Questions Re: PCM and AC

I have a 2000 GP with a V6 3800 Series II.

Recently had to replace my A/C compressor after several years out of service (moving to the high desert and open windows no longer qualify as A/C). Had a qualified mechanic/friend do the work.

Discovered that there appears to be a short in the PCM that provides ground to the fans associated with the A/C.

Note: everything else associated with PCM appears to be working fine.

Few critical questions:

Is it possible that the problem with ground to fans is a software issue in the PCM and can be solved by reflashing?

If not, can I make a straight swap with an equivalent PCM from salvage?

Would there still be reprogamming necessary to the newly installed, salvaged PCM?

If installed salvaged PCM, will car start and run enough to get me to a reprogrammer?

Wow, would any help on this be Greatly appreciated! I'm supposed to start moving next week, and dash is off and still-inop A/C is mocking my move to the desert!

How was it determined to be a PCM problem? What are your symptoms?

The PCM uses the engine temperature control sensor (ECT) to determine when to turn the cooling fans on.

What happens when the ECT is disconnected? With engine cool disconnect it. Start it up. Report back on what indication you get.

I thought the A/C low pressure switch controlled at least one of the fans...

I thought the A/C low pressure switch controlled at least one of the fans...

How do you figure? Please explain.

Well, I can't find a diagram, but on most systems, the PCM looks to the high or low pressure switch to determine whether to activate/deactivate the fan. Temperature and pressure are directly related in automotive A/C systems. But I'm not 100% sure without a schematic..

Thanks for the replies!

Corning, I think you have the idea -- as best I understand it.

I'm trying to convey what I was told by my mechanic friend, and the tech who at the shop where we purged and filled the AC system.

Was able to get the AC to work; felt the first cool air of hope. But the fans weren't coming on automatically. Could only get them to come on by providing a manual ground to them.

The ground was supposed to come from a signal from the PCM. When my friend traced the wiring back from the fan to the PCM, found no fault. The PCM simply wasn't responding and closing the circuit when it was supposed to. Hence, the fans never activated.

Is it possible that there could be a a single, isolated PHYSICAL fault within the PCM with just this circuit, while every other circuit managed by the PCM is working? Or could this be a software problem -- one that could be fixed by simply reprogamming?

At the moment, we're fixing on the idea that we need to replace the PCM.

bnaylor,

my above post answers your questions in part. Haven't had a chance to gather the info you requested, but will try to tomorrow.

Sounds like you're suggesting that the problem could lie with the engine temperature control sensor (ECT). That would make sense.

I apologize to all for being vague. I'm very limited in my auto skills. And I'm a teacher on teacher's pay trying to avoid the very large costs associated with just taking the car to a shop.

Well, I can't find a diagram, but on most systems, the PCM looks to the high or low pressure switch to determine whether to activate/deactivate the fan. Temperature and pressure are directly related in automotive A/C systems. But I'm not 100% sure without a schematic..

I have the GM service manuals for most '97 and up Grand Prixs. And the wiring diagrams do not support your theory.

But I see what you are saying. I agree the fans will turn on at some point with the A/C on and if A/C is operational. However, now what if the A/C is not physically turned on or not functional like low on refrigerant charge? Does that preclude my fans from ever working? I don't need the A/C on to get both my my cooling fans to operate. That will be based on solely on engine operating temperature. Plus the fans operate in pairs. They do not work one at a time. They work either in low or high speed. I've never had just one fan operating in my GTP or my two Regals.

The test I suggested is done with A/C off. With ECT disconnected both fans will turn on in high and run continuously. That will rule out output wiring from the PCM to the three cooling fans relays and both of the cooling fans.

According to the GM service manual wiring diagrams the PCM has output drivers that controls the operation of both fans via the cooling fan relays in the engine compartment fuse box. If the high/low pressure switch was flaky or the system low on charge then the compressor would not be working due to clutch not engaging.

