why wont my ac pump shut off??

You hit the nail on the head. The pressure switch should cycle the AC compressor off at the same time it cycles the fan on. If the pressure switch isn't the cause it could possibly be a stuck control relay for the compressor.

 

I would start at the relay, the H/P switch is probably working if the fan is cycling.

 

Mine did the same thing a few years back, fan was on-off-on-off-on most off the time and the ac pump same same, so I stopped at the KW shop in York,Ne and the guys in the night parts asked me what color my high press switch was, I had to go look. They handed me the right one and I happened to have the 134 hose to put in 4 cans of juice, problem solved. Make sure to put on a new o-ring with the switch.
J-J

 

Was that the one close to a firewall or next to a condenser?

 

The fan control should cycle the fan on around 250 psi. The compressor cutout switch around 400 psi. They only work in sync when a trinary switch is used to control both. Have you monitored your operating pressures with guages at high idle? Your compressor will cycle out from the low side switch unless the system is overcharged or the low side switch is at fault.

 

Either the Low Pressure switch is stuck closed or the compressor relay is burnt with the contacts stuck together in the closed position making for a continuous signal to the clutch on the compressor. An over charged system will also keep it on for long periods of time also, but you haven't mentioned that you've added refrigerant recently.

 

Mine was the switch out by the condenser, but the parts guy said that all of them were color coded, I had noticed they were different colors years ago but guess it never dawned on me that it actually meant something. If you get the right parts guy they will be very helpful, I have just a couple guys like that around and they have saved me a ton of money. Low freon level raises cain with stuff as well but you will know that as it will get harder and harder to keep it cool inside. I try to add enough freon till my spud thermometer reads a steady 40-42deg approx. To much and it will quit working also, not suppose to but if I get to much I just push the tire valve and let out some keep your face out of the way, bad for the environment right? Hope some of that helps, not an expert though.
J-J