another ECM issue

[Pbeanbag]

So I was taking a ride (first time with the wife) to get doors and my fuel pump crapped out. i could hear it running but no pressure. I replaced it and the truck ran great. However all of the sudden i am blowing ECM fuses . Once i was playing withe fuse with the test light and heard a clicking. Hit the key and she fired up. drove it today and the ECM fuse blew again'
two questions for you guys.....the relays on the driver's side firewall are they the ECM and related to the ECM circut because when i unplugged them the red wire with the black stripe on the left relay and the orange wire on the right one were showing ground on my test light. is that correct?
The previous owners turned my harness into a spaghetti nightmare.
I crawled around and checked wires with the test light between the posative cable and the B+ post and it did not go out. pulled the glove box , yanked those wires around, disconnected a 3 wire plug and it went out but the wires did not coincide with the orange or red they were pink. I know that the orange wire is always hot on these trucks, all the wires on the column are hot and bright. i keep looking for shafed wires and coming up empty. Any lues? Please ! i need to get to work

 

[Road Trip]

So I was taking a ride (first time with the wife) to get doors and my fuel pump crapped out. i could hear it running but no pressure. I replaced it and the truck ran great. However all of the sudden i am blowing ECM fuses . Once i was playing withe fuse with the test light and heard a clicking. Hit the key and she fired up. drove it today and the ECM fuse blew again'

Greetings Pbeanbag,

Welcome to the GMT400 forum.

Listen, the electrical systems were changed over the years. Help us help you by telling us
the year, model, engine, of your vehicle. (For example I drive a '99 C2500, 454/4L80-E)

Thanks!

 

[Schurkey]

Download the service manual set for your vehicle, from the links in the sticky section of the Engine forum.

The wiring diagrams are in there.

 

[Pbeanbag]

Greetings Pbeanbag,

Welcome to the GMT400 forum.

Listen, the electrical systems were changed over the years. Help us help you by telling us
the year, model, engine, of your vehicle. (For example I drive a '99 C2500, 454/4L80-E)

Thanks!

she's an '88 k3500 with 5.7 Liter but the block is gen 1 with tbi that bolts like a 78 and the head lights say its an 89? finding parts is difficult because the vin# doen't match the parts ,but an '89 does as well as the head gasket set is pre 88. also when i turn it on the ECM humms for about 20 seconds ( always did) strange huh?

 

[Scooterwrench]

Never heard of an ECM humming. Are you sure it's the ECM and not the fuel pump priming? If it is the ECM maybe whatever is making it hum finally shorted the rest of the way out and now it's blowing fuses. Did Chef Boy-R-Dee get under the dash too?

 

[Road Trip]

she's an '88 k3500 with 5.7 Liter but the block is gen 1 with tbi that bolts like a 78 and the head lights say its an 89? finding parts is difficult because the vin# doen't match the parts ,but an '89 does as well as the head gasket set is pre 88. also when i turn it on the ECM humms for about 20 seconds ( always did) strange huh?

Pbeanbag,

Thanks for the update, now we can get the correct FSM (Factory Service Manual) involved.

The overview of the wires fed by the ECM fuse is on this page, taken from the '88 Electrical Diagnosis manual:

You must be registered for see images attach

IMPORTANT NOTE: To keep your 36 year old work horse on the road, you should take
full advantage of the fact that we have a storehouse of GM FSMs, free for the downloading.
Go there & drink your fill: ('88+ GMT400 manuals)

This first page should give you a strong start, but you may need to dig deeper into this manual.

As for the 20-second hum, the only widget that comes to mind would be the Hot Fuel module
that the 8600+ GVWR trucks in your era got. More on that in a few.

If you have any further questions don't hesitate to ask. And when you find the root cause of
your problem make sure and take a couple of sharp photos and post them here so that others
can learn from your troubleshooting/fix.

Happy Hunting!

Cheers --

 

[Road Trip]

As for the 20-second hum, the only widget that comes to mind would be the Hot Fuel module
that the 8600+ GVWR trucks in your era got. More on that in a few.

