My 1990 C4 A/c is not working . The blower fan works. When I press the A/c button, the button light comes on, but I cannot hear the system "engage" like it used to. Similarly the engine revs do not drop when I press the button, so I assume the compressor is not going.
I have checked the fuse to the a/c compressor and it it intact.
I have had a flat battery since the last start up, when I thought the A/c worked, and recharged with a trickle charger , if this is relevant. Any suggestions?

http://www.pelicanparts.com/more_inf...77389-4024.jpg

__________________
This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: Porsche Air Conditioning Frequently Asked Questions

There is a system pressure switch that will stop the compressor from activating if the pressure in the system is too low. I'm troubleshooting A/C problems on my 959 right now, and I think that the issue on my car may be this pressure switch. I would check this out and make sure that continuity is constant across the switch. Otherwise, the compressor is not going to kick on.

- Wayne

I had a power wire fall off a terminal on the backup light switch. It powered the ac compressor clutch system