NOT SOLVED - Fuse 28 PCM-1 blows randomly

MRRSM

Lifetime VIP Supporter
Deja Vu ... (between you two) ...with even more On Topic insights from @TJBaker57 as well... :>)

 

Mooseman

Master Blaster
Moderator
Merged the two threads to keep it all in one place and avoid duplication. Thanks @MRRSM
 

gmcman

Guru
Just a thought, I'm also not 100% certain on the function of these coils......would there be enough in-rush current to blow this fuse, if all the coils stopped working for a second, then suddenly started firing/charging while at approx 2000 RPM or higher under higher cylinder load?

My thinking is at idle or cranking, each coil fires slow enough with reduced load, or the succession between cylinder firing is slow. What if the crank signal was interrupted briefly while at a cruise RPM, then the coils paused if there was a loss of signal where the PCM could not command a firing, then unload the coils again but at a higher RPM, much higher than when cranking?

Another way to see this would be a gradual, lower intensity sine wave of charging cycles, then nothing, then a sudden, more rapid, large intensity of coil firings for each cylinder under load causing higher currents through the circuit.

Like turning a power amp off then back on with the volume cranked up.

That may be a whacked way to think why this fuse is blowing but could this be a loose or faulty crank sensor or another circuit that can shut down the coils when conditions are met?

What circuits can satisfy the PCM to pull spark, especially while running...oil pressure, ignition switch?

Something to happen quick enough that it wouldn't throw a code. I know that unplugging a coil then reattaching while the engine idling could blow a coil, but that might be a different set of circumstances.
 
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gmcman

Guru
Fuse 28 is the PCM 1 fuse right? Are the injectors on this circuit also?

Have you tried to remove the fuse box and look for any loose wires, or damage to the harness from the PCM to the fuse box? Forgive me if that's been covered.
 

Mooseman

Master Blaster
Moderator
Fuse 28 is the PCM 1 fuse right? Are the injectors on this circuit also?
I believe so. That was determined early on in this post.

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.
I'd pull the fuse box and inspect the traces. I'd also be peeling all the wire looms and inspecting each wire.

So there's 3 circuits running off the fuse. Failing everything else, I'd split them up or isolate each one and splice in a fuse for each one. You could use the Centech or similar circuit test gauge to get a baseline of the amps it normally pulls and then put a fuse at the next highest amps. @TJBaker57 had done that but with all 3 circuits, it pulled just over 6A max.
 
Failing everything else, I'd split them up or isolate each one and splice in a fuse for each one. Y

In posts #12 and again in post #22 @c good speaks of having fused each coil. Not sure of the rest of the circuitry though.

Wonder about replacing that fuse #28 with an equivalent amperage auto reset circuit breaker. At least the thing might keep you from having to coast over to a safe spot, open the hood, open the fusebox and replace the fuse!
 

Mooseman

Master Blaster
Moderator
I'm talking about isolating and putting a fuse on each of the three circuits (coils, injectors and PCM).

I'm leaning towards the PCM. I haven't heard of coils causing this even when they fail nor the injectors. I can't even remember of a failed injector ever being mentioned. I'd be looking for another PCM in a u-pull yard cheap, put it in and do the security relearn.
 

c good

Active Member
Thread Starter
I'm actually serious. I love my Envoy....but I'm done chasing this gremlin. The dealership couldn't figure it out. My local automotive electrical specialist couldn't figure it out. I've spent months of actual under the hood time. Replaced 12 coils, pulled wire looms, chased wires to every possible system. I'm done. I really am just thinking about hard wiring the fuse. Carry a big fire extinguisher, and wait for the flame!

Smoked 5 more 15 amp fuses again today. Put in a 30 amp fuse and got home.

It was interesting in that an ABS light turned on for the first time. Even after I got it running again. Is there any chance the ABS is tied into fuse 28? If so, I'm not seeing it in the schematics.
 
:-( BUT did you do any of the "current measuring" testing that was suggested (and compared to tjbakers results) to see where vehicle stands? Even the simple "at idle" test measurement may tell you something about your "baseline electrical system" of that circuit.
 

Mooseman

Master Blaster
Moderator
Posts moved to this existing thread again. Please do not start new threads for the same issue.

As mentioned, did you do the suggested diagnostic tests?

