SOLVED! Fuse 28 PCM-1 blows randomly
- Thread starter c good
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Well that is unfortunate. Why? Because that might bring the PCM under suspicion. If the PCM has the power signals it requires, and a good ground but is not talking it may be done for.
While there at pin 19 check pins 20 and 21 also. 20 needs power at all times, even with no key. 21 should have power in Accessory, RUN, and START.
There is also one more power to the PCM, that is at connector 3 pin 17. That should have power at RUN and START.
Thanks T.J. Would it be safe to assume pins 20, and 21 are the two directly above number 19 ?
Also, where is pin 17 on connector 3? I will check those next. Thx...Cam
I'm actually OK with it being a PCM....at least that might explain my 10 year long problem.
If it indeed turns out to be a bad PCM, does anyone have any recommendations for a replacement? I've done business with PCM 4 Less North Carolina in the past for my transmission cooler. They seem to have a good reputation. But I'm open to suggestions.
What would be involved if I were to find one in a parts yard? Would it have to be reflashed and updated at the dealer?
Also, where is pin 17 on connector 3? I will check those next. Thx...Cam
I'm actually OK with it being a PCM....at least that might explain my 10 year long problem.
If it indeed turns out to be a bad PCM, does anyone have any recommendations for a replacement? I've done business with PCM 4 Less North Carolina in the past for my transmission cooler. They seem to have a good reputation. But I'm open to suggestions.
What would be involved if I were to find one in a parts yard? Would it have to be reflashed and updated at the dealer?
Yes, pins 20 and 21 would be adjacent to #19.
The connectors are numbered sequentially so #2 is in the center amd 3 is the opposite end from. Connector 1.
The numbering sequence is the same on all 3 excepting that connector 2 has an additional connector, #65 in the center and that is the primary ground for the PCM.
For spreadsheet users here is a link to a pretty good listing and diagrams of the various Vortec 4200 ECMs and pinouts.
docs.google.com
Seems like you had trouble with this scanner when your first got it as well, no? I made my statement that your PCM is not communicating based on your description of what is happening at the cluster. Before jumping to any further conclusions about the health of the PCM I would think a more thorough assesment needs to be made to rule out things like a communications issue.
The connectors are numbered sequentially so #2 is in the center amd 3 is the opposite end from. Connector 1.
The numbering sequence is the same on all 3 excepting that connector 2 has an additional connector, #65 in the center and that is the primary ground for the PCM.
For spreadsheet users here is a link to a pretty good listing and diagrams of the various Vortec 4200 ECMs and pinouts.
Vortec 4200 PCM Pinouts
Seems like you had trouble with this scanner when your first got it as well, no? I made my statement that your PCM is not communicating based on your description of what is happening at the cluster. Before jumping to any further conclusions about the health of the PCM I would think a more thorough assesment needs to be made to rule out things like a communications issue.
I have swapped in a junkyard PCM and after a security relearn (a half hour procedure) it ran OK. I am in a state the doesn't require emissions testing so a mismatched VIN would not be an issue for me. It didn't matter anyway as I swapped my old one back since
It was only done as a learning experience.
A junkyard PCM would need to have the same rear axle ratio and should be from the same year truck as well.
It was only done as a learning experience.
A junkyard PCM would need to have the same rear axle ratio and should be from the same year truck as well.
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I need clarification on some things here.
Some folks refer to "turning over" as firing and some mean cranking. Does it crank or does it not crank?
"All dash lights go out" when you turn the key FROM where TO where. I would guess from RUN to START and it is normal behaviour for many things to turn off while the key is in the START position because two ignition key circuits disconnect from power in that position.
The LEDs that light up: this would be the PRND321 display?? But the position indicator bar is missing??
There are clues in everything that happens at the cluster.
Hi T.J.
Excellent questions.
1.) It does not crank. I get nothing, Not even a "click" like you would expect from a bad selenoid contact.
2.) All dash lights go out when I turn key from run to start.
Correct...L.E.D.s light up but the position indicator bar is missing.
Thanks for not giving up on me..I'm not giving up on my Envoy until it's coughed up engine parts on the road!
And even then...I might just look for another engine!