I think it is a good idea to wait until the original poster weigh-in and clarifies or explains what his problem is.


Edit update:

bnaylor,

my above post answers your questions in part. Haven't had a chance to gather the info you requested, but will try to tomorrow.

Sounds like you're suggesting that the problem could lie with the engine temperature control sensor (ECT). That would make sense.

I apologize to all for being vague. I'm very limited in my auto skills. And I'm a teacher on teacher's pay trying to avoid the very large costs associated with just taking the car to a shop.

I would check into the ECT. If you have a digital multimeter you can check resistance to see if it is calibrated correctly. The ECT does not cost that much but try the easy test I suggested first. Good luck!

Thankyou, kind sir! Will do. And will let you all know.

Here is a link to the workings of the cooling fan system, with a schematic. It is for a 2001 Chevy Venture, but after comparing it to the schematic I have for the 2000 Grand Prix, it is almost identical (both GM, probably the same basic PCM).

Engine Cooling System (http://www.my-chevy-venture.com/engine-cooling-system.html)

Hope this helps some.

Take Care,

Good info GregA.

Here is the wiring diagram to show the ECT input to the PCM module. If the poster needs the calibration values of the ECT just let me know and I can send it via PM. I cannot post GM copyrighted material.

http://i5.photobucket.com/albums/y186/lizzywiz/ect.gif

Plus the fans operate in pairs. They do not work one at a time. They work either in low or high speed. I've never had just one fan operating in my GTP or my two Regals.
!
Not so on mine. I have noticed at times only one fan runs and later the second one cuts on. Maybe the GT's are different.

Good morning,

Just getting started on the day, but have digested all that has been said, and have a question.

First, I can see how you all are pointing away from the PCM as the culprit, and from what you're saying, that makes sense.

However, I've never had a problem with engine overheating -- just the A/C problem. So -- before testing/observing ECT and fan operation -- it seems that the sensor that may not be functioning properly would be related to the AC pressure function.

So, referring back to the desciption of the engine cooling system that GregA provided, the problem would be A/C refrigerant pressure.

So, the question: assuming no problem with PCM, what sensor should be functioning as the A/C refrigerant pressure rises?

Doesn't that seem to be the place to look in my situation?

Not so on mine. I have noticed at times only one fan runs and later the second one cuts on. Maybe the GT's are different.

What month was your GT manufactured in '97? GTP and GT/SE fan turn on is the same. There may be a few seconds of delay between fan #1 and #2 turning on based on engine operating conditions but I don't see it or get that. Both fans draw a lot of current at turn on so I can see a possible delay but not just one fan operating all by itself. However, I get both fans in the following vehicles tested:

'97. 5 GTP with A/C in MAX/Hi letting engine idle to operating temperature I saw both fans turn on within a split second of each other.

'99 Regals LS (L36) just like a GT the same indication as above.

Maybe think you may have a problem or missed something? Methodology?

If you'd like I can test my '01 Regal GS since it is above the 2000 model year but I am quite sure the same results will be obtained.

Good morning,

Just getting started on the day, but have digested all that has been said, and have a question.

First, I can see how you all are pointing away from the PCM as the culprit, and from what you're saying, that makes sense.

However, I've never had a problem with engine overheating -- just the A/C problem. So -- before testing/observing ECT and fan operation -- it seems that the sensor that may not be functioning properly would be related to the AC pressure function.

So, referring back to the desciption of the engine cooling system that GregA provided, the problem would be A/C refrigerant pressure.

So, the question: assuming no problem with PCM, what sensor should be functioning as the A/C refrigerant pressure rises?

Doesn't that seem to be the place to look in my situation?

To start what exactly is your problem. What is the complaint?

Is the A/C cooling at all?

Right now your situation is not clear or convoluted.

BTW - If you have an A/C problem get it properly diagnosed and tested. If you get any A/C problem resolved your cooling fan issue that you raised may be moot.

