1987 Pontiac Firebird, a/c compressor not coming on.

[GMP440]

Compressor not turning on. System has charge.
When you turn on system max a/c and a/c the cooling fans come on which is normal. Switch moved to other setting , defrost , etc everything else is working.
Used jumper wire from battery to compressor. Clutch activates ,compressor works and spins.
Pulled off electrical plug from pressure switch on accumulator to check voltage at plug. No voltage at plug when the system is on.
Don't know if its a bad relay or diode that is causing me not to get voltage to the pressure switch plug.
This is where I am at. Still trouble shooting. Any help is appreciated. Thanks. George

[Tom Greenleaf]

Welcome George. Testing my memory for this. TMK LPCO right on accumulator goes directly to clutch coil from there but no power there. It's just an on off switch set for a pressure to cut out nothing more. Power to that follow the wire and any info you have for where a relay is for it or maybe fuse also maybe inside not under hood? That wire just may pass thru a WOT (wide open throttle) switch to give more power cuts out compressor with VERY low intake manifold vacuum on purpose. Any vacuum leaks or at that switch would show low vacuum and not send on power to LPCO at the accumulator. Anything to do with vacuum for those just rule out everything starting with hose almost has to have been totally done already once and time again it doesn't last well.

When time please list out just what you have here, all custom, altered then which engine and compressor. What refrigerant are you using it came new with R-12 (the real Freon) best if super nice it still does if possible for you but don't go back now,

[Cornbinder89]

Well, I think you've done well so far. I does look like the feed to the pressure switch is the problem, without feed, you can't expect it to work. With a vehicle of that age, it is just as likely to be a wiring harness problem (corroded plug, broken wire corrosion in the harness) as a component (relay, switch etc). SO the solution is going to lie in getting the wire diagrams a tracing out where it system works and where it doesn't and keep working from both ends until you find where the problem is.
I've been out of automotive for so long, but going from memory, it would have an R-4 compressor and a simple cycling switch on the accumulator. The fact the fans come on, suggest that the dash control is working as it should, but the problem is between that point and the cycle switch.
You can start from both ends, may be wire a buzzer into the feed wire to the pressure switch and start wiggleing the harness to see if you can find a break, the buzzer will let you know if you make connection. Pinched harness on something this old is a possibility, as is corrosion. If someone used a test light a punctured the insolation anywhere along the line, you could have a broken wire that will be very hard to find.
Good luck and get the wireing diagrams, with out them it will be much harder.