AC Compressor kicking off and on
 

I checked the freon today and added a can. Reading the ac pressure gauge, I see about 40 which is normal and around 100 on the pressure "high" side. The car will cool if I'm running about 50 mph or higher. At lower speeds the ac will barely cool.
After turning off the car, I would expect the high and low side to equalize to around 50 but it doesn't. I still stays at 40 on the low side and around 100 on the high side.
Any ideas a too what component may be failing to cause this?
Could it be the onboard computer fouling up?
Any ideas welcome. Thanks to any one for help.

Is there a pressure switch on this thing that could be responsible?

I FIXED IT!!!!
So I called the local repair shop and the guy asked if I was using Freon from AutoZone. I said yes. He then said the problem was that I should have pulled a vacuum on the system first and he said he would do so for $100. He also said the Freon from AutoZone was not good either. I immediately knew he was full of s#@!.
I've topped of ac systems a bazillion times without issue. The only time you need to pull a vacuum on the system is if the system had been opened which mine has not. The Freon from AZ is the same as any other. This guy would have taken my money and would still not have fixed the problem.
I tried disconnecting the wire to the compressor and ran a test wire from the compressor to the battery. The compressor clutch engaged and stayed that way while I ran the car. The ac was running really cold as it should. While the car was running, I disconnected the pressure switch to get some voltage readings to see if the switch was turning off and on. It looked like it was operating normally. When unplugging the pressure switch, the electric cooling fan turned off as it must obviously be part of the pressure switch circuit. I let the car run for about 10 minutes when the pressure relief valve on the compressor blew and a slight amount of oil spewed out. No more than a thimble full I would estimate. This was due to overheating from the fan being off. I then turned off the car and hooked all wiring back as it was before this all started. I then started the car and it started operating normally. No more compressor kicking in and out and it was cooling very good. If I accelerate the engine the compressor will kick in as it supposed to.
I have a theory, not sure if it correct or not however, here goes. I’m old school and have a 67 GTO. The compressor is designed in such a way that the compressor oil will stay in a concaved are of the compressor and will not enter the Freon lines. On newer cars, the compressor shape has changed from the old days and now a certain amount of oil will enter the line thus occupy some of the space that would normally be all Freon. When the valve popped of and spewed out the oil, the system now has more line room for Freon to flow freely. Now when I turn off the engine, the Freon equalizes evenly between the high and low side as it should.
I could be way off base here however it all works now so I can’t complain.

I guess I qualify for old school knowledge. The old compressors had a sump to store oil to keep the compressor lubricated. New, R134a systems, rely on the oil moving thru the system. The pressure relief valve popped due to high pressure, not overheating. If the cooling fan isn't running, the pressure cannot be reduced as it travels thru the condenser by removing the heat from the Freon. There is a possibility that the AZ charge up kit did have oil in it and there was too much oil in the system. But your high side reading of 100psi is lower than I would have expected to see. It is not uncommon for the system to not quickly stabilize after shut down. Buy messing with the pressure switch, you may have fixed a intermittent problem there.

The thing to question is, why was it low? A/C is a closed, sealed system that should not require service unless you have a leak. Look around for an oil stain on the A/C components. Find oil accumulating on the outside, you probably found your leak.

The location of the pressure relief valve on the compressor
 

Where is it?

I started having the same issue after being serviced at a body shop. The high and low sides equate very slowly after turning off A/C and the high side gets really hot. (I couldn't remove a coupler for a while after I turned off A/C.)

I suspect the mechanic added too much refrigerant (because pressures are rather high) or too much oil. I also want to release some oil/refrigerant without recovering refrigerant, if possible.

Initially, I also suspected the pressure switch. I couldn't test it properly, but even when I bypassed the high/low switch with a shorting wire (while watching pressure build up by a gauge), the clutch disengaged. Something else is turning it off. I went back to a service manual and found three more suspects: the engine coolant temperature sensor, the evaporator temperature sensor, and the thermal protector on the compressor. Now my primary suspect is the thermal protector. It makes sense that the compressor gets really hot and that it turns off without any apparent pattern with pressure. I think I can diagnose this possibility by monitoring the voltage to the compressor. (The thermal protector is after the A/C relay.) But even if overheating of the compressor is the problem, I still need to figure out why the compressor overheats.

Thanks for your help.