A/C Compressor not coming on - 84 Volvo 242

You can check it with an ohm meter, it's just a switch. Be careful not to damage the capillary tube for the thermostat that controls it. Turning it fully clockwise should close the contacts regardless of the state of the thermostat. If the compressor is no longer turning on the charge may have leaked out, I don't recall whether the 240s have a pressostat to sense that.

Connect the 2 wires at the receiver-drier, and see if the compressor comes on then. If so, then the switch is bad (very unlikely) or the refrigerant has been lost.

Dave I do not know where the a/c compressor is on the 242, but if you can get to it, when you think that it should be coming in, ie a/c full on, VERY CAREFULLY give the end of the clutch a tap with a thin wooden stick. If it clicks in, your clutch may be worn beyond its limits. It might be possible to adjust it in situ if this is the problem. Graham

It's right on the front on the lower-left side of the car. Easy to get to like all the rest of the accessories on a 240.

While we're on the subject, what's the deal with the red end of the AC thermostat? What's that represent? War Emergency Air Conditioning? Temperatures so cold as to be dangerous?

It bypasses the thermostatic switch, forcing the compressor to stay on (assuming no fault conditions). It can cause the evaporator to ice up, no damage but it will stop cooling until the ice melts and it could leak on the carpet.