PCM, dealer says it's "broke" but everything works fine.

[Integraoligist]

So I had the Envoy in the shop getting the HUBs changed under warranty (Vehicle One Aftermarket warranty) and I told them to fix the Cruise Control too, because it never works.

Well they change the Cruise Control switch, that didnt do anything, so they swapped the Powertrain Control Module with another Envoy and it works fine.

So they said the there is something "broke" in the PCM and I need a new one in order for the cruise to start working.... to the tune of $900

Ah, yeah, first off why isnt that under warranty? O yes, because thats only for the Ultra Premium warranty, of course...

Secondly, how can that module be "broke" but not throw any codes or cause any other problems other then stopping the cruise from working? Wouldnt it cause all kinds of issues if it was bad?

Thanks all!

 

[marshall@pcm]

What year is yours?

We can sell you stock one as well as tuned ones. Depending on the year, stock ones would be anywhere from $50 - $150.

With a 1 year warranty, as well.

 

[TB360]

What year is yours?

We can sell you stock one as well as tuned ones. Depending on the year, stock ones would be anywhere from $50 - $150.

With a 1 year warranty, as well.

since you sell pcm, if you program it with my vin and everything, do i still need to see dealer for additional programming or just perform the re-learn process?

 

[Sparky]

Just the CASE and security relearn process.

If I was ever faced with a PCM issue I'd get a PCM4L one in a heartbeat, especially given how much the dealer wants for a STOCK tuned PCM!

 

[The_Roadie]

Well they change the Cruise Control switch, that didnt do anything,

Incompetent diagnosis. It's almost never the switch. Unless they troubleshot the switch properly and saw that it wasn't asking the PCM for CC engagement.

so they swapped the Powertrain Control Module with another Envoy and it works fine.

Interesting, but not conclusive

Secondly, how can that module be "broke" but not throw any codes or cause any other problems other then stopping the cruise from working? Wouldnt it cause all kinds of issues if it was bad?

The PCM may indeed have a fault that could kill just the cruise control and not be easily noticed, but the dealer should have done the proper CC diagnostic.

Many items on a list of about 12 things can suppress the CC operation. Not just the engagement switch, but a hair-trigger brake switch, over temp, many PCM codes like misfire, and so forth. It's so difficult to diagnose WHY a CC stops working that the GM PCM designers did something clever. They keep a special place in memory where they remember the last few reasons WHY the CC was told to disengage. A Tech II tool can look at the engagement switch function and make sure it's enabling the CC. Then you drive along, attempt to engage the CC, and the PCM memory keeps track of the reason for the failure. Whether it's a pending SES code or something else.

To guess that it's a bad PCM, even to the point of swapping with another vehicle, is a sign of an untrained shotgunner, not a trained diagnostician using all the power of the Tech II tool.

 

[Chickenhawk]

The Roadie knows what he is talking about. Cruise generally shuts down if there is a fault detected elsewhere, plus PCM failures are so rare on our platforms as to be almost non-existent.

If your PCM actually is bad - which I highly doubt - then deal with the PCM4Less folks.

 

[ScarabEpic22]

Bills absolutely right, the PCM stores the last few CC attempts. A Tech II (and I believe EFILive/HPTuners) can read that area and tell you what the condition(s) for disengaging it were. Dont throw parts at it, not worth the cost. Make the dealer do the correct diagnostic, you already paid for it!

 

[Voymom]

My Toyota Camry had an issue with cruise control a few years ago. We tried replacing the Vacuum hosing that went to the CC and it didn't help. We changed our 02 sensor and it worked fine! Not sure why?? It would engage/turn on but it would shut itself off if it ever hit a hill or over drive.

 

[Iahawkeye]

I am not for sure if this will help, but I had an issue with the cruise and climbing hills. It would downshift into drive as I climbed the hill, but not shift back into overdrive when I hit the flats. I did a "global reset" by disconnecting the battery leads and then holding them together for 30 seconds or so. My cruise has worked flawlessly ever since.

