SOLVED! Fuse 28 PCM-1 blows randomly

c good

Active Member
Original poster
Dec 8, 2011
407
Engine stops and won't even turn over when it happens. Replace fuse,, starts right up and runs no troubles. I've got 60 miles on since it happened no troubles. 5 years ago I had a similar problem and it was never figured out 100% but was told it may be a bad coil so put all new OEM Delphi Coils in it and drove it without any problems until last weekend. How long should coils last? Any other ideas on why #28 should blow? Thanks, c good
 

Sparky

Moderator
Dec 4, 2011
12,798
Check the wiring harness for the coils. Might not be a bad coil but just bad wiring someplace rubbing.

My factory coils lasted 14 years and 215k miles, so they can last a long time.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
That or the fuse panel itself. If there's corrosion there, it can increase resistance and blow the fuse.
 

Sparky

Moderator
Dec 4, 2011
12,798
Well, technically increased resistance in of itself will never blow a fuse because higher resistance = lower current :wink:

I=V/R
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
OK, I've been out of college for a long time. Still worth investigating though.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
According to this post on TV:

Primary resistance (between the two terminals in the connector) is usually around .7-1.7 ohms and secondary (between spark plug terminal and one of the terminals in connector) is usually around 7500-10500 ohms.....
 

c good

Active Member
Original poster
Dec 8, 2011
407
Thanks to all that gave input on this pesky problem....Please consider it SOLVED for now......It was another bad coil...this one was only 4 years old. It was a Duralast from Autozone. It was shorting out and blowing the fuse. My recommendation is to stay with Delphi or BWD for replacement coils. I tested resistance at the the three pin connection on suspected bad coil. With the black test lead on the black wire pin and the red test lead on the pink pin, it read out at 29.07 K ohms. Definitely bad. It should read 0! Installed a new Delphi Coil Pack and everything is good again. Symptoms were hard starting at first, hesitation and stumble off of idle, then complete shut down while driving on the interstate at 70 mph. No restart possible. Blown #28 fuse. Lucky to coast to an offramp and have tools and extra fuses to get me home. I will keep tools and an extra new coil in my travel pack from here on out. Hope this helps anyone else with this problem.
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
I absolutely stay away from unknown make house brand parts. Good work on finding it.
 
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c good

Active Member
Original poster
Dec 8, 2011
407
Ughhh....Not solved. Fuse blowing again. Took it to the dealer....Will keep everyone posted....
 

JimInRadfordVA

Registered Member
Feb 29, 2016
195
Radford, VA
Any update on this?
 

c good

Active Member
Original poster
Dec 8, 2011
407
Well, here's the latest. Dealer had it for two weeks, could not isolate the bad coil....did not charge me because they said they would be able to diagnose it when I brought it in. Decided to build in a fuse "trap" to each coil. If any one coil would have shorted out...it would blow only the fuse to the bad coil.....no blown fuses with 150 miles of driving.

Had a trip planned so replaced all the coils...(again) luckily they were all on lifetime warranty so replaced them all under warranty as the parts store had no way of testing them either.

Put a 1000 miles on it so far since all new coils...no problems and running perfect....will update if any more surprises.

I guess the take away from this whole nightmare is that the coils do go bad.....and will short out the PCM fuse 28 and leave you stranded when they do short out. Keep lots of fuses and a couple of extra coils in the vehicle. Buy lifetime warrantied coils and keep your receipts. They cannot test them so there is no way for them to tell you if you have a good or bad coil....
 

c good

Active Member
Original poster
Dec 8, 2011
407
Gremlins are back. Fuse 28 blew again. Put in a new fuse and it's running fine again for the last 5 days. Very Frustrating and dangerous. Made it 30 months this time. Am I missing something? Maybe not a bad coil after all? Nobody seams to be able to tell me definitively how to test these coils and what the resistance values should be. Not even the manufacturers. Has anyone had a PCM that went bad with this type of failure? Thanks to all for any input. c good
 

budwich

Guru
Jun 16, 2013
1,737
kanata
I do believe your injectors are also on the fuse. Perhaps you may want to try a "testing adjunct" so that you can readily read the current at the fuse during certain times (ie. cold start, 30 minute run, etc) to get an idea of how much current is flowing thru the fuse at some instance. You might be able to do that with some form of "add a fuse type circuit adapter" (dual tap with a "loop"). This will allow you to readily insert a multimeter (or clamp on meter) in the circuit at times to get a reading (assuming you have a meter that can take up to 15 plus amp... most do just 10... clamp ons can usually do more ). Just a test / investigation suggestion.
Of course, bad coils need to be addressed first. Having said that, the loss of a coil regularly potentially needs more attention.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Thanks Budwich....Here is another part to the equation. Just bought a new code scanner and it won't connect to the module. It says to check the connection which I've done. I also checked the new scanner on another vehicle and it recognizes it right away. I wonder if I've got a bad PCM?
 

