A/C Compressor wont kick on unless jumped in fuse box using a paper clip. Compressor does not kick on with relay in place. Freon levels are between 35-40psi on low and 190-200psi on high side.

Pull a relay out and use paper clip to jump. Usually cross corner. Watch compressor clutch and see if engages when key is turn ahead. Don't start car. If it does good. Now take alligator clips and check relay. Clip each wire to negative and positive battery post. Then clip negative to negative relay pin. And touch positive to correct pin. If it doesn't click do it another relay so you know your doing it right. If determined relay is good. And your compressor clutch did engage when testing it. You are low on freon.

Had a car with same problem. Was temperature sensor. Do you know how to check relays? And do you know how to use paper clip and jump relay switch in fuse box? You need to jump relay to make sure air compressor is working to fill with freon.

I need year--make--model?
You removed the fuse and used paper clip in the fuse socket? I'm just trying to understand what you did with paper clip?

The low pressure switch is just a sense input to the PCM - powertrain control module > Grantit , it is needed for the PCM to know the pressure in the system . But the PCM must energize the compressor clutch relay for the compressor to engage . Finding the compressor clutch relay in the under hood fuse / relay box an testing there would be the best place to start . Pull the relay out of the fuse box , look on the bottom ! You should see four set's of number's ,30 & 87 plus 85 & 86 . 30 & 86 should have B+ voltage . Use a multi-meter to test . 85 goes to pin # 1 connector 3 at the PCM . The PCM supply's a ground here to energize the relay if condition's are right . You could jump pin's 30 & 87 an if the compressor clutch coil is good ,the compressor clutch will engage . Video's on youtube showing how to do these test .

Could be a fuse or a relay. You may need a wiring diagram to track the power to the compressor clutch.

low pressure switch is bad unplug jump across wires wit paper clip or such and see if it kicks in just test cant run that way replace switch /check relays

well first did you put enough freon? usually takes about 1 1/2 lbs to 2 of freon. now was it completly low. if it was you have to jump compresor the plug is on freon canister its that bottle looking thing with 2 lines coming out of it. disconect plug and jump with paper clip with car running put freon while holding paper clip in the jumper. and you should be alright.

The power to run the compressor clutch coil goes from the battery, to the AC Clutch Relay, somewere in the engine compartment fuse box, then it goes to the positive side of the AC compressor clutch coil; goes thru the coil, activating the compressor, and then goes to complete the circuit through another switch (high pressure protection switch) in the back of the AC compressor and finally reaching negative or Ground.

Disconnect the AC compressor clutch connector and using a 12 V bulb with 2 wires (test lamp) check the following:

1.- connect 1 lamp wire to battery neg; with the other check both pins at connector with the AC in the RUN (AC and ignition switch on ) you should have power in one side.

2.- connect 1 lamp wire to battery positive and with the other check both pins at connector, lamp should light getting the ground from the clutch connector thru the high pressure compressor switch to ground.

On the other side what makes the relay close and let the power flow to the clutch works with the low pressure switch, (if the freon pressure is too low it opens and kills the ground to the relay coil and that kills the compressor), that low pressure switch is in the low pressure side of the freon tubing, the fat pipe that comes from inside the firewall into the accumulator/dryer, a big aluminum cylinder sitting on the right side of the engine compartment in GM cars.

To know if the low press switch is the cause you can unplug it and connect both legs with a paper clip for a minute or two, no more, you don't want to damage the compressor. anyway if jumping the switch makes the compressor come alive and kick in, you either have low freon pressure or a bad switch; it's time for the pressure gauges and check how the compressor is pumping and that is better left to a pro.

By the way you said that your system is a R-12 system, have you considered switching to R-134a the new "ozone friendly" freon? to do that you need to evacuate the system with a recycling machine and pull vacuum in your AC, if it holds the vacuum for a while, then you can be sure that there are not leaks and then dump 1.5 or 2 pound (check the labels in the engine compartment) of R-134a into your system. it will be a good idea to put some fluorescent leak detecting dye in the system to check for leaks, just in case..

check the low side switch located on the ac receiver drier for 12 volts if so jump it with a paper clip if the compressor comes on then the relay is good if not the relay is in the fuse junction box under the hood.Most likely the pressure sensor.