Three years ago my AC stopped working. I hot wired the compressor clutch and found appropriate suction-side pressure (periodically adding R-134) and function, so was driving it that way until now – with a switch that would connect 12 V from the cigarette lighter through a wire glued to the exterior of the car to the AC clutch.

This fall the AC clutch failed (bad pulley bearing). Found compressor operational, but replaced it with a reconditioned unit anyway because I had already purchased it. Found only a couple of ounces of oil left in the system and it was all in the evaporator - guess one should add oil with freon if you have a leaky system. With new O-rings everywhere, I think my leaks are gone.

After learning more about AC while doing this, I’ve come to appreciate that it is not a great idea to continue driving with a hot-wired AC clutch. The pressure switch on the suction side was working. Found a switch connected to the high pressure line that was open at zero pressure (I presume this is a high pressure cut-out switch - and that it should be closed at less than ~300 psi -correct?). The local Volvo dealer told me after some fumbling that it was a pressostat and wanted an unreasonable amount of time and money to provide a replacement. I replaced it with a Porch part with appropriate specs. Also found the AC fan relay had failed. Would activate, but high current side remained open. Found corrosion (but continuity) in the wire bringing high current into this relay. Replaced the relay with a GMC part with appropriate specs, and fastened a lug above the corrosion on the wire.

What happened after thus restoring the original configuration is this: the AC fan ran always when the ignition was on (even with the AC switched off). The AC clutch didn’t engage, but I saw 0.3 volts appear on the clutch after the AC was switched on – guess I have bad soldier joints in the CCU.

Here is something very strange. I then disconnect the wire that carries about 0.1 amps to activate the AC fan relay. Now the AC clutch activates as expected following ~10 second delay after AC switched on! The clutch also works if I hot-wire the low-current side of the AC fan relay. I’ve been poking around the postings here, hoping to find a definitive description of the AC wiring for my 740 – but apparently it is not to be found – and I’m not anxious to pay Volvo to see it.

Now finding that turning up the temperature setting on the console with the AC on only runs hot water through the heater while running the compressor continuously. I was hoping that a thermostat would be used to cycle the compressor to maintain a specific temperature. Is there no AC thermostat in the 740?

Am I over looking anything important here?

John Mattox, North Carolina, 1991 740 with 168k miles.