1987 Pontiac Firebird; a/c compressor not turning 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