1991 740 NA Air Conditioning -Intermittent, no leak

[SkyVolvo]

My '91 740 only blows cool A/C on occasion. It used to be regular, then it only worked sometimes, then only rarely. A year or so ago, I topped the system off with Enviro-Safe, and it was beautiful - all functional, all the time.

I drove the car down from Denver to Nashville and gave it to a relative, and this summer, the A/C got squirrely again. My first thought was that it needed a little extra charge, so I added a little, thinking that maybe it was just shy of what it needed. This did nothing, and it looked like it had PLENTY of pressure. It was another few weeks before I was able to even take a look at it again, and in the interim, it occasionally cycled on and blew cool.

The next time I looked at it, the pressure still seemed very high, so I wondered if it actually had too much pressure for the compressor to kick on. I released a little bit, and behold - the compressor started cycling. I eased it back up to the right fill for the outside temperature, sat in some lovely, cool AC for a minute or two, turned the AC on and off a few times for good measure and went inside.

Anyway, I went back out to move the car a little later and no AC. The compressor was not engaging. It clearly does work, and the system clearly has refrigerant in it. It just doesn't work on command. Is this likely to be a clutch gap issue, some type of relay somewhere, a switch - what am I not looking at that I should be?

Thanks!

UPDATE: Measured the clutch gap at .030 inches (.762mm)

[93Regina]

The compressor was not engaging.

With a wiring diagram, one follows backwards from AC-compressor.

On a 240, for instance, there is a pressure sensor; so at that junction, one injects 12 Volts, and listen for AC-clutch...no clutch noise, then go direct to compressor with 12 volts.