check your voltage at the compressor while running. I suspect you may have low voltage which, after a while your clutch starts slipping. I found mine ~11.4V, on my 88. It doesn't have a relay on the firewall. I wired one in next to the battery and ran the green positive wire to it and ran battery voltage to the compressor. Now I have ~14.0 there. I was going to change the clutch but decided to wait and see after I did this. This may or may not be your issue but, just a thought.

Here's the thread I posted showing what I did. You can go to the last 2 pages if you want to skip all the other stuff, HTH.

Edit: forgot the manuals' link.
 
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