A/c prob 01 monte carlo SS

Mechanically, a/c works fine The problem is that the contactor won’t engage unless it is jumped.

Problem originally began when a/c would stop blowing cold air for a few minutes and then would kick on again and then repeat the process. After about a week, it wouldn’t come on at all. I wasn’t too concerned as I thought it was a low charge or a pressure switch.

Uh, uh.

Everything has been checked. Everything. I even swapped out the HVAC dash controller to no avail. Also checked for bad ground but didn’t find one.

I’m very familiar with a/c mechanically. Dealer tells me that juice is getting to the BCM but not to the PCM and suggests that I replace the BCM and if that doesn’t work replace the PCM. (And yes, he was wearing a bandanna covering half of his face at the time.I paid $100 for that “diagnosis”. I will skip the suggestion that I gave HIM.)

A check with my mechanic and he thinks there is a communication loss between the two computers which conforms with the dealer diagnosis.

It is true that the PCM ordinarily sends juice to the contactor from the PCM based on other input such as sensors, acceleration, trannie input and so on. So, it IS possible that the PCM is the culprit but I am hesitant to pay good money for a computer that might not solve the problem.(I noticed that the a/c radiator fan isn’t working but that make sense as it works with the a/c.)

The only obvious part I haven’t checked is the coolant sensor but am not getting any dash warning or check light.

It’s been three months without a/c and that ain’t good in Arizona.

Any suggestions?


It sounds like a problem with the PCM. This is what controls the operation of the compressor clutch relay.


Was it possible to “command” the AC compressor to engage using the Tech2? For $100.00 you should know this. Elaborate a little on what must be jumped to make things work,if just for my benefit.

Where is AZ? I am in Tucson.

Serial data from BCM must be present

All of this can be checked with the Tech2.

True, Tester. But,it could also be some faulty sensor that makes the PCM THINK that there is a problem. What confuses me is that there is juice to the BCM but according to both analyses it is either not getting out of the BCM or not getting into and through the PCM. Again, the dealer says to replace the BCM (cheaper) and if that isn’t the problem, replace the PCM. It confounds me why the problem can’t be further isolated between one or the other.

Wish I could read a complicated schematic to see whether the various sensors and which or all of them go through the BCM or directly to the PCM.

After much searching, I found one problem similar to mine where the mechanic claimed that the PCM was the culprit. But, in that case the guy permanently ran a switched circuit directly to the clutch relay. Not something I would even consider. And, doing that certainly didn’t prove that the PCM was actually the problem.

Need to get to an appointment, Oldschool. Will touch bases later today.

I think here is a schmatic of your ECM its the on that is marked Fig. PCM References - 1 of 2© (2001): http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?chapterTitle=Engine+Control+Systems++2001&partName=Wiring+Diagrams&pageId=0900c15280244228&partId=0900c15280243879 I think I would check for voltage coming out of the ECM for the A/C Comppressor Clutch Relay Control pin on the ECM

Glendale area, Oldschool. Moved down from Wisconsin 26 years ago.

I would think that the dealer was using the Tech2. My mechanic was using an expensive scanner. Neither could isolate the problem; both said that BCM was getting juice but it either wasn’t getting out of the BCM or into the PCM. One or the other. The scanner showed lack of communication between the two.

I, like many others, am tired of replacing parts and finding out that they weren’t needed.

Example one of many. A couple years ago I was heading to Reno and just outside Vegas the gas guage stopped working. When I returned to Phoenix, two mechanics thought it was the sending unit and one thought it could be in the computer. I pumped in a tank of Dextron II and that problem cleared up within thirty miles of driving the car.

Thanks for that site, Skypilot. There is no juice from the PCM to the clutch relay. I can jump the clutch and everything works. Perhaps that schematic will show what other sensors etc are involved in signalling the PCM.

Thought about flushing and re-flashing the PCM but for the same amount of money ($100) I can always end up ordering a tuned and tweaked up PCM from ZZ motors aleady set to my VIN. (Core charge is additional $75). If it turns out the PCM isn’t the problem, that tweaked PCM is worth the money and at least I’ll get something in return for the investment. It’s surprising what they do to the PCM before sending it out included in the price.

Any further info is appreciated. As to those who have already suggested possible remedies, thanks much.

We are back to this"jumping" thing that you have not fully explained. If you are jumping the low pressure switch I understand,but jumping the clutch? are you really saying you are providing power to the clutch from some other source? There is a difference between jumping and providing another source of voltage.

I just thought maybe you are really powering up the relay (or just jumping it) and you are calling this “jumping the clutch”

Sorry. I knew I had forgotten to answer one of your questions, oldschool.

Guess “jumped” to me means something different than it does to you.

The first thing that I did when the problem originally arose and I found that it wasn’t a matter of a low charge was to apply juice directly to the clutch. It engaged and the compressor pumped cold air.

Then I began to go through the electrical circuit and even replaced the pressure switch and then the HVAC dash control.

Mechanically, nothing is wrong with the a/c.

Could be in the BCM; could be in the PCM. Might be a sensor that won’t allow the PCM to close the clutch relay. Might be a gremlin or a dead mouse under my dash or maybe my ex-wife put a curse on me.

I’ve already paid “experts” almost $200 to tell me what the problem is and all they can say is we can replace this or that and if that doesn’t work then replace this or that.

This or that would be fine with me if this or that didn’t solve the problem and they didn’t charge me for this or that unless it did.

Unlike lawyers, doctors, politicians and mechanics, in my business if we don’t deliver we don’t get paid.

If I sound ticked off, oldschool, that’s because I am. Living in Arizona as you do, you already know that living without a/c is bad enough but living WITH a/c that doesn’t work is almost as bad.

I believe you have already answered the question if the mechanic can talk to the BCM with the Tech2,and the answer is, he can’t,am I correct? You must establish communication with the BCM,have you priced one? I think you can “google” around and find rebuilt BCM"S. I never replaced one for this though,always security system or keyless entry failures.

You know by looking at the schematic there are some splices in the circuit.

Fortunately, the cost of those computers have really dropped. As an example, I could pick up a BCM for around $50 and a PCM for $99. The dealer price is $236 for the former and $983 for the latter.

In regard to your question, indication is that there is no communication between the two. But, they can’t tell me whether it is a problem with the BCM or the PCM.

It looks as though I will end up buying a PCM and if that isn’t the problem, go for the BCM. The only reason that I have been stalling is that I’m not fully convinced that it isn’t due to a faulty sensor shutting down the PCM circuit to the clutch relay.

If I solve the problem, I’ll post the result on the board in case anyone else experiences something similar.

Thanks to all who suggested possible remedies.


The mechanic CAN talk to the BCM but no further.

Did the pcm fixed your problem?

Understandably it’s difficult to notice the way out of date signature as it is very dim.

The post is 10 years old and quite likely the OP has gotten rid of that car a long time ago unless he’s like me and keeps them forever.