Join Date: Aug 2010
Location: Magnolia, Texas
FWIW, I feel your pain.. I'm experiencing the exact same problem with my '91 740 Turbo. Because it is *really* hot here in TX, I've been running with a long jumper wire clipped to one of the wires going to the low pressure switch on the accumulator. Yes, it does make a difference which one, because you do not want to bypass the switch and just have the compressor run constantly. I first ran a jumper wire from the battery to the wire that goes to the compressor, just like you did, except I had the key off/car not running. I unplugged the connector on the low pressure switch on the accumulator and used my test light to see which prong was getting current. I then stripped a small bare spot on the wire going to *the other* prong (the one that was notgetting current) and connected my long jumper wire with alligator clips to the battery. If you do this, you can run the AC and stay cool while you try to solve the original problem. Of course, it's a PITA to have to clip and unclip the jumper wire every time you shut the car off for more than a few minutes, but it does beat sweating.
Right now, I'm thinking the easy/quick fix is going to involve replacing the control panel, which includes the on/off switch, fan switch, etc. I pulled one out of a wreck today and hopefully will get a chance to install it sometime in the next few days. I'll let you know how that turns out...
P.S. After re-reading your original post, I realized that you mentioned that the AC did not cool when you jumpered the compressor directly from the battery, although the compressor did come on. This would almost certainly indicate low or no refrigerant, and in this case you most definitely do not want to run the compressor much or you risk seizing it from lack of oil, which circulates with the refrigerant. If you hook a jumper wire as I described above, the compressor will probably not come on because the low pressure switch will not allow it. In my case, about 3 days after running the AC with the jumper it started blowing hot air again. I thought my jumper had come loose, but all connections were good. Long story short, I had developed a leak which caused the low pressure switch to shut down the compressor, saving it from frying. I had to find/fix the leak and recharge the system to get cool again and resume my search for the on/off switch problem. Sounds like you may be in the same spot.
Last edited by Joseph/TX; 08-01-2010 at 02:46 AM.