budwich

Guru
Jun 16, 2013
1,737
kanata
Anything is possible... but having it work with another vehicle does not fully exonerate the reader unless it is the same vehicle (ie. same year, model, make) as the platform can have "differences" that makers of readers don't always do a "good job" with. This is true with a number of blue tooth elm327 devices. Having said that, there are a few tests you should do at the connector to ensure some form of electrical integrity at the DLC connector before pointing the finger at the PCM.
Others (tjbaker especially) has done a lot of electrical testing of the PCM to quantify its interfaces as part of another set of threads with similar problems (ie. unable to connect). It could be a bad PCM but there are a number of things in the electrical paths that need checking before going that way unless you want to "bind part swap" the PCM.
 
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TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
Not knowing the year this may or may not apply.

Fuse #28 serves ignition coils as you know and fuel injectors as @budwich stated and also C1-19 of the PCM. This fuse is energised through terminal C of the ignition switch once the RUN position is reached.

PCM and BCM communications are established before the key even reaches the ACCY position. Thus it would seem unlikely to be an issue with PCM communications.

If the vehicle is 2007 or older the code scanner would need to support the SAE J1850 VPW protocol. If it is known to do so then my first look would be at the serial data line from splice pack SP205 to the terminal 2 of the DLC.
 

c good

Active Member
Original poster
Dec 8, 2011
407
I've replaced all the coils a couple of times over the last 11 years. Good quality Delphi coils aren't cheap! Nobody seems to have a way to test them so I've just replaced them all! That seems to solve the problem for a year or two. I've cleaned all the ground locations....including the one under the passenger side shift tunnel/panel. Today I was driving and #28 blows. It's really dangerous because it simply shuts the engine off when it happens. Luckily I was able to get over to the right and onto the shoulder safely. I always care spare 15 amp fuses. Pulled the old fuse out...confirmed it was smoked....put new fuse in and it started right up. Frustrating....I love my Envoy...but if this can't be figured out....it's going to the wrecking yard....I couldn't sell it knowing it might hurt somebody. I've read possibly everything out there about this problem but I'm all ears for any suggestions. Thanks All, Cam
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
Were I in your shoes I think I would do resistance checks of all that is served by that fuse. 6 coils you already replaced, what of the 6 injectors on that same fuse?? The only other thing is a circuit to the PCM, an unlikely suspect.
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,325
Ottawa, ON
A possibility is broken wires in the harness itself. Have a look at this video:


He found a broken wire which only killed Cyl. 6. What if yours is shorting?

And check the schematics to see which circuits that are powered by that fuse and check them. Try to do a shake test of the harness while it's running.
 
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budwich

Guru
Jun 16, 2013
1,737
kanata
OP... in your first post, you mentioned the work that was done along the way but you didn't actually state the problem that you were attempting to solve while doing the work. Now you indicate that the fuse has blown. Is that what was happening before or were you just getting "running issues" (ie. misfires, no fires, etc)? Just want to clarify fault history.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Hi Budwich,
It has been the same problem since I've owned the Envoy. 11 years now. I will be driving, and it will just turn off. If I go to restart, the dash lights all will come on but starter will not engage. I pull fuse #28 and replace it with a good fuse, and it starts right up. Sometimes it will run for months, even years with no problems.

I had it at the GMC dealership the last time it happened. They had it for two weeks....wanted to charge me $250.00 and they couldn't find the problem. All they said was they "thought" it might be a bad coil. I told them I wouldn't pay them a dime if they couldn't figure it out. I was willing to pay them more if they could just figure out what the problem was but they said there was nothing else to do except "possibly" replace all the coils.

