Cadillac Repair: 1983 cad dev 4100dfi air cond, fuse block, blown fuse

Question
when I apply 12v to a/c compressor terminal, the compressor operates o.k. but not otherwise. cannot find blown fuse. p/w & pdl work ok, so don't think fusible link is blown. Is there a relay somewhere? blower motor &dampers work o.k.

Answer
Hello Larry,

      When checking the a/c for proper operation hooking up the gauges is one of the first steps that needs to be done. That way you know if there is enough freon pressure to safely operate the compressor. And also that adding more freon won't cause problems.

       I would expect you have at least 80psi on the low and high sides when the compressor isn't on. If you have 20-25psi then the compressor shouldn't turn on at all and you will need to add freon and check for a leak in the system. The good thing with R12 freon is where there is a leak it most times shows up as a damp oil spot.

       If you have enough freon then there is a 25amp a/c fuse in the fuse block by the parking brake pedal. The compressor relay is located under the hood on the RH side up by the windshield. Also in that same housing at the top is a power module. It controls the a/c compressor and also the blower motor. You will recognize it because it sits on top of the housing and has a single large black ground wire coming from the top along with a wire connector. Also on the evaporator core tube there is a compressor cycling switch which is nothing more than a relay that is activated by temperature...mainly cold allows the compressor to turn on and as the temp of the tube warms up the compressor turns off. Try tapping on it with a screwdriver handle and see if the compressor turns on or just remove the 2 wire connector and jumper them together and if the compressor turns on then the cycling switch is bad and needs to be replaced.

       See what happens with those checks and let me know if you need anymore help. Bill