The only possible correlation between the ABS and PCM could be a bad ground. Check what the code is for the ABS, which may provide a clue.
 

gemsheba

Registered Member
I would replace your ignition switch. It takes 5-10 minutes. Quite a few things from the BCM go through it. Mine went bad and killed the BCM signal to power the blower and thus no AC.
 

c good

Active Member
Thread Starter
Ist is at idle 1.7 Amp. Next is 25mph. Got it to go up to 5.3 amps while accelerating on a hill. No spikes. Put 30 miles on it this afternoon. Will try again tomorrow.
 

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Ist is at idle 1.7 Amp. Next is 25mph. Got it to go up to 5.3 amps while accelerating on a hill. No spikes. Put 30 miles on it this afternoon. Will try again tomorrow.

So your baseline draw seems nearly identical to mine. On the one hand I keep thinking it has to be one or more of rhe 7 or 8 pink wires that feed off fuse #28. But then it's been years, you would think that by now the wire or wires would have enough damage to set a hard, permanent condition.
 

c good

Active Member
Thread Starter
So your baseline draw seems nearly identical to mine. On the one hand I keep thinking it has to be one or more of rhe 7 or 8 pink wires that feed off fuse #28. But then it's been years, you would think that by now the wire or wires would have enough damage to set a hard, permanent condition.
I would have thought so also T.J. That's why it is so frustrating. I'm thinking of cutting in line fuses into the pink/hot wires feeding the PCM1, Coil Packs, and Injectors as suggested by Mooseman. What do you think would be the correct amp fuse for each circuit?
 
I would have thought so also T.J. That's why it is so frustrating. I'm thinking of cutting in line fuses into the pink/hot wires feeding the PCM1, Coil Packs, and Injectors as suggested by Mooseman. What do you think would be the correct amp fuse for each circuit?
That might be a good idea.... maybe. You can use 15 amp fuses. But before you go there...
In your "journey", you have never replaced any cabling (ie. coil or injector harness), right?
Have you ever tested the resistances of the "aggregate" injectors along with the individual injectors? I am thinking the "failed coil" symptom is a sign of problems elsewhere that is taking out the most "sensitive" component on the link.

As a side question, what year of truck are you having (it is not obvious to me).
 
Fuse 28 is the PCM 1 fuse right? Are the injectors on this circuit also?

Have you tried to remove the fuse box and look for any loose wires, or damage to the harness from the PCM to the fuse box? Forgive me if that's been covered.
I don't think we ever got an "reply" to a "fuse box investigation".
 

c good

Active Member
Thread Starter
Hi Budwich. It's a 2002 Envoy SLT 4x4. 180 k miles. I have not replaced any wiring harnesses. Before I cut in fuses, I'm going to test all the coils and finish cleaning all the grounds. I've done a couple grounds, including the one on passenger side transmission tunnel. Next I'll start peeling back the wiring harness and physically trace the pinks and look for obvious shorts. That's why I was thinking I would just put in a heavier fuse or hardwire the fuse and just keep a good fire extinguisher handy. Maybe a battery quick disconnect too. If it sparks a little, it will lead me to the short! 😀
I don't think we ever got an "reply" to a "fuse box investigation".
I haven't done
I don't think we ever got an "reply" to a "fuse box investigation".
I haven't done that yet. I'm hesitant to disconnect the battery due to the potential failure of the AC actuators and ruining another C.D. player that I just replaced courtesy of Matt.

I'm going to look for a way to plug into my cig lighter/12 v port that will save memory if I have to disconnect the battery. Then I'll look under the fuse box.
 
OK... just wanted clarification of what was and what wasn't. I think that your early strategy with fusing the individual COP's was good. The problem is that you needed to run that way until you got a failure... which as you point out, could be a while. Without a fail condition, you are back to guessing which circuit path / circuitry is the root cause.

IF you are concerned about "depowering", I would then potentially try the "resistance checks" of the injectors, which is done by disconnecting them which can be done while powered.

Have you ever serviced the injectors / wiring there of?
 

c good

Active Member
Thread Starter
Hi Budwich. I'm second owner but have owned it 11 years now. Previous owner is a friend. He does not remember having any injectors serviced or wiring. I bought it at 148K miles. It now has 188K miles.

I took it out for a drive yesterday and got about 40 miles on it staying on my local surface streets. I finally popped the fuse while I had the Circuit tester on. I put in a 30 amp fuse and restarted it with the circuit tester on. It was spiking up to 27-28 amps. I didn't run it for long like that. It sat for 10-15 minutes. I put a new 15 amp in and restarted it. I drove it for another 15 minutes home and the amps never went above normal.

