SOLVED! Fuse 28 PCM-1 blows randomly

gmcman

Guru
Dec 12, 2011
4,540
@c good

Did you ever prove pin#19 and while observing a DMM, move all the wiring around to look for a momentary loss of signal?

Reason I ask, is you didn't follow up this question earlier.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Hi Gmcman,
I did not get to that point. All the inside electrical went haywire and I started thinking BCM. I got fed up and took it to the GM shop. The tech did some troubleshooting and also thought the BCM was bad.

He unplugged the BCM, plugged everything back in and now it just started back up with no codes and everything running perfectly. Maybe it was just a loose or bad connector. I'm picking it up today, but am not convinced that's all it was. Will keep everyone posted.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Went to pick it up, talked to tech and service advisor. Was told it's running perfectly. Paid the $160.00 walked out to Envoy, click, pop...#28 fuse blows again. They were embarrassed. So it's theirs again. Not my problem.
 

budwich

Guru
Jun 16, 2013
1,737
kanata
Not quite the "off wall scenario" posted earlier, but almost. Surely, IF they were able to plug into the vehicle, they could easily determine what's up including what's happening at each module on the network... of course, assuming they aren't using a "torque and elm327" as their primary tool. :smile: It would seem you got them on a "$160 hook" with the "its all fixed"... :smile:

One question, which PCM did they have access to? and another question, what did you mean "click, pop #28".... does that mean that just turning the key to start caused #28 to pop?
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
It will be interesting to find out, if ever, what they find.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Not quite the "off wall scenario" posted earlier, but almost. Surely, IF they were able to plug into the vehicle, they could easily determine what's up including what's happening at each module on the network... of course, assuming they aren't using a "torque and elm327" as their primary tool. :smile: It would seem you got them on a "$160 hook" with the "its all fixed"... :smile:

One question, which PCM did they have access to? and another question, what did you mean "click, pop #28".... does that mean that just turning the key to start caused #28 to pop?
I kept the original PCM in place as the one I ordered made no difference. I did not want to introduce another variable.

Yes, simply turning key to start pops fuse 28. Instantly! Pops it so loudly one can hear the snap from the driver's seat with the fuse box cover off.
 
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budwich

Guru
Jun 16, 2013
1,737
kanata
it would appear that you now have some "hard fault" ... for a little while (does that make a "soft hard fault"... :smile: ) Hopefully, they made some electrical measurement at strategic points to see what's up. I guess the question is... currently, is the condition duplicatable / repeatable.... meaning, it happens now every time? Having said that, without doing any disturbing outside of pulling the fuse, hopefully, they did a resistance measurement towards the output leg at the fuse, got a reading and then did a reading / watched as the key was turned got another reading and then finally, with key left turned, carefully disconnected the cable connector at the PCM (one of interest / involving ign1) and got another reading. That should have provided them with a "map of electrical interest" with a degree of isolation of the circuits involved.... or they could just replace your entire cable and PCM structure .... :smile: With your current "turn the key, gets a pop", it should be easier to find but of course, nothing is ever easy.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Update. Tech called me this morning and was pretty sure it is a bad PCM. He mentioned that when he plugged in the old PCM it would draw 50 amps! I took the new/used PCM in from PCM of N.C. that I purchased last week to him. He put it in, did the security relearn and the Envoy is running again for now.

Time will only tell if this is indeed the problem. I will be driving it close to home for awhile again but I'm pretty sure this Gremlin is finally put to bed. (Fingers Crossed) Thanks to everyone for your knowledge, input, and patience during this loooooong ordeal! Hopefully this archived thread will help someone out in the future when they suffer the dreaded popping Fuse #28 problem! Gotta LOVE the GMT Nation crew!
 
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MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,917
Tampa Bay Area
Fingers Crossed...

One More Question... (Una Pregunta Mas...)

During the call from the Tech.. .Did he (...or she) specify that the PCM of NC Custom Calibration in the New, Replacement P-10 PCM was over-written with a GM OEM Factory Updated Calibration... or not?
 

c good

Active Member
Original poster
Dec 8, 2011
407
The tech (He) made no mention of the PCM from PCM of N.C. being overwritten with any modifications. During my conversation with Mark from PCM of N.C. , Mark mentioned that their PCM would be flashed with the most recent stock GM parameters for my year and model Envoy. This was not a new PCM, but a used one, with my VIN over written, and a one year warranty.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
It doesn't matter if PCMofNC used a modified stock base tune however the dealer can recognize if it's a stock tune or not if it has the wrong checksum and could have reflashed it just based on that or just a shotgun attempt at fixing it. That's why you never bring your vehicle in to the dealer with the modified tune in it especially if they're doing work of this type on it.
 
