That's the only thing I can think of based on the diagram. Mine is a '68 but the only major differences are the compressor location and hose routing. My assembly manual diagram is a little tattered, but from what I can tell the main power line is controlled by a switch located on the temperature control. When this connection is made, it feeds power to the thermostat control switch, which then feeds power to the compressor. How about taking a 12v light bulb, connecting one end to ground and one end to the compressor side of the theromostat control switch. You could easily run a lenght of wire into the passenger compartment and observe if the compressor power shuts off in conjunction with the compressor clutch. I do not know if the GM compressor has an internal means of disconnecting the clutch if the gas (freon or R134)pressure is too high, but that could be a possibility depending on the light bulb test.