I parked it in my driveway for a few hours and went to take it for another drive and it would not turn over. The key would turn but I had nothing going to the starter. I turned the key a few more times and it clicked and tried to start but the battery was almost dead. Something like a parasitic draw was happening. I jumper cabled the battery. Let it charge up for 15-20 minutes and it started right up. I parked it in my garage. Put a battery charger on it overnight.

Took it for a drive today. Had the battery checked and it tested good. (Although it is 9 years old. It is an Interstate/Costco 100 month battery.) I keep fresh electrolyte in it and it's always on a float charger. Drove it around the rest of the day. 40-50 miles. Not any troubles.

It's driving me nuts! Someone mentioned a key switch. Any possibility that might be it?

'm thinking the C.O.P.s have been a red herring this whole time. Will test them with my multi meter tomorrow. My next step is to cut the inline fuses into each of the 3 system hots. PCM1, Injectors, C.O.P.'s......will keep everyone posted. Thanks everyone for your patience.
 
:-( hmmm... interesting about the "popping" amperage. When you were observing this, what was the actual "ranging"... meaning... you saw indications of "high amp" and then where did "bounce" back to... ie. normal of a couple of amps or what?... basically, I am asking you to give a verbal description of the "waveform" of the current that you were seeing on the meter. Not sure how readily you can get at the injector harness and connectors but there are resistance tests for the area, both as a whole and individual. You might want to look at those.

Your "in line fuse" (of 15 amp) will at least give which "path" is the one that is the "next route" for attention. You can go from there. It appears that the problem is coming more often if I understand your posts. Of course, the other problem is "disturbing" the "environment" to cause checks and readings may change the potential root problem as it exists. Tough battle.
 
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Mooseman

Master Blaster
Moderator
The ignition switch can be a source of electrical gremlins however have never seen one blow fuses. Usually gives no start or no power conditions.

The parasitic draw could be a clue to a failing component that may spike the current draw.

You should take the fuse box apart and inspect the underside.
 

MRRSM

Lifetime VIP Supporter
Best Practices on "How To Test for Parasitic Draw..."

 

limequat

Hobbyist
Lots of good info in this thread.

To summarize, on PCM I Fuse 28 we have 6 coils, 6 injectors, and the PCM.

The PCM draw should be in the milliamps, as well as each of the coils. The majority of the load comes from the injectors. Each injector will draw approximately 1 Amp at 100% duty cycle. At redline, the injectors will approach 90% DC. So the the ~6 Amp readings are spot on.

The 20+ Amp readings while running are telling. This informs us that the problem is not current inrush (A PCM might draw 50 Amps on startup, but only for milliseconds).

So it can only be one thing: an intermittent short to ground. It's doubtful that the short would be in any of the injectors or coils - else the engine would run poorly.

30 amps is about the point where wires of the gauge used on Fuse 28 will start melting. @c good , I think your answer may be coming soon. I like the idea of using fuses to isolate the problem. I would do a 3A or less on the coils and PCM, and 10 on the injectors.

Finally, I've seen many harnesses come in that have wires worn bare from regular operation. The most common problem area is where the wires exit the coil connectors. It could be as simple as rain falling on a bare wire, that clears up when the engine is warm.
 

MRRSM

Lifetime VIP Supporter
,,,as was mentioned back in Post #20 by @Mooseman and *winkled* out 'On Camera' by Paul Danner.
 

c good

Active Member
Thread Starter
Test all my coils today. This one didn't seem right. Substituted it out with a spare that tested in line with the others.

The test to the secondary, where it attaches to the spark plug showed zero continuity. All the other 12 coils I have read around 5.5

Drove around for 30 minutes. So far so good.
 

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c good

Active Member
Thread Starter
UPDATE. More miles on the Envoy and Fuse 28 is still good. I won't consider this "solved" yet....but...there is a pattern. The one coil I found with the different value seems to have been the culprit. After previous replacement of coils over the years I would get a few years between troubles. So...moral of the story....if you have fuse 28 start popping. Do a thorough test on all of your coils. This seems to be the most common source of the problem.
 

c good

Active Member
Thread Starter
After driving over 100 miles with all the good coils, there is definitely better performance. The one bad coil never caused a miss and that is why it was so difficult to figure out. One would think a coil is either good or bad. Unfortunately not the case when these start to fail. This one would apparently just start to short internally as it heated up and draw so much amperage that it would blow fuses.

