the previous posts might have it covered, but there is a 3rd possibility. pull the cover from underneath the glovebox. there's a white relay that's screwed to the ac case. it's the compressor delay relay. when the ac is on and the car is started, this relay kills the compressor until the alt starts charging. if all is well, the comp comes on when the red light goes out. check the harness for stiffness. old, tired compressors draw too many amps and fry the harness. it runs from the ac switch on the dash to the relay and out to the accumulator sw. to find it in a factory diagram, look in the ac section. it's never in the charging section. good luck, chuck.