Can you find the output schematics of circuits for the AC controller? Before digging deep into hard to reach wiring and parts, I'd start with the main AC module. See which is the output wire/circuit that leads to the compressor, and what voltage it should be. Check that first before spending large amounts of time or money on other items.
The sensors and switches are more reliable than the electronics of modules. If guessing the module is more suspect, and there are many sources for repair services for them. I'm just suggesting to eliminate that possible expensive and common issue, which will be more likely as the parts get older.
Again, I fixed my 91 Lincoln same issue, which was the AC controller(EATC), by not fixing the EATC itself, but adding a relay which uses the existing insufficient signal. I don't claim it's the best answer for every case, but the high current through the old model controllers was a bad(old) idea. Repairing those old electronics is hard to find help with, and expensive, my cost for the relay was about $8, plus my time(about 15 minutes(plus prior 2+ hours of diagnosis time)).
So be sure your AC controls are producing the proper output signal to the AC clutch. If it's right, then you are narrowing down the problem to the wires and components in between, on that circuit.