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Blckshdw

Likes lights and stuff
Moderator
Nov 20, 2011
10,228
Tampa Bay Area, FL
That's why you never bring your vehicle in to the dealer with the modified tune in it especially if they're doing work of this type on it.
:iagree: When my transmission ate itself a couple years ago and got replaced, the dealer reflashed my PCM back to stock. Luckily mine was a mail order tune, so putting it back was easy.
 
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c good

Active Member
Original poster
Dec 8, 2011
407
So far so good. Envoy running great. I did notice the lower steering column cover was hanging down a little. I know the tech was in there poking around because he had the cover off during one of my stops at the dealership.

I also had the lower cover off a week before when trouble shooting and replacing the ignition switch as part of my troubleshooting tasks. So I know how to take the cover off and put it back on correctly. It's tricky but if you're careful and know how its designed no problem.

Well, the tech muscled the thing and broke 4 of the tabs and bent 1 or 2 of the others.

So I took the cover back up to the dealership and told the service manager I wanted a new lower cover. He called the tech up and he admitted he had struggled with it and did indeed break it. So my new lower cover will be in tomorrow. No charge. And I will be the one installing it! Correctly!
 
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c good

Active Member
Original poster
Dec 8, 2011
407
Update. Drove the Envoy 60 miles today. (without the steering column cover on) The new cover won't be in until next week.

Envoy is starting and running better than it ever has. I had previously posted in another thread, about strange starting issues in the past. How long it would take to start, how it started, etc.

The PCM swap is like a whole new vehicle. Starts right up, no hard starting, no hesitations, idles smooth and perfect rpm. I have my original old AC/Delco C.O.P.'s in it and it is running perfectly.

At this point, I'm not sure if the bad PCM ruined my other coils or if they are actually bad. As you might remember I swapped out all the coils at one point to eliminate any bad coils. So it turns out all the original AC/Delcos that I kept were fine after all. Right now they are staying in. I'm not changing anything!

I think it has been a PCM going bad for a very long time. It's frustrating that over 11 years, two previous dealerships and a specialty automotive electrical specialty shop, they were never able to diagnose it. But....in their defense, it was an intermittent problem. I guess the complete failure was the only way it could have finally been figured out. Will update as I put more miles on it. Fingers still crossed! Thx All!
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
So we can mark this one solved?
 

c good

Active Member
Original poster
Dec 8, 2011
407
Do you still have the "bad" PCM? An autopsy might be interesting.
I do have the bad PCM. I took it apart and did a cursory inspection. I didn't see anything glaringly obvious. Nothing burnt or smelling funny. If anyone knows of a place that does rebuilds, I would be interested in sending it in and having it as a backup. I'll post some pics of it tomorrow.
 

azswiss

Active Member
May 23, 2021
361
Tempe, AZ
Given that the vehicle ran, albeit poorly, with the old PCM I would have been surprised if visual inspection revealed any obvious defects. First step in failure analysis is to test the PCM at the unit level. Subject to what is found, detailed fault tracing dives down into component level assessments (board, discretes, IC's, connectors, etc.).
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
Probably not worth the trouble or expense of fixing. Getting another one at a PnP yard would be cheaper and easier. The vast majority are reliable. Only heard of a handful of them going bad, including yours.
 

limequat

Hobbyist
Dec 8, 2011
517
V=IR. PCM was drawing up to 50 amps. If correct, the resistance between power and the case should be less than an ohm. I would expect visible damage at that point. @c good , care to throw a DMM on it for fun?
 

c good

Active Member
Original poster
Dec 8, 2011
407
V=IR. PCM was drawing up to 50 amps. If correct, the resistance between power and the case should be less than an ohm. I would expect visible damage at that point. @c good , care to throw a DMM on it for fun?
If anyone could walk me through the steps, I'm willing to take some time to troubleshoot it. I have a DMM but just don't know how I would go about testing the PCM with all the connections.
 

MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,917
Tampa Bay Area
With the errant PCM installed... it might be more revealing to conduct THESE Voltage Drop Checks and determine if there are any problems with your Wiring. If they exist and go on undiscovered, they might also wind up ruining the replacement PCM.

These Instructions for WHERE to hook up the DMM in the Basic Circuitry and what to look for in the way of Voltage Drops along with the Detailed Explanations and the Diagrams are very straightforward and will be quite helpful.


Now... THIS is Important...

THIS is Known Good, Classic Coil On Plug Waveform with All The Parts Identified. Use this to improve your understanding of what come next:

IGNITIONPATTERNANALYSIS.jpg

In order for you to get to the bottom of the reasons WHY you acquired Second Degree Burns on your Fingers earlier in this situation regarding the Ultra Hot Tops of any particular Coil On Plugs... You will need to understand the phenomena called:

Mutual Inductance

In a Nut Shell, this concept involves what happens when a Coil On Plug is Firing on instructions from the PCM to add Current to the Primary Side of the Coil as needed... and the Secondary Side of the Coil is UNABLE TO FIRE... (OPEN CIRCUIT)

Whenever this happens.. .that excess High Voltage on the Secondary Side of the Coil does NOT simply Disappear.... It continuously Oscillates Back and Forth in the Circuit looking for an Escape Route and as it moves back and forth, it can BODY SLAM the Primary Side of the Coil and generate ENORMOUS HEAT.