I then took it to an Electrical specialty shop. He had it for 3 days....couldn't figure it out because it wouldn't blow the fuse again. Intermittent problems are the worst to diagnose. He wiggled harnesses, tested continuity, etc...etc....He still charged me $175.00 for nothing.

He suggested I build a "short trap" to each coil. So I bought pin connectors, and wired an in line 15 am fuse to each coil. If a coil was shorting out it would have blown that fuse and identified the problem. I ran it for months that way. Never had a problem. So I remove all the extra wiring, buttoned it all up and drove it for two more years before it happened again yesterday.

I'm beginning to think it might be PCM related. Will pursue that next.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Mooseman, That is an excellent video. And yes I also thought about a short in the wiring but it would probably cause a miss. The engine runs perfectly. Up until it just quits from the fuse blowing. Then I install a new fuse and it runs perfectly again. Doesn't even store any engine codes. Completely random. It's just crazy and frustrating. I'm leaning more towards a PCM. I did notice the rpm drop after hooking up the red wire. I'm wondering if there might be a break similar to the one in your video (excellent info, thank you) I might have to pull the harnesses apart and look carefully at that area.
 
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TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
You guys are always coming up with stuff that makes me curious so then I have to do some test or another!! :wink:

So I just hooked up a circuit load tester, bungeed it to the wiper blade and did a quick run to see what sort of amps might be normal on that fuse #28 circuit of 6 coils, 6 injectors and the PCM Ignition 1 input. Idle at about 1.8 amps, neighbothood streets up to 3 amps, hard acceleration to 5000 rpm hit 6.1 amps.

20210722_150524.jpg
 

c good

Active Member
Original poster
Dec 8, 2011
407
You guys are always coming up with stuff that makes me curious so then I have to do some test or another!! :wink:

So I just hooked up a circuit load tester, bungeed it to the wiper blade and did a quick run to see what sort of amps might be normal on that fuse #28 circuit of 6 coils, 6 injectors and the PCM Ignition 1 input. Idle at about 1.8 amps, neighbothood streets up to 3 amps, hard acceleration to 5000 rpm hit 6.1 amps.

View attachment 101266
That is excellent info. Can you show me how you wired it into the fuse buss? What is the brand and model of tester? Hard to see but is it Emtech circuit tester?
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
That is excellent info. Can you show me how you wired it into the fuse buss? What is the brand and model of tester? Hard to see but is it Emtech circuit tester?

Close! It is a Cen-Tech circuit tester. Harbor Freight. I wish they made one designed for our smaller fuses. I scoured the web looking for an ATO to Mini adapter. Found nothing. So I fashioned my own adapter(s) using 3/16 female spade terminals and some strips of 0.7 mm aluminum sheet flashing I have laying about. I fairly well ignore their stated 10 second test time since I never even approach the 30 amp stated capacity.

PXL_20210723_000117091.jpgPXL_20210723_000130576.jpgPXL_20210722_184849361.jpg
 

MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,917
Tampa Bay Area
Answering your PM Here...

 

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c good

Active Member
Original poster
Dec 8, 2011
407
Answering your PM Here...

Thank you Sir! Could you clarify a couple of things. When it states in step 3, "place one lead on the center prong of the coil packs electrical connector and the other lead on each of the spark plug terminals"...."spark plug terminal" means the spring portion inside that physically attaches to the top of the spark plug?

Also, in step 4..."place both leads on each spark plug terminal" Wouldn't placing both leads on the same terminal just get zero resistance? It would basically be like connecting the leads together for perfect continuity?
 
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c good

Active Member
Original poster
Dec 8, 2011
407
Close! It is a Cen-Tech circuit tester. Harbor Freight. I wish they made one designed for our smaller fuses. I scoured the web looking for an ATO to Mini adapter. Found nothing. So I fashioned my own adapter(s) using 3/16 female spade terminals and some strips of 0.7 mm aluminum sheet flashing I have laying about. I fairly well ignore their stated 10 second test time since I never even approach the 30 amp stated capacity.

View attachment 101267View attachment 101268View attachment 101269
Sweet set up. I'm going to try this also. Thanks for the info.
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
Sweet set up. I'm going to try this also. Thanks for the info.

I thought it might be good to establish a baseline of a sort to which one could compare.