Excellent questions.
1.) It does not crank. I get nothing, Not even a "click" like you would expect from a bad selenoid contact.
2.) All dash lights go out when I turn key from run to start.
Correct...L.E.D.s light up but the position indicator bar is missing.
Thanks for not giving up on me..I'm not giving up on my Envoy until it's coughed up engine parts on the road!
And even then...I might just look for another engine!
Hi T.J.
Excellent questions.
1.) It does not crank. I get nothing, Not even a "click" like you would expect from a bad selenoid contact.
2.) All dash lights go out when I turn key from run to start.
Correct...L.E.D.s light up but the position indicator bar is missing.
Thanks for not giving up on me..I'm not giving up on my Envoy until it's coughed up engine parts on the road!
And even then...I might just look for another engine!
[1] If indeed the PCM is inactive you would not get anything from turning the key to START. The PCM has to recieve a signal from the ignition switch terminal D, yellow wire, through the crank fuse #17 amd then it negotiates with the BCM with regards to the vehicle security system. If the PCM is not active, as evidenced by what you describe on the cluster plus your code reader being unable to connect, then you would get nothing from turning the key to START. These vehicles are rolling computer systems, the ignition key does not actually send power to the starter solenoid or anything like that. Turning the key to START only registers a request to start with the BCM and PCM. It is these modules that actually do the deeds.
[2] Indicator bar missing. The transmission range switch is connected to the PCM. It is messages from the PCM on the serial data bus that cause the cluster to display the indicator bar. So that is another indication that either the PCM is not operating or the PCM is not communicating.
What does the "Service Engine Soon" light do? Is it on with key at RUN but not cranking? Does it stay on when you turn the key to START? Does it not come on at all?
@TJBaker57 's Link to the Vortec 4200 PCM Pin-Out Data in Post #84 is "Off The Hook"...Superb" ... So I Downloaded it as a PDF and dropped it in this reflection. Suggest the Mods might want to keep this one in the appropriate FAQ.
Thomas... Awesome Stuff... Dude!
Thomas... Awesome Stuff... Dude!
when you turn your key to ON only and leave it there, what displays are showing on your dash? You might post a picture of that to help.
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I cannot say who is responsible for the spreadsheet. It might be Calvin Nelson aka "Nivlac57" of Youtube fame and the Vortec 4200 Wiki...
Vortec4200wiki
Welcome to the Vortec 4200 Wiki. This is centered around the Vortec 4200 platform. Here you will find lots of information on the performance and modification of Vortec 4200 engines.
sites.google.com
Mooseman
Moderator
Added link to the 4.2L FAQ. Good stuff.
Also added the link to that Wiki.I cannot say who is responsible for the spreadsheet. It might be Calvin Nelson aka "Nivlac57" of Youtube fame and the Vortec 4200 Wiki...
Vortec4200wiki
Welcome to the Vortec 4200 Wiki. This is centered around the Vortec 4200 platform. Here you will find lots of information on the performance and modification of Vortec 4200 engines.
PCM woke up this morning. Engine will start but sounds like it's running on one cylinder. Fuse 28 getting low amps. Dash all back to normal for acc/run/start.
The battery is only a few weeks old but it seems like it won't hold a charge....I had it on the auto shut off charger all night. That may have been the reason the PCM woke up. It barely turned the engine over but it did start to run, just poorly as mentioned. No codes stored.
Might just be a bad battery. I'm going to return and install a new one. Then check my alternator output. I'm Always open to input. Thx...
The battery is only a few weeks old but it seems like it won't hold a charge....I had it on the auto shut off charger all night. That may have been the reason the PCM woke up. It barely turned the engine over but it did start to run, just poorly as mentioned. No codes stored.
Might just be a bad battery. I'm going to return and install a new one. Then check my alternator output. I'm Always open to input. Thx...
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Mooseman
Moderator
A bad battery can cause all sorts of weird gremlins however blowing fuses is not one of them, or is it? Been a long time since college electronics but if the voltage is low, it may try to pull more amps? Another possibility that the PCM is bad is it's not commanding the alternator properly thus killing your battery.
