dreaded blown #28 fuse strikes again.....Pulling out my hair!

c good

Well-Known Member
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

Well-Known Member
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.
 

Mooseman

Moderator
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

Well-Known Member
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.
 
OP
C

c good

Well-Known Member
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.
 
OP
C

c good

Well-Known Member
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.
 

TJBaker57

Well-Known Member
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
 
OP
C

c good

Well-Known Member
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

Well-Known Member
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 Supporter
Answering your PM Here...

 

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OP
C

c good

Well-Known Member
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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OP
C

c good

Well-Known Member
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

Well-Known Member
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

Well-Known Member
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

Well-Known Member
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

Well-Known Member
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

Well-Known Member
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.
 

MRRSM

Lifetime VIP Supporter
To answer your Question from Post #11:
 

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MRRSM

Lifetime VIP Supporter
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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