PCM burning out

[Bradley G]

Got a PM from one of the long time members.
His trouble is, the PCM keeps burning out.
I did not get any details yet, as to any changes, he may have made to his car.
The only thing that comes to mind, is a faulty ground.

[David Morton]

First off, look for the chafing harness condition. Where the wiring harness stretches across the left valve cover at the back of the head, it can rub through the plastic loom and into the wires. I saw a thread on this just after I bought my 2004 back in May 2004 and at 500 miles I could see it wasn't going to last, so I wrapped it with a heat shield sock. You have to grab the harness and twist it around to make sure it isn't rubbing there. Just looking at it won't do.

If that's OK a bad ground is certainly the next thing I would look for. Check the wiring diagram for the PCM ground circuit(s) and make sure those are good, as well as the chassis, frame and engine grounds. But in case that doesn't get it look for a bad circuit that the PCM is controlling.

General Motors (and I suppose all of the other manufacturers) have been using ECM/PCMs to control solenoids, motors and relays, which are all basically a tightly coiled wire that generates a magnetic field, to move a piece of iron; a check ball in transmission TCC or shift solenoids, a gear spindle in idle air control motors or a spring loaded switch in a relay.

The engineers have always known what happens when the ECM/PCM shuts off the current to the solenoid. And here's what reveals the cause of some ECM/PCMs "frying". The magnetic field collapses across that small coil of wire and induces voltages approaching 100,000 volts! So the engineers designed solenoids that have a diode or resistor in parallell with the coil that let's this voltage spend itself uselessly going in circles inside the solenoid until the field is gone. But if the diode or resistor goes "open" there is only one place for that 100,000 volts to go, back up the wire to the ECM/PCM. Now this problem isn't even in the PCM at all, it's down inside the transmission, or at the throttle body, or in the relay, wherever that open diode or resistor is.

So the manufacturers were tearing their hair out over vehicles that had this conditon I described. Lemon Law buybacks that had had seven ECM replacements and still were frying because the TCC solenoid had an open diode in it or some such other bad ECM controlled circuit.

So to solve this diagnotic nightmare the ECM/PCMs have been redesigned to prevent most of this by internally protecting the ECM/PCM with a quad driver, but not all circuits are protected by quad drivers. Quad driver modules make ECM/PCMs more expensive, so look at the wiring diagrams and search for the circuits that are still protected the old fashioned way, with diodes or resistors. Then try to find the bad circuit component and fix that before you replace the PCM. You're gonna need a Fluke meter.

Could be a bad control circuit or a bad ground. Easy enough to make sure your grounds are good, an hour at the most. But if you're buying the PCM, and you don't find an obvious bad ground, I'd break out the wiring diagnosis manual and start looking for a bad control circuit. A solenoid, switch or relay could be the cause.

[WolfeBros]

First off, look for the chafing harness condition. Where the wiring harness stretches across the left valve cover at the back of the head, it can rub through the plastic loom and into the wires. I saw a thread on this just after I bought my 2004 back in May 2004 and at 500 miles I could see it wasn't going to last, so I wrapped it with a heat shield sock. You have to grab the harness and twist it around to make sure it isn't rubbing there. Just looking at it won't do.

If that's OK a bad ground is certainly the next thing I would look for. Check the wiring diagram for the PCM ground circuit(s) and make sure those are good, as well as the chassis, frame and engine grounds. But in case that doesn't get it look for a bad circuit that the PCM is controlling.

General Motors (and I suppose all of the other manufacturers) have been using ECM/PCMs to control solenoids, motors and relays, which are all basically a tightly coiled wire that generates a magnetic field, to move a piece of iron; a check ball in transmission TCC or shift solenoids, a gear spindle in idle air control motors or a spring loaded switch in a relay.

The engineers have always known what happens when the ECM/PCM shuts off the current to the solenoid. And here's what reveals the cause of some ECM/PCMs "frying". The magnetic field collapses across that small coil of wire and induces voltages approaching 100,000 volts! So the engineers designed solenoids that have a diode or resistor in parallell with the coil that let's this voltage spend itself uselessly going in circles inside the solenoid until the field is gone. But if the diode or resistor goes "open" there is only one place for that 100,000 volts to go, back up the wire to the ECM/PCM. Now this problem isn't even in the PCM at all, it's down inside the transmission, or at the throttle body, or in the relay, wherever that open diode or resistor is.