I saw some talk about resistance checks of coils. I also have been curious how these new fangled coil modules operate. I learned auto mechanics in 1974-76 so as I thought, my understanding of coils doesn't really apply to these modules. I looked online but found nothing that I considered reliable as it simply does not jive with my own observations. I have 4 surviving original Delphi coils and two replacements that are roughly two and seven years old and not the same brand as well. I also have a used spare from the Upull yard. I have nothing that measures anywhere near the often cited low resistances for the primary, measured from the center PCM driven control terminal and ground. Or any other measurement for that matter. All the Delphi coils and the replacements measure about 11.?k (11000 - 12000) ohms from center terminal to ground. About the same regardless of metering polarity as well.

20210723_131635.jpg20210723_131700.jpg20210723_131714.jpg



And with reversed meter polarity....

20210723_131831.jpg20210723_131838.jpg20210723_131845.jpg
 
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TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
While I was at it with the air intake resonator off I measured each injectors resistance. All 6 individually rounded off to 12 ohms.

I only see these as useful in terms of comparing across a set of coils or injectors.


And with all this said, I'm leaning towards @Mooseman in thinking wiring harness trouble somewhere. Doesn't necessarily mean a short to ground but could be a short to another powered circuit or a sensor.

It makes sense that you would have no codes to help in this particular problem because that fuse provides Ignition 1 power to the PCM so the very instant the fuse blows the PCM shuts down in the same way it does when you turn the key to ACCessory. It's not going to set a code in the ACCessory position, and with fuse 28 blown that's where it thinks the key is.
 

budwich

Guru
Jun 16, 2013
1,737
kanata
so as you can see by the very good "baseline" that Tjbaker57 provided, what appears to a be "normal current draw" is significantly less than the 15amp fusing. At this point, most "regular" DMM's provide for 10 amp draws on their current range. I would hook up a normal DMM just to see what running at idle is drawing on fuse 28 just to get started. It might give you some 'direction" without you doing much.... of course, check your DMM ratings first.
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
I did notice during my run down the highway that total current draw varied most directly with engine rpm, with an added load from the injectors if you are into the pedal.

As @budwich points out the draw is generally well below the rated fusing. I was most curious about what the total loaded amps might be. Static circuits are fairly straightforward but these coils and injectors can be considered pulsed width I guess and I haven't the foggiest notion how those loads would be calculated.

I also wondered about how many devices might be active at one time. Clearly only one coil is firing at any given time and perhaps an injector pulsing at the same time and quite likely some overlap as well. Too many eggs in the pan for me!
 

budwich

Guru
Jun 16, 2013
1,737
kanata
You might consider digging under your fuse box (below the top layer holding the fuses) to look at the tracking associated with fuse 28 and surrounding points. There is a possibility that there might be an underlying issue therein.
 
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MRRSM

Lifetime VIP Donor
Oct 22, 2015
5,917
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To answer your Question from Post #11:
 

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MRRSM

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Oct 22, 2015
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This is just a BIT O/T ...but am I the only one who is beginning to think that TJ is actually one of the Yautja? I mean look at these images and compare... The Yautja not only have the BEST Alien Technology and Weapons in the known Universe... they also wear those same Cool Sandals and carry their very own Hi-Tech Automotive Diagnostic Kits on their Left Foreams, too!
 

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c good

Active Member
Original poster
Dec 8, 2011
407
Excellent write up and photos TJ. I appreciate your efforts and your time.
 

c good

Active Member
Original poster
Dec 8, 2011
407
Hi TJ. So far so good but this is typical for this problem. It will run for months or years sometimes and suddenly just blow #28. I am going to clean a few more ground points and check all the coils in the Envoy plus the add'l 6 coils that I still have after replacing them all. I Appreciate the follow up and will keep you posted. Thx...c good
 

TJBaker57

Guru
Aug 16, 2015
2,074
Colorado
Hi TJ. So far so good but this is typical for this problem. It will run for months or years sometimes and suddenly just blow #28. I am going to clean a few more ground points and check all the coils in the Envoy plus the add'l 6 coils that I still have after replacing them all. I Appreciate the follow up and will keep you posted. Thx...c good


Understand the variability. I used to run a CNC controller that would do that, run fine for years then repeatedly blow one of the circuit fuses. Might blow 4 or 5 in a month then not do it again for years.
 
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