I am trying to revive the a/c in my 940. it hasn't worked for years. But everything else runs so well, I thought I'd give this a shot. Coolant has been serviced and no apparent leaks. Low pressure switch has been checked. There is power to the head unit but compressor is not getting power.

 

So, the compressor kicks in when shorting the switch?
If so, and the refrigerant is still there, if that is what you mean by "coolant", the problem is in the control unit in the dash.

 

So how much pressure is in the low side? If all the electrics are okay and it's just low on refrigerant it should kick on when you short the tabs on the pressure switch together. If it does, then you just need a charge.

 

I just finally got my system up and running in my '93 940.

What I found odd is, when you first turn the fan on, it takes several seconds - up to 10 & sometimes longer - for the compressor to kick on. I figure they engineered it this way to let the engine settle from fast idle, before putting a big load on it. That threw me for a loop when I first charged it up.

I ended up replacing everything except for the evaporator, and high side hose from compressor to condenser. Running R12 - works fantastic.

 

How much freon is in the system. Did you put a set of gauges on it. If you pull the plug on the low pressure switch and put a jumper in the switch does the compressor turn on. Did you try to pull the wire off of the compressor and jump it with a hot 12V wire.