It was explained to me that when the battery leads were held together it drains out all residual power in the modules and clears issues that might not be sending a code. Anyways, this might help with your cruise issue. Good luck.

 

[The_Roadie]

It was explained to me that when the battery leads were held together it drains out all residual power in the modules and clears issues that might not be sending a code.

Whoever told you that was joshing you. Or didn't understand the way electronic things work. Sigh..... not your fault, but I can't let misinformation like this become GMTN folklore. It's just that - campfire folklore.

 

[Iahawkeye]

Whoever told you that was joshing you. Or didn't understand the way electronic things work. Sigh..... not your fault, but I can't let misinformation like this become GMTN folklore. It's just that - campfire folklore.

I really do want to understand, so... please dont take this as me arguing.

It was the service manager at a local chevy dealership that told me about the global rest. I was having some problems with the encoder motor not engaging correctly or finding its correct position and they could not figure it out. So he suggested the reset. Sure enough, the encoder motor has worked flawlessly and so did the cruise.

I am not sure why it happened, but it did. He explained that by disconnecting the battery, would not clear out all of the codes. By connecting the two leads, it allowed for a complete grounding of the system or something similiar to that. I am not an electrical engineer (if i remember correctly, that is your field of expertise), but it does seem logical for some electricity to be stored in the capacitors and circuits - I am reaching back to college physics on that one. Do you have a suggestion for why it worked after the fact?

I am sorry for the hijack, but my experience was related to the cruise and possibly the pcm. Thanks for the input.

 

[The_Roadie]

Oh, heck no. I love to explain a bit more in depth. I can tell when folks want to argue physics with me.

You're right that there are indeed many capacitors that can act as miniature batteries and keep a module alive for a few seconds or even minutes after the battery is disconnected. And many (not all) of them are connected to the 12V power pins and those capacitors can be discharged faster by shorting out the battery cables on the vehicle side. Sensors are often powered by a 5V output from the PCM, BCM, HVAC controller, ABS controller, or TCCM. But codes are generally of two types - ones that get stored in non-volatile (meaning permanent) memory, and those that are real-time codes. Non-volatile codes like almost anything emissions-related, will never get cleared by a battery disconnect or by discharging the capacitors. If they were, then anybody with a SES light and a code who needs to pass an emissions test would simply disconnect the battery and clear the code, then scoot in for the emissions test. The EPA wouldn't allow that, so codes are stored in protected non-volatile memory and can only be cleared by fixing the underlying problem. Or by a code clearing scan tool, but then the emissions test will still fail because a freshly-cleared code won't have accumulated sufficient "drive cycles" after the code was cleared for the EPA to consider it legitimate. So you can't clear codes with a scan tool around the corner from the test facility, then scoot in and pass. You have to drive around for a couple of days, and if you haven't fixed the underlying problem, the code will come back.

In the case of the HVAC controller and the TCCM for your encoder motor problem, those modules don't have non-volatile memory for some of the parameters (they do for others), so a loss of power will trigger a recalibration process. That recalibration process fixes quite a lot of encoder motor and HVAC actuator problems (not all), but the modules would be triggered to run a recalibration process by even a minute of no power. Shorting the battery cables together isn't necessary. It won't hurt, but it's not the real reason the encoder motor got fixed.

30 minutes of battery disconnection should be identical to 30 seconds of battery disconnection combined with a power cable short. No capacitors in the modules are going to retain an effective charge through either of those two procedures.

I hate to admit it, but I worked on designing a test system for integrated circuits, that was repurposed to test Ford PCMs, around the 1993-4 time frame. Was able to learn (under a non-disclosure/secrecy agreement) almost everything about the internals of Ford engine control modules at the time, 95% of which are the same sort of circuits used in everybody's PCMs, since they all have to pass the same sort of EPA tests. So that's where I get some my "insider knowledge" of PCMs, even though I didn't specifically learn GM or GMT360 PCMs or how to tune them. Sigh.....

 

[Iahawkeye]

Thanks for the tutorial, professor Roadie, your explanation was great!