95 with a 3.4- Does the a/c compressor have a compressor pressure sensor located on the compressor itself? Cant see one from the engine bay looking down. Compressor will only engage when jumped at the relay. Low side pressure is fine and blows cold when jumped. Also will engage if I probe the red wire on the 3 wire connector to the high side pressure sensor (?), but then will cut off again as soon as I drive the car and not turn back on unless jumped. I don't think this car has a low pressure switch (?) but read somewhere it may have a sensor on the compressor itself. Thanks

 

The ac clutch relay is controlled by the PCM. Have you checked to see if any DTC have been set? I've attached a pdf from the 95 service manual that may help.

 

Thanks. No haven't checked for any codes. Tried a reset and the compressor still does not turn on (clutch engage) unless I jump the relay or probe the red wire at the high side pressure sensor connector. This all started when I added 1oz of oil and some refridgerant when the system was a little low. The compressor began to short cycle and hasn't been right since. Now it wont turn on at all without jumping. I wanted to test the compressor pressure sensor if it has one as well.

 

The only pressure switch I could find in the schematics was the one in the evaporator line. The diagnostic chart in the pdf above goes through how to test the circuits for the pressure switch, PCM, dash switch and clutch relay.

 

The high side pressure sensor is sometimes called a low pressure switch/sensor. As far as I know, you are looking at the only a/c pressure sensor on the engine.

How were you using the meter leads, measuring voltage or resistance? Was the negative meter lead at engine or chassis ground?

The fact that putting a meter lead on the red wire at the sensor causes the relay to come on seems to indicate there is too much pressure or there is a problem with the connector causing the pcm to think there is too much pressure. If you can tell me the voltage there, I can tell you the indicated pressure. Or you can calculate it yourself. Calculate the raw data (N) this way: N = volts x (255/5). Then calculate the pressure in psi by: P = (N x 1.875) - 18.75

 

I was able to verify there is no 'pressure in compressor sensor' on the compressor. I had to get up under the car to see the back of the compressor and nothing there. No harness, sensor etc. The 3.8 or the 5.7 may have one and perhaps that's why a part (compressor sensor) is listed. So yes looks like the only sensor is on the high side line. A friend of mine was doing some diagnostics to test the high side sensor and when he probed the red wire with ground to chassis, the compressor clutch engaged and the compressor ran. He said the sensor was working but I don't know what the voltage was. Obviously there may be some issue with that sensor or connector. The pressure in the system is normal, 35 and drops a bit with throttle. For now I'll have to keep jumping the relay for a/c and will update when it's figured out. Still need to check for any codes as well. This car has OBD1 with a 2 connector so I cant just run to the zone etc and don't have the scanner myself. Really appreciate the information!

 

35psi when the compressor is on or when it's off? 35 psi is too low if it's off, ok is it's on.

 

With car running compressor on. Drops just a bit with throttle.

 

I suspect that the high-side pressure sensor or it's connector is bad. You probably have a DTC 66, but that will not turn on the SES lamp