1991 Buick Riviera A/C Comp. Clutch not engaging

I have voltage to the Coil of the relay that controls the Clutch on the compressor. According to the ONLY electrical print I have, the ground/common side of the relay goes to the PCM. When jumping the two terminals that goto the CONTACTS of the relay, the clutch engages. When jumping the common side of the COIL to the chassis, the clutch engages.

I am led to believe that some other CONDITION must be met for the pcm to complete the control circuit on the common side of the coil for this to work. I do notice the Print shows an A/C low side temperature sensor and a high side as well, connected to the bcm. Maybe there is an issue there?? Any help would be greatly appreciated.


The compressor clutch is indeed controlled by both the Low and High Side Temperature sensors. Essentially these sensors, in reality, convert pressure to
electrical resistance. If you back probe the sensors the voltage should read between 2.4 and 3.5 volts. Anything outside this range is indicative of a defective
sensor; assuming of course that the +5v reference voltage exists.

Also it is possible to via diagnostic mode to actually view temperature the data “real time”. Upon entering diagnostic mode, select BCM -> BCM Data -> Scroll using the fan speed control to 28. If the reading is below 10 (indicating 10 degrees Celsius) then the clutch will not engage. Assuming that the system pressure is good, this is a pretty good indication, again, that the sensor is bad.

Hope this helps…:slight_smile:

I have done exactly as you say with the Tech2 but with the Tech1? (which is what you use on a 91) a BCM on a 91? I don’t have the book in front of me but if you say so.

Put a set of gauges on it and check the Freon pressure…After 18 years, it tends to leak out…Low pressure will disable the compressor to prevent damage.

Agreed completely. Odds are this issue is not as complicated as you may think.

Well, I DO have a book and YES it does have a BCM. After FURTHER digging, I found that the low pressure temperature sensor was plugged in backwards as was the high pressure side. And by the way, I had already checked the pressure and filled it to spec. Thanks Weigel

I been able to jump these switches they are either open or closed,I am having a hard time with the plugged in backwards idea.

Do you know what a transducer is?


A vacuum valve used to transfer the electrical signal from the amplifier into a vacuum signal. This signal regulates the power servo unit in automatic tempeture control units.

I did read pressure switch when you had written temperature switch. The temp switches are located on the line between the condensor and the evaporator,the high side temp switch is on the condensor side of the orfice tube and they are BCM inputs. I don’t see a clutch inhibit feature on the temp switches like I see on the pressure switches. The manual clearly spells out the clutch inhibit feature of the pressure switches,and lead to a idea that the temp switches are for automatic temp, control,is this a feature of your 91 Buick? It would probably safe to conclude a temp switch permently fixed in a inhibit range would keep the clutch from engaging.

I have done much more work with the low pressure switch,I really can’t recall a temp switch replacement.

Actually, it is a Friends car, but we work together at a tire plant and I am an electical troubleshooter there. The sensors that I was refering to are called out in the manual as high/low side temperature sensors. To the best of my ability, I have assertained that they are transducers. They are supplied with a 5 VDC signal, and depending on the temperature/pressure, the send a feedback voltage to the BCM. So, the 5 VDC is your source/reference and the output is your feedback. A transducer has to be WIRED correctly before you ever get anything out of it. The BCM communicates with the PCM as well. Then only thing that i know for sure is, that the low side temperature can not be TOO low and the high side temperature can not be too HIGH. If so, the clutch does not engage. Prior to switching the plug around on the sensor, the diagnostics on the car showed that the sensor was reading -13 degrees celsius.

I was giving a defination of transducer as used in the AC sysyem,transducer is also used in many EGR systems and cruise control systems. My electrical work is very much automotive (with a HF component) so I am not sure how the term gets tossed around in other fields. But I see no vacuum influence at the switches themselves,how about the signal out of the BCM into a transducer regulating vacuum for mode and temp control?

I’d still like to hear the defintition of “checking the pressure and filled it to spec”.

So what are the high and low side pressures and at what RPM?

Other questions might be the story behind filling the system; how low was it, was it empty, if empty was the system evacuated, etc.?

This was originally an R12 car. Has it been converted to 134 or what?