At this point, I'd probably throw a used PCM at it. Shouldn't be that expensive from a PnP yard.
At this point, I'd probably throw a used PCM at it. Shouldn't be that expensive from a PnP yard.
If I install a PCM from a salvage yard, doesn't it have to be reprogrammed with VIN and C.A.S.E. relearn? I'm also thinking it might have a parasitic draw or direct short problem, yet fuse 28 is now NOT blowing. What is best way to test for parasitic draw?
As long as you get a PCM from the same year truck AND has the same gear ratio as yours you can use it. It will require a 30 minute relearn for security and the battery would have to be on a good charger because the key will need to on for a little more than a half hour for the relearn. True, a CASE relearn would be best and the VIN wouldn't match BUT it would do fine for test purposes.
I gather you are in California?? Then you would need the VIN changed but you could wait on that while testing out the truck.
Use a clamp type current meter so you do not have to disconnect the negative battery terminal (for fear of messing with the HVAC actuators, etc.). With the key off place the clamp around one of the battery leads (making sure to encircle all wires coming off that lead) and measure the current. For completeness sake, do this on both terminals and compare results, they should be the same. Typical draw is going to is in the range of 20 and 50 milliamps. If you find a draw that is more than this then you can try to isolate the culprit by systematically removing fuses (again, leaving the HVAC stuff for last, if necessary).
When doing your current draw tests be aware that anything that awakens the vehicle network will increase the draw until such time that the network goes back to sleep mode. Hitting a keyfob button, opening a door, etc. Removing and replacing certain fuses will also awaken the network for a time.
So here was my day. I got a new battery from Costco. I kept power to the system with jumper cables while I swapped out the battery. I got it to fire up but still only running on one or two cylinders. So....I swapped out some old coils one by one and got it to running on all 6 cylinders.
Something damaged 4 previously good coils. It started up, idled fine, was going to take it for a spin around the block....got 2 feet out of my drive way....and POP....Number 28 fuse blew again.
I pushed back into my driveway with the help of a neighbor (slight uphill). Put in a new fuse. When I turned the key to run position the throttle body mechanism started making a high speed clicking/buzzing noise. I unplugged it and it stopped. I plugged it back in and it no longer makes the clicking/buzzing noise.
Now, none of my power locks work, my key fob doesn't work. I have very weak interior lights. Headlamps and tail lamps are good. Everything prior to today was fine.
There are some major electrical gremlins that seem to point more and more to the PCM. So...I ordered one from PCM of N.C. today. They will program my V.I.N. into it. Mark said that most of the earlier models don't need the C.A.S.E. relearn
I paid a little extra so it will be here by this coming up Wednesday. Are there any tips I should know about while installing it? I would like to keep power to my A.C. actuators to avoid any failures.
Something damaged 4 previously good coils. It started up, idled fine, was going to take it for a spin around the block....got 2 feet out of my drive way....and POP....Number 28 fuse blew again.
I pushed back into my driveway with the help of a neighbor (slight uphill). Put in a new fuse. When I turned the key to run position the throttle body mechanism started making a high speed clicking/buzzing noise. I unplugged it and it stopped. I plugged it back in and it no longer makes the clicking/buzzing noise.
Now, none of my power locks work, my key fob doesn't work. I have very weak interior lights. Headlamps and tail lamps are good. Everything prior to today was fine.
There are some major electrical gremlins that seem to point more and more to the PCM. So...I ordered one from PCM of N.C. today. They will program my V.I.N. into it. Mark said that most of the earlier models don't need the C.A.S.E. relearn
I paid a little extra so it will be here by this coming up Wednesday. Are there any tips I should know about while installing it? I would like to keep power to my A.C. actuators to avoid any failures.
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Serious Gremlins indeed! Before you connect up the new PCM, I would systematically check all power & ground pins to make sure there are no surprises.
Edit: While you are at it I would also inspect the battery cables/connectors and vehicle & engine block grounds.
Only the Paranoid Survive!
Edit: While you are at it I would also inspect the battery cables/connectors and vehicle & engine block grounds.
Only the Paranoid Survive!