Hello Pbeanbag,

If you read through the factory manual there's a short blurb about how the "Hot Fuel Module"
was added to the HD trucks to increase the Key On fuel pump prime from the standard 2 seconds
to 20 seconds. For what it's worth I've attached a couple of pages from the electrical manual - the
first is a shot of the fuse block, and the second shows how the Hot Fuse module is placed in the
circuit.

And to wrap this up and put a nice bow on that mystery 20-second hum, here's a link where
the Hot Fuel module is literally described & dissected for your viewing pleasure. (Hot Mess of a Hot Fuel Module)

Of course, to confirm that *your* 20 second hum is the same as the official source of
the 20-second fuel pump run, just pull the associated fuse and see if the mystery sound
disappears.

Again, best of luck finding the cause of what's blowing your fuse.

 

[Schurkey]

Hum when you turn the key, from the general area of the ECM?

First Guess: The Recirculation/Fresh air actuator is going from recirc to fresh. When you shut the key off, it hums again as it goes from fresh to recirc. Contrary indication is that the recirc actuator should only hum for five-someting seconds not twenty.

 

[Pbeanbag]

Never heard of an ECM humming. Are you sure it's the ECM and not the fuel pump priming? If it is the ECM maybe whatever is making it hum finally shorted the rest of the way out and now it's blowing fuses. Did Chef Boy-R-Dee get under the dash too?

yes Chef Boy-R-Dee lives there. and when the truck was first running i was going 15 mile runs and never aproblem (even with the humming ECM) only after the pump replacement did it start. and I only have one fuel tank

Hello Pbeanbag,

If you read through the factory manual there's a short blurb about how the "Hot Fuel Module"
was added to the HD trucks to increase the Key On fuel pump prime from the standard 2 seconds
to 20 seconds. For what it's worth I've attached a couple of pages from the electrical manual - the
first is a shot of the fuse block, and the second shows how the Hot Fuse module is placed in the
circuit.

And to wrap this up and put a nice bow on that mystery 20-second hum, here's a link where
the Hot Fuel module is literally described & dissected for your viewing pleasure. (Hot Mess of a Hot Fuel Module)

Of course, to confirm that *your* 20 second hum is the same as the official source of
the 20-second fuel pump run, just pull the associated fuse and see if the mystery sound
disappears.

Again, best of luck finding the cause of what's blowing your fuse.

Yes Roadtrip you were correct. the ecm is not the humming it is a device of some kind ( im assumig its the hot fuel module) i pulled the speedo fuse and the ECM fuse and it still hummed. thanks ! now how do i know if its bad? i did find a bare wire coming from the transmission up to the harness and fixed it. hope that was the problem as all i did wa replace the return fuel line and pump.

 

[Road Trip]

Yes Roadtrip you were correct. the ecm is not the humming it is a device of some kind ( im assumig its the hot fuel module) i pulled the speedo fuse and the ECM fuse and it still hummed. thanks ! now how do i know if its bad?

Pbeanbag,

Glad you are still motivated to make everything right and working on identifying
the source of the mystery noise. Unidentified electrically-powered noise
sources in an older vehicle is not a good thing, so let's figure out how best to find this
so that we can make an informed decision about whether it's OK vs we really need to fix this?

If I understand you correctly, you removed the fuse feeding the ECM and the noise
persisted? The good thing is that we now know that the ECM is not the source of the 20-second humming.
By the way, you mentioned the SPEEDO fuse, but how about the GAGES fuse?
Did the 20-second humming persist after the GAGES fuse was removed?

****

How about we try to figure this out from the other end? What if we were to take full
advantage of the 'Divide & Conquer' electrical diagnostic capabilities that your fuse block
gives us thanks to it's layout?

What I would like you to do is to methodically remove each fuse, one at a time, until
you discover the pulled fuse where the 20-second hum disappears. Then report which
fuse it was back here, and with the help of the Service Manual we will be able to identify
which electrical device it is in short order.

****

If I misunderstood your message then please set me straight. Otherwise, start pulling
those fuses at your earliest convenience, share what you find, and we'll figure out
the rest of the mystery & decide what the best course of action will be.

Thanks for the update. Looking forward to your next update.

Happy Hunting --