I have a 1993 Pontiac Bonneville with an AC problem. Assumed it was low on Freon (R12) at first, so I took it to a mechanic who checked it out, said the Freon was good and he also looked at the relays and said those where good, he thought there was maybe something wrong under the dash.
I did a little testing myself, pulled the connector on the AC clutch off, when I turned the AC on and I would get 13.5 volts on the connector. This to me at least suggested that everything was working, dash switches, pressure switches, and relays, otherwise why else would there be voltage at the clutch when I turned the AC on??? So I decided to change out the AC clutch, managed to change the clutch without too much trouble, without taking the pump off the car, put the new clutch on and turned the car on. Turned the AC on and got nothing, pump is not turning, I can turn the pump by hand by the way so it is not froze up.
My problem now is that I have a check engine light on! Code reader shows code 26 and 56 (quad driver errors?). I rechecked my voltages at the AC clutch and I am no longer getting voltage at the AC clutch when I turn the AC on, I'm guessing this has something to do with the codes?
Now What? Any ideas or theories would be appreciated!! Thanks!

On original test, did you check for power AND ground?

Both codes refer to the quad driver....not sure how many are used in this ECM....

Not sure if one of the quad driver'* runs the A/C relay(grounds it)....

Checked all the fuses in the relay center? Tried swapping out the A/C clutch relay?

You really need a scan tool, to see if the ECM is getting a "Request" from the HVAC control head....if it is, then is the ECM grounding the relay?

On the original testing I did, I hooked the positive lead to one wire (green wire) and the negative lead to the other wire (looked like a black wire) and when I turned the AC on the voltmeter would read 13.5 +/- volts. I would assume this covered the power and ground?
Yes, I have visually checked all the fuses in the relay center, they all looked good. I tried swapping what I believe is the A/C relay (first relay from the passenger side) with another relay no change.
As far as getting it checked out professionally, I had a terrible time just finding a place that would look at it in the first place, nobody wanted to touch the AC in this car. Finally found an older guy that said he would check out what he could. Haven't tried taking it to the dealer shop yet (price yikes!).

Also if it is of any help when you turn the AC on you can hear the engine change and the RPMs go up. The AC pump does not turn though.

That seems like the ECM got a "request".....

So now it'* a matter of is the ECM grounding the relay, or is the relay bad, or is the fuse bad, or is there an opening in the wiring?

Been working on this some more and testing as much as I know how to do. I solved the check engine light problem, there was a blown fuse in the fuse panel on the passenger side of the car - never knew that was there before!
Supposedly the mechanic tested all the relays, pressures switches, and actual Freon pressure. I'll recheck the relays just to make sure.
I manually hooked the cultch on the compressor straight to the battery and the clutch kicked on and was blowing nice cold air in the car.

I had to replace my pigtail as it was a ground problem on the pigtail itself.
I used a new pigtail with a diode, available from a local A/C shop.
While testing, did you have a separate pigtail, wired directly to the battery?

I just used some alligator clips to the battery and the clutch. I did have a switch and inline fuse on the positive side, something I setup a long time ago to test wiring.
I was wondering if there was a diode in the circuit somewhere, I don't have a wiring diagram, so running a bit blind here trying to figure out the wiring.
Thanks!

Two things to check....the A/C clutch relay and the pressure cycling switch on the accumulator....

Thanks for all the help, turns out the problem was the connector that goes to the AC clutch. After a lot more testing, I came to the conclusion that that was the only possibility, hooked up the clutch connector and the clutch with some wire and alligator clips and the turned the AC on and it worked! Not sure how or why the connector is bad, but after bending the crap out of the connectors it now works!