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Mooseman
Moderator
Normally you would totally disconnect and power down the truck before disconnecting the PCM, you seem to want to keep power on to help prevent killing HVAC actuators, which I think is just delaying the inevitable. You should pull all the PCM fuses. You'll need to do the security relearn.
PCM security relearn
If after running it a while it throws a crank/cam correlation code, it will need a CASE relearn. Most high end scanners should be able to do this so you don't have to go to a dealer.
PCM security relearn
If after running it a while it throws a crank/cam correlation code, it will need a CASE relearn. Most high end scanners should be able to do this so you don't have to go to a dealer.
Whichever method you choose to replace the PCM, here is a quick reference for the P10 PCM power input/supply circuits. Note that of the 4 circuits, 3 are already disconnected when the key is in the OFF position leaving only a single powered source.
Had the key on for a few minutes and was plugging and unplugging the coil packs. While I was doing this the Throttle Body began to chatter again.
Also, happened to touch the top of the C.O.P. on cylinder number 5 and branded my finger with the cooling fins. For some reason I don't think they are supposed to get that hot. All the other ones were completely cool.
Also, happened to touch the top of the C.O.P. on cylinder number 5 and branded my finger with the cooling fins. For some reason I don't think they are supposed to get that hot. All the other ones were completely cool.
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The last coil I lost died on the interstate. It was hot like you said your #5 was.
Addendum: are you monitoring your battery voltage during these happenings? A drop in voltage or an unstable voltage could cause a reset which would make the throttle actuator cycle.
Addendum: are you monitoring your battery voltage during these happenings? A drop in voltage or an unstable voltage could cause a reset which would make the throttle actuator cycle.
I haven't been monitoring battery but I have been keeping my Amp meter hooked into fuse bus 28. When I turn the key to run it goes to 13 amps for 1 second then immediately drops to zero without popping the amp meter inline 15amp fuse.
Here is a great video posted by @MRRSM from Eric O. at South Main Auto on analysis of a coil failure (A Closer Look At A Coil On Plug Mis-fire) that may have some relevance to the problem at hand.
To make a long story short, the development of a high impedance in the secondary coil current path (likely between the Hi side of the Secondary coil and 12V+) can lead to coil damage due to internal current oscillations & primary/secondary mutual inductance (check out the South Main video at 21:35 for an overview of the coil circuitry and the mechanism for coil damage due to heating).
Combining this failure model with @TJBaker's analysis above and the fact that multiple coils are dying suggests there may be something adverse happening between the common coil Hot connection and the fuse.
The challenge is going to be that a normal resistance measurement is a DC measurement whereas impedance is dependent on frequency (or rate of current or voltage change).
To make a long story short, the development of a high impedance in the secondary coil current path (likely between the Hi side of the Secondary coil and 12V+) can lead to coil damage due to internal current oscillations & primary/secondary mutual inductance (check out the South Main video at 21:35 for an overview of the coil circuitry and the mechanism for coil damage due to heating).
Combining this failure model with @TJBaker's analysis above and the fact that multiple coils are dying suggests there may be something adverse happening between the common coil Hot connection and the fuse.
The challenge is going to be that a normal resistance measurement is a DC measurement whereas impedance is dependent on frequency (or rate of current or voltage change).
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This may seem to be a bit of a reach... but this information is quite technically sound and should be factored into your Solution Calculus:
Excessive Carbon Build Up inside of the Upper Cylinders can often increase compression enough to pass the proper "Quench" level and force atmospheric conditions inside of the Cylinder just BTDC that will make driving the Spark Gap Jump VERY DIFFICULT.
If your present Spark Plug Specs and/or Gaps are not within limits, this along with the presence of Excessive Carbon Build Up may be forcing the involved COPs to work so much harder to bridge the Spark Gap during the Secondary Coil Firing Events... that it risks shortening the life expectancy of those involved Coil On Plugs.
The presence of excess Carbon in the upper cylinder chambers will absorb Atomized Fuel like a Sponge and can cause Incidental Lean Air to Fuel Ratios. Such LEAN A/F Conditions will increase the work that COPs have to perform to create enough of a Spark to Bridge the Gap.
