I have an issue with the AC in my '91 740 turbo wagon. After reading a bunch of threads, I decided to try re-soldering the MCC board. I removed the MCC and melted the solder joints that I saw in the pics in another thread, and a few others where wires connected the board. It didn't look like any were cracked or loose. I didn't think new solder would fit on the joint without making a mess. Put the MCC back in and it still doesn't work. The compressor has recently been replaced, and I am pretty sure I have pressure, and the pressure switch has been checked and is OK. Where do I go from here?

 

You have to make sure that control unit is ok... before exploring further. Get one from a junk yard.

 

I'd check for voltage further down the chain before doing anything. There's a pressure switch near the firewall that should have +12v coming in, and you can jumper to its output to see if the clutch engages on the compressor. (Three big circles in the pulley will be spinning not sitting still.)

 

I have another MCC from a junk yard. I already tried it and it didn't work. I was thinking that the circuit board issue might be a problem in BOTH my MCC units.

 

I'll check the voltage further down the chain. I suppose it could even be a bad wire or connection after the MCC?

Thanks for your suggestions!