RTH - when I was troubleshooting the one on my '85 model I ran a switched "hot" wire from the radio power supply to the "supply" side of the evap temp switch. I could turn my comp on/off by making the switch. Seems that the power is fed from the pushbutton unit and mine was not working so until I replaced the pushbutton unit this is how I turned the comp on/off. The power goes from the pushbutton unit to the evap temp switch and then through the pressure switch to the Klima relay at least on the '85 model. Try putting power on the wire between the evap temp switch and the pushbutton unit. Make sure the wire from the pushbutton unit to the evap temp switch is disconnected first. You probably have a bad pushbutton unit.

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Jim