So at this point, performing an ACDelco Top Engine Cleaning procedure with all of the Spark Plugs removed for a three hour soak in the Foaming Flavor of this Special Solvent on such a high mileage engine would not hurt a thing.
Allow me to suggest that you watch THIS Man's Lecture on this issue of Secondary Ignition Wave Form Analysis and you'll find out what is going on here to get a better idea about what might be "Killing Your COPs":
Excessive Carbon Build Up inside of the Upper Cylinders can often increase compression enough to pass the proper "Quench" level and force atmospheric conditions inside of the Cylinder just BTDC that will make driving the Spark Gap Jump VERY DIFFICULT.
If your present Spark Plug Specs and/or Gaps are not within limits, this along with the presence of Excessive Carbon Build Up may be forcing the involved COPs to work so much harder to bridge the Spark Gap during the Secondary Coil Firing Events... that it risks shortening the life expectancy of those involved Coil On Plugs.
The presence of excess Carbon in the upper cylinder chambers will absorb Atomized Fuel like a Sponge and can cause Incidental Lean Air to Fuel Ratios. Such LEAN A/F Conditions will increase the work that COPs have to perform to create enough of a Spark to Bridge the Gap.
So at this point, performing an ACDelco Top Engine Cleaning procedure with all of the Spark Plugs removed for a three hour soak in the Foaming Flavor of this Special Solvent on such a high mileage engine would not hurt a thing.
Allow me to suggest that you watch THIS Man's Lecture on this issue of Secondary Ignition Wave Form Analysis and you'll find out what is going on here to get a better idea about what might be "Killing Your COPs":
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Definitely not a reach. Any info is always appreciated. I guess a compression check will be next on my to do list.
My PCM arrives Wed. If I am able to get it to turn over I will check compression before running it for very long.
My PCM arrives Wed. If I am able to get it to turn over I will check compression before running it for very long.
gmcman
Member
- Dec 12, 2011
- 4,660
Make sure the small springs are inside the coil boot.
Also, what was the status of your battery cables? Were the small tabs still intact, that contact the lead side terminal?
Have you tried to probe pin#19 while wiggling the battery cables, and wiring harness...noting any change or fluctuations in voltage?
Also, what was the status of your battery cables? Were the small tabs still intact, that contact the lead side terminal?
Have you tried to probe pin#19 while wiggling the battery cables, and wiring harness...noting any change or fluctuations in voltage?
in your "travels", have you inspected the lower layer of the fuse box in the engine compartment to ensure the traces are proper / good?
I have not yet, but I will before installing the new PCM. Thanks for the reminder.
New PCM installed. Did security reset. Still no start. No overhead lights, no power locks, no sound system. Would a bad body control module cause this type of trouble?
Roger that...
...and using either THIS Kit (...or one like it...) can Help to sort things out:
Brian from 'Briansmobile1' YT Channel shows a similar Kit in use and what it can do for you during Broken Automotive Wiring Searches:
...and using either THIS Kit (...or one like it...) can Help to sort things out:
Brian from 'Briansmobile1' YT Channel shows a similar Kit in use and what it can do for you during Broken Automotive Wiring Searches:
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You need to post pictures of your dash... with key in ON only, and then with key in START so that people have an idea of what's happening. Further, just in addition to checking voltages at your PCM, there are similar checks for those at the BCM. Are you seeing any "anti-theft" indicator?
You are all awesome and I have appreciated every bit of information.
I have decided to throw in the towel and take it to a GM dealer. They have been there for a very long time and have some excellent techs. They should have a diagnostic done on it by Monday.
I will keep everyone posted. Thank you again for all of the help.
I have decided to throw in the towel and take it to a GM dealer. They have been there for a very long time and have some excellent techs. They should have a diagnostic done on it by Monday.
I will keep everyone posted. Thank you again for all of the help.
limequat
Member
- Dec 8, 2011
- 520
I feel like whenever this happens, the dealer comes back with something completely off the wall. Like, "We replaced the trunk carpet." WTF? "Yeah, static builds up back there, then discharges to the rear fuse box. Ya gotta keep ur carpet grounded."
Good luck @c good, it must be tough and a relief at the same time.
Good luck @c good, it must be tough and a relief at the same time.
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