I need to know how to jump the ac low pressure to get my ac compressor to come on.

You really should have Manifold gauge set for high and low pressures before messing with AC system. If system was empty, It still needs refrigerant to fill the system. There is a sticker on your hood or firewall or ac evaporator housing showing AC Capacity. Another thing when AC system is being charged, you usually have to jump the low pressure sensor in order to get it in. Please don't attempt this with out gauges.

Either connection is bad to low pressure sensor or pressure sensor is faulty.

You could have several problems. If your pressure readings (freon) are low it will not kick on the clutch for the compressor. Do you happen to know what your pressure readings are when you do jump it or what they are (low or high pressure side) when it isn't engauged? Also you could be dealing with a relay or a sensor that detects your pressure readings. Really knowing your pressure readings would be a start before I can begin to help.

The low side pressure switch in the AC low pressure line signals the PCM (computer control) to close the AC compressor relay and raise the idle speed. If the system is low on refrigerant charge the relay will not allow the system to engage the compressor, this is because the oil that lubricates all the works is suspended in the AC refrigerant, if that is low then the system gets no lubrication and will quickly seize up..

This could be a few things. The freon could be low, one of the switches (high or low pressure) could be bad, or you could have a bad compressor clutch. You can test the clutch by checking for battery voltage and ground at the two pin connector with the A/C on if you have both power and ground, then the clutch is bad. You really should start by hooking up pressure gauges to the A/C ports to check the pressures, and these vary depending on ambient temperature and whether the A/C compressor is engaged or not. I am willing to bet that the freon is low though and may be just as simple as recharging the system.
Hope this helps. Good luck!

Hi, the way to get the compressor to come on is to short the connector to the pressure sensor on the return line. The sensor is along the same line you were trying to charge on. Just remove the connector and use a large paper clip to connect the terminals inside the connector. Please let me know if you have more questions, and thanks for using FixYa.

it most likely has a leak from an improperly installed o ring etc.

The AC system has a low and high pressure sensor. If your refrigerant is too low (most likely) it won't run to prevent ruining the compressor. If he system is plugged the high pressure sensor prevents ruining the compressor or blowing up lines.

If the refrigerant is low you have a leak, maybe small, maybe large, so simply recharging the system is a temporary fix lasting anywhere from minutes to years. Need to find leaks and fix them eventually. After repair the system needs to be evacuated and recharged. Best left to qualified personnel with the proper tools.

When you jump the switch all you are doing is forcing the compressor to kick in and work. It could be low on freon which will also keep it from kicking on which in that case you need to have it charged so does not mean your "sensor" (switch) is bad or that your compressor is bad.

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..