The hi/lo pressure switch is located by the accumulator. The PCM provides a reference voltage of a little less than 5 volts to it. The sensor in turn provides a varying voltage back to the PCM for processing. If the system charge is good as confirmed with proper gauges at the low and high pressure ports and in range of the low and high threshold then it should be OK.

If the pressure is too low or too high then the compressor will not engage.

And I would not rule out a PCM problem.

Okay . . .

I just went back to the tech and asked for clarification of the situation. Here it is:

The A/C compressor is not turning on.

He was able to get it to turn on by providing a ground in the fuse block in the engine compartment.

He said the problem could be one of four things:

1) the PCM is bad
2) there is a short in a wire
3) the sensor in the accumulator is bad
4) the on/off switch for the A/C system is not working (even though the light comes on when switch depressed)

My mechanic friend followed the wire from the fuse block to the PCM and found no short. Hence the diagnosis that the PCM is the culprit.

Have to admit that part of the problem is that my mechanic friend is notoriously hard to nail down. When tools are in-hand and the hood is open, he's great. But since early in the week, have had great difficulty in getting a solid troubleshooting session/discussion to wrap this all up. Hence my turning to you kind folks.

So, while my friend has turned his eyes on the PCM, I am compelled to make sure that all options are pursued. I feel confident that Possibility # 2 is not in play, since he Did test that out.

So . . .

While I new stand ready to pursue Possibilities #3 & #4 (when I can lasso my friend), I'm back to the original thrust of my first email.

If the problem utlimately does turn out to be the PCM . . .

Is it possible that the problem with A/C compressor is a software issue in the PCM and can be solved by reprogramming?

If not, can I make a straight swap with an equivalent PCM from salvage?

Would there still be reprogamming necessary to the newly installed, salvaged PCM?

If installed salvaged PCM, will car start and run enough to get me to a reprogrammer?

*****

I'm sorry for the mention of the fans, which sent you all off doing extraordinary work coming to my assistance.

No problem nothing out of the norm. We're here to help and there are times threads have to work out and get loads of information and a bunch of people weighing-in until we get to the meat of the situation. But I believe the issue is narrowed down.

If you can illiminate or prove the pressure switch is not the cause of your problem then the PCM is a possibility since it is the brain of the operation.

It could be a software issue and just needs re-flash but that will need to be done by a GM dealer ($$$) unless you have PCM programming capability like a Powertuner, etc.

Yes, you can make a straight swap with another PCM like from a salvage yard or off of EBay but make sure it is for your year and model Grand Prix. There are programming variances between the model years and models such as GT or GTP.

Sometimes when you swap out a PCM a crank variation learn is required which also must be done at a GM Dealer on a Tech II machine. But I have swapped out PCMs and the case learn was not needed. I was able to drive the car. Also, the PCM must be programmed with your VIN number and some other data. Otherwise, what happens is a Tech II machine will not allow the proper diagnostics screen to be presented whenever you take the car in for future servicing. For example it may default to diagnostic for a 1999 model, etc.

Like any problem it is good to pursue all options because in the end it is your money and time.

A follow-up to the A/C saga.

All is well and all is COLD!! All ready for the high desert.

Thanks to all who jumped in to help, and particularly, to bnaylor.

Finally broke down and took it into a shop with the necessary computer diagnostics to track down the problems.

Turned out there were two elements (and neither of them was the PCM -- it was fine.)

Had a blown fuse to the A/C clutch relay circuit, and a faulty wire to the refrigerant sensor.

On a side note, also found something totally unexpected: a rat's nest and droppings up under the dash!! Have no idea how that happened (car never sat for long at any time.) Did find out, though, that (at least here in the San Diego area) rats apparently often times bring snails up onto hot engines to cook them before eating. A rat's approach to escargot!

Anyway, my friend was able to handle the mechanical end of replacing the compressor. Nowadays, though, apparently can't replace the role of computer diagnostics.

Thanks again!!