In retrospect it did cause a little hesitation off of idle. Barely noticeable at the time. But after driving it now it is so much smoother. No hesitation whatsoever., I will make a note if I start to feel a little hesitation, and then it starts blowing fuse #28 to go straight to the coils and start testing them again.

It is interesting to note that this was one of the newer "better quality" coils I purchased in 2018 made by Delphi and purchased at Autozone. I have a mish mash of BWD, Factory Original Delphi, and a few Pep Boy's "Standard" brand C.O.P. that is in it for now.
 
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c good

Active Member
Thread Starter
Hi All.....Envoy back to it's old tricks. Fuse 28 Blew again and left me stranded 35 miles from home. No getting it started this time so at least whatever is happening is no longer intermittent.

Won't turn over. All dash lights turn on but gauges are all not reading or reading incorrectly. My fuel gauge is showing empty and light is on but I just put 15 gallons in it. When I turn the key...all the dash lights go out. I get no shift indicator when I turn on the key. Just the led's light up.

I connected my code reader and it keeps saying...Linking Error...please try again. So, it's not receiving a message from the ECM even with a fresh and good fuse #28.

I though it might be a bad ignition switch so I went and bought one and put it in....no luck. It does the exact same thing.

I'm stumped. Any ideas? I'm actually thinking I might have a bad ECM.....Any thoughts?

It's dangerous when this happens...just dies in the middle of the road. Luckily I was driving along the coast and stalled right in front of one of the nicest Hotels in San Diego...The Catamaran. Called AAA....hung out and got a free water from the Valets...and watched some beach scenery....if you know what I mean! :smile:

Thanks for any and all input....Cam
 
Take your multimeter, pull off connector one at the PCM (the blue one), test for 12 volts at pin 19 with key in RUN. This is the ign 1 signal to the PCM from fuse 28. Without 12 volts here the PCM will not talk.

Editing: wrong picture added. Corrected

PXL_20210930_024701283.NIGHT~3.jpg
 
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gauges are all not reading or reading incorrectly. My fuel gauge is showing empty and light is on but I just put 15 gallons in it.

The PCM is responsible for the fuel gauge, the engine temp gauge, the pil pressure gauge, the PRND321 indicator and some other things I might be forgetting. Your PCM is not awake. It requires the 12 volts from fuse 28 to pin 19 of connector one to wake up and operate.
 
FWIW,,, My guess is that the cause of the blowing fuse has been an intermittent short inside the harness leading from fuse 28 to PCM connector 1 pin 19. Each time it blew the fuse it further damaged the wire. Finally it has severed that wire and you now have no ign 1 power to your PCM.
 

c good

Active Member
Thread Starter
Thank you for the Excellent info TJ. I will check that first thing tomorrow morning. Will let you know what I find out. 👍😎
 

c good

Active Member
Thread Starter
Should I disconnect battery for this? What is the trick for unplugging the connector from the PCM?
 

Mooseman

Master Blaster
Moderator
Just unscrew the bolt and pull off. In the same idea as @TJBaker57 , check for continuity and short to ground from that pin to fuse 28.
 

azswiss

Registered Member
I have been following this thread for a while now; really enjoying watching the collective knowledge base tackle this problem. One question though: assuming the coils are the victims rather than the perpetrators, what is causing the damage @c good is observing?

One mechanism that comes to mind is excess current routing thru the coil primaries due to an unintended/alternate current path (e.g. floating ground at the base of the primary).
 
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MRRSM

Lifetime VIP Supporter
Pull the Canister Vent Valve from the Driver's Side of the Engine Block and also test it for continuity and function...and as mentioned in Post #20 by @Mooseman... examine the adjacent Purge Valve Wiring for any Dead Shorts in the wiring harness. The reason for all of this? The Canister Purge Valve shares the Circuit bearing the #28 Fuse.

EDIT:

Dammit... Scratch that (...these Old Eyes of mine...) the Fuse involved with this Circuit is #26... Sorry about that...

EVAPFUELCIRCUIT.jpg
 
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MRRSM

Lifetime VIP Supporter
Also... was just wondering if THIS Thread deserves 'blending' into this one as well, since it is a fairly recent posting from 2019 ...and it is DOT (Dead On Topic):

 

c good

Active Member
Thread Starter
I have seen this thread from Rolo in the past. The fuel pump runs on a different circuit than fuse 28 IIRC correct?
 

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