So the manufacturers were tearing their hair out over vehicles that had this conditon I described. Lemon Law buybacks that had had seven ECM replacements and still were frying because the TCC solenoid had an open diode in it or some such other bad ECM controlled circuit.

So to solve this diagnotic nightmare the ECM/PCMs have been redesigned to prevent most of this by internally protecting the ECM/PCM with a quad driver, but not all circuits are protected by quad drivers. Quad driver modules make ECM/PCMs more expensive, so look at the wiring diagrams and search for the circuits that are still protected the old fashioned way, with diodes or resistors. Then try to find the bad circuit component and fix that before you replace the PCM. You're gonna need a Fluke meter.

Could be a bad control circuit or a bad ground. Easy enough to make sure your grounds are good, an hour at the most. But if you're buying the PCM, and you don't find an obvious bad ground, I'd break out the wiring diagnosis manual and start looking for a bad control circuit. A solenoid, switch or relay could be the cause.


I agree with David. The condition he describes is called back EMF (electro motive force) It is the same principal that makes an ignition coil work. A diode across the coil or solenoid won't conduct in one direction and it shunts the voltage to ground in the other direction.

I also think the chafed wire harness by the firewall is another good suspect area. We have documented failures on this board where the transmission was shifting erratically because of this.

[Bradley G]

Thanks for your responses.

I will pass the detailed info along, to the member who PM'ed me and If I get any feed back, I will let you know what it was!

[Rider90]

Thanks for your responses.

I will pass the detailed info along, to the member who PM'ed me and If I get any feed back, I will let you know what it was!
Braaadd, are you having PCM problems?? :D

[Bradley G]

No Jason,

I got a PM yesterday from a member.I have never seen him post.
Not sure why he contacted me.
Just trying to give back.:coolman:

Braaadd, are you having PCM problems?? :D

[MMM2003]

I have an extra PCM available fo sale. It's coded with the latest MM Fxxx code. Let me know if your friend is interested.

Thanks,

Peter


No Jason,

I got a PM yesterday from a member.I have never seen him post.
Not sure why he contacted me.
Just trying to give back.:coolman:

[Bradley G]

Thanks Peter,

I got another PM yesterday from the same fellow.
He thanked me for the help, which I in turn thank you guys!
He gave me his phonie #, and turns out that is just what it was.:confused:
Anyway, He is having similar difficulties, after it was apparently fixed.

I have an extra PCM available fo sale. It's coded with the latest MM Fxxx code. Let me know if your friend is interested.

Thanks,

Peter

[Dennis Reinhart]

Got a PM from one of the long time members.
His trouble is, the PCM keeps burning out.
I did not get any details yet, as to any changes, he may have made to his car.
The only thing that comes to mind, is a faulty ground.
He called me they have changed out four PCM's the car has never been chipped of flashed it has over 33k on it so far they have done this under warranty, there telling him if this PCM goes bad he is SOL, this is a Ford dealership I told him he needs to contact a Zone rep, I also told him about the Chafing wiring TSB, now the way he desribes the problem is the car stalls then will not even crank that is not a PCM issue, but then he says after it sets it will crank but will not start that of course can be a Pats or PCM issue, but even chafed wires will not melt a PCM that could be related toa CCRM issue I am trying to help as much as I can.

[KilledKenny]

Tell them also to take a look at the connectors under the master cylinder. Around here in the salt belt we have had some corrosion issues in those connectors. My own car had an issue at those connectors also. Mine was pretty weird, drive down the highway my starter motor would engauge intermittently when ever the a/c clutch would enguage. Traced it to a couple wires corroded in that connector. Other vehicle I have seen fuel pump intermittently inop, and pcm not powering up properly causing no starts with pats lamp flashing. Just a little FYI. :)

[Bradley G]

Thats great of you guys' to help out!
I forgot to call him back with his new #

[HwyCruiser]

Electrical problems can be a real ***** to diagnose. I had a Mitsu Eclipse where the headlights would turn on whenever the key was removed from the ignition. The dealer spent a week finding a bad crimp on a connector pin behind the glove compartment.

The really bad part of the deal is they wouldn't even give me a loaner because they said they could button it up any time I needed the car and I could bring it back later. I knew better to even touch the keys until they fixed it because they wanted wash their hands of it.