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Did you let the comp run for a while with a low charge and then take the reading AFTER the comp was shut off? Or were you reading off of the gauge when the compressor had shut down?
The best way to verify pressure is to put the low pressure gauge on the system FIRST. Don't even turn the thing on. See what the reading is. THEN jump the switch briefly and watch the pressure go down, which it should. If it does NOT go down, un-jump the switch or shut the system off. It should not escalate from a system off reading, rather it should go down. On a hot day, with the system running and THEN shut off, the suction side will eventually equalize with the rest of the system, making the reading very high. The only accurate way to read is with the system running. Otherwise it is virtually meaningless.
If it escalates when you first turn it on, then there is a major problem somewhere. Usually a frozen evaporator or orifice would lead to a VERY low suction side reading, which the pressure switch would already have compensated for. There would have to be a compressor problem, or a big overcharge on the system to have 120 with a running compressor. But on a very hot day, after the system has just been run and then shut off, the low side could creep quite high once it was shut down.
Even if the charge was somewhat low. After picking up heat from the cabin and being shut off, it has nowhere to go with the heat. That's why a lot of A/C blowouts happen once you shut the car OFF.
Also, please forgive me if I've reviewed things you already know.
IF perhaps it is running, and at 120psi, then as I mentioned before it could be overcharged, or the condenser could be ineffectively exchanging heat. In that case, the high side pressure should theoretically shut off the system altogether, though, so that would be less likely to be the case.
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1992 940 wagon, 72k
make people envious; smile often.
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