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I had two problems with my '93 940 AC. It wasn't getting very cold until I had made a fast run down the highway, then it stopped cooling totally.
I fixed the second problem by removing and resoldering the AC control board in the dash. The AC compressor clutch wasn't pulling in which led me to this repair. There are lots of references to this in the archives.
For the second repair, once the compressor was turning, I bought a r134 fill tool which had a pressure guage on a hose which connects to the low pressure side of the system at the accumulator on the firewall. This showed the low side pressure to be about 25 psi when the compressor was operating. It should be about 40 - 45 psi. The 'tool' has a place to connect a can of r134 and a trigger valve to let the r134 into the system. I let a little bit of r134 at a time into the system with the AC running and let the pressure stabilize. When the pressure with the compressor turning reached 40 psi, I quit. The AC now runs very cold.
I used a can of plain r134, no sealer, conditioner, or oil. I felt fairly certain that the AC system had just slowly leaked down since I've had the car a few years with no AC service needed.
The proper method of recharging is to evacuate all r134 then add the needed amount by weight.
There was also a factory TSB showing how to improve AC cooling at slow speeds by changing the electric fan circuit to make it run when at idle or slow speeds. I do not have this change in my 940.
I'm no AC expert but this is what got my AC working this summer.
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