Too much coolant?

My AC used to work just fine and dandy in my '97 GMC sonoma (I had it serviced last summer). Now that it's starting to get really hot it doesn't seem to work real well. If I'm driving (ie pressing the gas pedal) the AC will be strong enough. But sitting in traffic or just driving around town, the AC isn't powerful enough for the heat of mid-day.

I just got back from AutoZone and bought a can of coolant. Followed directions etc. and when I hooked it up my pressure gauge on the can read 85psi. That's way too high right? What do I do now?

I trust you actually mean refrigerant. Coolant is what goes into the radiator and engine block, often referred to as antifreeze.

The 85psi reading is just with the can connected to the gauge, or the guage connected to the low pressure side of the system? That is high for the low pressure side, but could just be due to a low charge, I suppose. You really should monitor the low and high pressure sides to get a good feeling for if there are other issues with the system besides a low charge.

You also should make sure that the radiator cooling fan and clutch is working properly and that the condensor is not restricted.

Does the compressor clutch cycle at an idle?

-Rod

In brief, if the engine wasn't running, you can easily have a "low" side pressure of 100 psi on a properly charged system. Once the compressor starts running, the pressure will than drop and will cycle between about 25 psi and 45 psi. The compressor comes on only at pressures at or above 45 psi and will shutoff at 25 psi, strictly speaking only about it being controlled by the clutch cycling switch. Strange as it may sound, but.... the low side pressure DECREASES as the the compressure runs till it hits 25 psi, turns off and waits till the pressure rises to 45 psi to come on again. Yes, it's kind of screwy. :screwy The only time the compressor will be turned off outside of those conditions, is when the compressor discharge/high side pressure gets too high.

For me, the best clue that you gave that indicates you have a partially discharged system, is the statement you made about the engine rpm and rate of travel.

RBNR I keep falling asleep!?!

It worries me that in the US you can so easily buy R134a and attempt to recharge your own AC.
How much is needlessly wasted and destroying the ozone layer because of this?

The compressor comes on only at pressures at or above 45 psi and will shutoff at 25 psi, strictly speaking only about it being controlled by the clutch cycling switch.

All above is true if you have a 4.3L 6 cylinder engine-- but I'm guessing your Sonoma is a 4 cylinder- which means you have a variable stroke V-7 compressor - that DOES NOT CYCLE by design-- so low charge is not recognizable by the continuous cycling that is seen on a CCOT system...

Test pressures are necessary for further diagnosis..l

It worries me that in the US you can so easily buy R134a and attempt to recharge your own AC.
How much is needlessly wasted and destroying the ozone layer because of this?
The last few cans of do-it-yourself refrigerant, that I have seen, advertised themselves as being chlorofluorocarbon-free (and non explosive!!) so its possible it was not R134a