Obtained this 1994 940 from a friend. It belonged to his mother. It was sitting for about 7 years and as far as he knows, it ran when she parked it. Got it running after redoing the fuel system, replacing a broken timing belt and taking care of a bunch of other little things including a bad fan relay. It's running pretty decent now, but the AC isn't working. I don't know if it worked when she parked it.

The clutch wont engage. It has enough pressure to activate the hi and lo side pressure switches. I removed the control unit and checked for the infamous bad relay solder joint. All solder joints looked fine. I even removed the 9 volt relay and tested it. I re-did most of the large joints anyway. I put it back in the car. To my delight, the clutch engaged and the AC was blowing cold air. I turned the AC off a couple of times and it came back on. The high side pipe was ice cold and the radiator fan was running. Then I think I might have turned the heat on to see if it would work, as it never worked before. It still didn't. I wasn't surprised. I think I have a problem with the pneumatic valve. But anyway, when I put the control back to cool, the compressor never kicked on, nor the radiator fan. I double checked both pressure switches, they were closed. I even jumpered them... still no AC. I can jump the compressor and it will blow cool air, but the radiator fan doesn't come on.(Although it works as it should to cool the engine.) I even removed the control unit again and double checked it. All the solder joints are solid. All the switches work. I can change the vents with the **** and the recirculating button works. It could be the control unit, but I want to be sure before I spend the cash for another one. Any ideas?

 

off the top of my head, there's a single wire that comes out of the dashboard HVAC control unit, runs through the pressure switch on the side of the reciever/dryer, then runs through another pressure switch on the evaporator manifold, and straight to the compressor. 12V on that wire, going through both pressure switches (one is high pressure, one is low pressure), and it powers the compressor clutch directly.

what is this 9V relay you talk about ?

 

I have previously tried jumping the compressor as you suggested and the compressor will run, but no radiator fan comes on. It is The # 10 wire (green/red) on the control unit. I actually unplugged the control unit connector, pushed a small wire on the # 10 terminal and plugged the connector back in, started the engine and then touched the wire to the cigarette lighter hot wire that I had disconnected. I turned the AC on and the clutch engaged, but the radiator fan never came on. It is just like attaching a hot wire directly to the compressor. I tried this before I had the the short success of making the AC work, I might add that I am getting power to terminals 12 (continuous) and terminal 8 (when the engine is running) at the control unit. The relay I am referring to is the small 9 volt , single contact (N.O.) relay that is soldered on the control unit board.

The wire you mentioned that runs to the compressor continues on from there to the system control module. Terminals from there continue on to the hi and lo pressure switches and to the fan circuit.

 

This tread seems to cut to the chase. (Did you ever search and search without finding the answer to your problem until you post your own thread?? ) Although the original poster solved his issue by adding R134A, he initially thought it was an electrical issue similar to mine. Another member ran him through the basic checklist. It fits my situation. Looks like I probably need a replacement control unit.

https://volvoforums.com/forum/volvo-850-16/no-ac-compressor-does-not-turn-all-19395/


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Just an update. I arrived home from work and saw that the eBay control unit had been delivered. I plugged it in and now my AC is operational! I do need to hook up my gauge (no Hi pressure port, so yes gauge!) and check the pressure though.