I have a 1991 745T with a system converted to 134a. I had a problem with the control on mine while on a vacation several years ago. I ran 12v to the compressor and it kicked right on. Thought I was pretty smart until down the road I realized it wasn't cooling like it had been. I raised the hood to find the receiver dryer encased in solid ice. With a constant 12v to the compressor it will no longer cycle on and off. I disconnected the 12v feed and evidently hadn't done any damage because after repairing the broken solder joint on the climate contrl circuit board I was back in business.

There is a small relay that is attached to the board by solder joints. It evidently overheats and joint breaks. After removing the climate control:

open up the unit and look for a bad joint like this one:


Resolder the joint and you should be back in business.

Randy