I read the FAQ and wasn't quite sure on this still. I have a tag hanging off a silver cylinder in my 91 940 SE that says 134a, is that where I connect the recharge kit and is that all I need to do to recharge the AC?

If the system is R-134a (converted or original), then the charge fittings should only let attach a R-134 hose. The fitting with a blue cap on the accumulator (silver thing) is one to use. To fill the system, run the AC on high & invert the supply cannister so that you are feeding liquid into the accumulator.

If you over fill the system, bad things will happen including possibly destroying the compressor. Pressure will not tell you how much freon is in the system (other it is empty). The only real way know is to drain the system & fill by weight. AC is one system that really need to know what you are doing to work on.

Did you or the PO change the system over to R134A? The 91 940SE came with R12 at least both of mine as well as my 92 960 came with R12.

91 940SE sedan and wagon

Ambient/pressure temperature are not accurate in the low side while the a/c is running. It is best to let everything even out in a cold engine and then connect the gauge in the low side and read preassures.
R-134A
68PSI 68F.
70PSI 69F.
72PSI 71F.
74PSI 72F.
76PSI 73F.
78PSI 75F.
80PSI 76F.
85PSI 79F.

Pretty easy process, how long before the recharge kicks in? I imagine it has to cycle through?

Yes, probably. There should be only one fitting that takes the blue low side charge hose off your gauge set. But the line you charge should connect (follow it back)to the driver's side port on the back of the compressor.

It is my understanding that to add R134a to a system it is necessary to know what the low side pressureis correct.

I found a recharge kit that has a gauge which has a marker to set according to the ambient temperature. The pressures on the gauge were also backgrounded with green or red, depending on the ambient temp. A good Investment, IMHO.

Good Luck,

Bob

:>)