So it becomes very important to investigate WHY that COP was unable to Fire the Spark Plug due to an OPEN CIRCUIT that can not only Damage the COP,... but Feed energy back into the PCM as well and likewise create problems there... even if there is a Voltage Limiting Circuit in place for its protection.

If you watch THIS Video and listen to Eric "O" from SMA and "Crazy Ivan from PHAD (Pine Hollow Automotive Diagnostics) explaining all of this in Very Fine Grain Detail from around 13 Minutes onward... it will open up opportunity for you to run further Diagnostics on your GM 4.2L COP and Ignition System using either a PICO-Scope Model # 2204A or a Hantec Model # 1008C and dial in these problems.

Please ignore the fact that the Vehicle under their scrutiny is a Dodge as It's THE PRINCIPALS are what is important to understand here that really count ...and the way to use the OHM Meter feature of the DMM to Check the Coil Resistance gets discussed at around 27 Minutes into this Diagnosis:

 

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TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
If anyone could walk me through the steps, I'm willing to take some time to troubleshoot it. I have a DMM but just don't know how I would go about testing the PCM with all the connections.

The simplest thing would be to test the resistance between pin 19 of connector 1 and either the ground terminal at pin 65 of connector 2 or the pcm metal case. On a normal P10 PCM I see an average of 10.9 k ohms there. As Limequat stated if it were in fact drawing 50 amps from fuse 28 the resistance you would expect to see there would be less than one ohm, maybe as little as one quarter ohm!
 

limequat

Hobbyist
Dec 8, 2011
517
The simplest thing would be to test the resistance between pin 19 of connector 1 and either the ground terminal at pin 65 of connector 2 or the pcm metal case. On a normal P10 PCM I see an average of 10.9 k ohms there. As Limequat stated if it were in fact drawing 50 amps from fuse 28 the resistance you would expect to see there would be less than one ohm, maybe as little as one quarter ohm!
Agreed.
I've never pulled apart a P10, but generally any module is gonna have some input circuitry and then a power supply that cuts the 12V down to 5V or 3.3V or less. The fault would likely be at or before the power supply.
To sink 50 amps without burning, the short would have to be the equivalent of ~10 ga wire, so...
 

azswiss

Active Member
May 23, 2021
361
Tempe, AZ
Agreed.
I've never pulled apart a P10, but generally any module is gonna have some input circuitry and then a power supply that cuts the 12V down to 5V or 3.3V or less. The fault would likely be at or before the power supply.
There are going to be input protection diodes so testing should be done twice, first with the DMM negative lead (black) on the ground pins or the case and the positive lead (red) on the input pin. Flip the leads and measure again; there should be a difference.
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
Wouldn't have guessed that there were two boards.

I was looking at these and other pics just last night. I need to have another look inside, as I didn't remember two boards myself. I think the one is just gathering pins and passing them through but I would like to see the underside of that one again.

I have a few P10s collected for experimentation, scanning for PIDs, DTCs, etc. Plus, you never know when you might need a phone stand...
 

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c good

Active Member
Original poster
Dec 8, 2011
407
Well. Another 200 miles on the Envoy. I'm afraid of jinxing it but I think we can mark this as "Solved"......Running better than it ever has and definitely getting better fuel mileage. I haven't dug into my PCM yet, but will keep posting on this thread if I figure out what went bad on my old one. Thanks again to everyone for helping solve this.
 

c good

Active Member
Original poster
Dec 8, 2011
407
I was looking at these and other pics just last night. I need to have another look inside, as I didn't remember two boards myself. I think the one is just gathering pins and passing them through but I would like to see the underside of that one again.

I have a few P10s collected for experimentation, scanning for PIDs, DTCs, etc. Plus, you never know when you might need a phone stand...

I prefer the narrower, more space efficient option for my phone 😆
 

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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
SHHHHHHHHHHH! You'll jinx it!
 
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c good

Active Member
Original poster
Dec 8, 2011
407
SHHHHHHHHHHH! You'll jinx it!
Hee Hee! :smile: So far so good. I'm gaining confidence in it more everyday. I'm actually shopping for new tires now. It really is running better than it ever has. That PCM was going bad for a loooooong time....unfortunately it was so intermittant nobody could diagnose it and it never stored any codes. It had to have a hard failure before anything definitive could be figured out....along with alot of great input from everyone here.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Update. 970 miles since the PCM replaced. All is well. Still keeping my AAA premier 200 mile towing program intact. Don't want to jinx it.....but....so far so good. Putting some more miles on it for Thanksgiving trip. Will keep you all posted. Happy Thanksgiving to all.
 

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