ECM B fuse blows instantly

Hi All!

I have a 2002 Gmc Sierra 2500hd 4x4 8.1L

Bought truck knowing there is a electrical issue.
I was told, truck was parked at store working fine. came out and would not start again since.


So here is my situation
battery connected and no key in ignition.
If i try to install a fuse into #30 (ECM-B) it blow instantly.
I have pulled out the entire wire harness and inspected and re-dressed.
In process found oil pressure sender broke in half.
I replaced OPS.
While wire harness was removed fuse would be ok.
Plugged everything in with a good fuse in place and all was well until i hooked up the grounds.
There is 3 ground connects. two aft of engine and one left front of motor.
The one left front is 2 wires to one ring terminal and the two aft are single.
All of them a small gauge black with white stripe and in harness to fuse block.
With none of the devices connected touching the grounds to the engine will still blow the fuse.

Anyone seen anything fun like this before?

thanks for your time and thought
 

NJTB

Silver Supporter
I had to read this a few times, but what I'm reading is that all the components on the main harness are disconnected and the fuse blows when you hook up the grounds?
 

Reprise

Lifetime VIP Supporter
Try pulling the fuel pump relay. Put the title of your post into google and it'll be listed right at the top.
 
OP
S

saltymisfit

Member
I had to read this a few times, but what I'm reading is that all the components on the main harness are disconnected and the fuse blows when you hook up the grounds?

Yeah. If any of those 3 grounds on the engine harness touch the engine it blows.
 
OP
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saltymisfit

Member
Try pulling the fuel pump relay. Put the title of your post into google and it'll be listed right at the top.


I did that as well as pull bed and disconnect and fuel pump.
Previous owner replaced fuel pump just before selling it to try and fix it.
I inspected that harness all the way back as well.
 

Reprise

Lifetime VIP Supporter
I've got nothing else, offhand (sorry). Will see if I can find anything else (and if you've scoured the web thoroughly, let us know, so we don't spend a lot of time duplicating efforts. Thx)
 
OP
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saltymisfit

Member
Thanks so much!
Yeah I have totally searched far and wide everywhere on the web.
Since all the tubing on the wire harnesses is brittle and junk I have completely removed both the chassis and engine wire harnesses. I don't see anything yet. Definitely going to clean up the grounding spots because looks kind of junk.
But in the back of my mind something's telling me that it's in the dash. But I might as well reprotect all these wiring harnesses to rule it all out.


I have two manuals but I can't seem to find anything with proper diagrams of locations of grounds. I can find them for the smaller v8s but nothing for this 8.1 l
 

Reprise

Lifetime VIP Supporter
Use the link in Mooseman's signature. There are manuals there for the GMT800s, and I think there's some stuff there on the 8.1L (engine, if not electrical). I'll see what I can find out (but don't expect anything before morning, as I still need to hunt / catch / kill dinner for tonight) :biggrin:
 
OP
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saltymisfit

Member
Yeah you! Get that protein!

Yeah I got those manuals That's awesome!
Bummer is is there's no wiring for the 8.1 and the colors are different.

Thanks so much for your time
 

TJBaker57

Well-Known Member
This reminds me of a gadget I bought back in the 70s for chasing down short circuits. Essentially it was a signal flasher in series with a beeper of sorts and what was basically a compass type of meter. You hooked this gadget in series with the shorted circuitry and then used the pulsing meter to follow the magnetic field that pulsed on the shorted circuit.

Just googled and found this on amazon, looks like the same thing I bought like 40 or more years ago!!

 
OP
S

saltymisfit

Member
This reminds me of a gadget I bought back in the 70s for chasing down short circuits. Essentially it was a signal flasher in series with a beeper of sorts and what was basically a compass type of meter. You hooked this gadget in series with the shorted circuitry and then used the pulsing meter to follow the magnetic field that pulsed on the shorted circuit.

Just googled and found this on amazon, looks like the same thing I bought like 40 or more years ago!!

Oh heck yeah that thing's super old school!
I actually just ordered it 👍🏽

Made my way into the interior and removed the dash and have found tons of mouse poop and already found two wires that have nibbles and exposed wire. They're not part of the circuit that's popping but I'm pretty sure if I trace the instrument cluster wires for the oil and for the fuel between the firewall to the gauge I believe that's where I'm going to find my damage. But it's too late in the day and I got other jobs to do tomorrow so we'll see how it goes on Thursday.

Thanks for telling me about this awesome vintage short finder 🤙🏽
 

NJTB

Silver Supporter
Sounds to me a component on the harness may be shorted, and since you're putting a new one in plug everything in one ar a time till the fuse blows.
@TJ Bakers suggestion is great, I've seen those used.
Thinking further, the rodent infestation sounds like there may be a power wire shorted to a ground wire.
 

Reprise

Lifetime VIP Supporter
Back; sorry for delay.

Unfortunately, *everything* I can find suggests an issue with the fuel pump and / or the subharness that connects it to the rest of the truck (it's supposed to be a plug-in connector on both ends (the pump is, obvously). And you reported that you checked the pump and its wiring.

Since the truck is a 2002, the connector is likely like my '03 was -- a 2 x 2 plug (4 wires; 2 stacks of 2 wires, each side). GM changed the plug later on, and when you purchase a new pump module (regardless of source / brand), a new 4 in-line connector is almost always included, to splice in place of the old plug. Did you find this when you examined the wiring at the pump end? If so, how was it spliced in? The best is soldering, but a lot of people get leery about introducing heat (or flame) sources near gasoline. At the other extreme... if you found black electrical tape at the splice, check under it, to ensure there's a solid connection there, and not some bare wire twist job.

With the pump, people reported that the failing pump caused a short, and wound up melting the wires on the harness / connectors. This should be easy to see, if it happened (you'd probably have found it already, TBH)

Finally... there are a LOT of crappy pumps out there. Carter, Airtex, anything made of Chineseium... you get the point. And Carter pumps are US-made. But they still fail a lot after a year to 18mos.

*IF* you need another pump...
This site is strong on using GM / ACDelco / Delphi parts. I don't always agree with that, but when it comes to fuel pumps, people seem to have better experience with them (or Bosch, who is / was one of GM's suppliers.) The other option is something like a Walbro, or a Quantum, etc. -- people who make high performance units (you can get a standard capacity units from them, too). I know Quantum sells the complete modules (I have one in my Sierra).
So... I know you may have checked through these things already; maybe there's some new info that you didn't see. And you've found some potential issues behind the cluster, so that's another avenue for you to check with. Keep us posted.
 

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