another ECM issue

[Pbeanbag]

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 --

hi guys, yes i pulled the wrong fuse. i got confused on the diagram you sent me. So...today i removed the gauges fuse and the hum was still there. next i pulled the heater fuse and it went away . So, as someone earlier replied it is probably the door actuator. also each time i removed a fuse the truck ran still. now i am not sure if i even have a hot fuel module. here is a pic of Chef Boy-R- Dee lmao. i did mention that i found a bare wire from the tranny harness and taped it up and put convulated tubing over it. it drove around the block about 3 minutes and was ok. still i will wait until my buddy is around to tow me back before venturing out on the freeway. you guys have helped me a lot and i am very grateful. i will keep you all posted as to what i find so others can learn from my experience.

 

[Road Trip]

hi guys, yes i pulled the wrong fuse. i got confused on the diagram you sent me. So...today i removed the gauges fuse and the hum was still there. next i pulled the heater fuse and it went away . So, as someone earlier replied it is probably the door actuator. also each time i removed a fuse the truck ran still. now i am not sure if i even have a hot fuel module. here is a pic of Chef Boy-R- Dee lmao. i did mention that i found a bare wire from the tranny harness and taped it up and put convulated tubing over it. it drove around the block about 3 minutes and was ok. still i will wait until my buddy is around to tow me back before venturing out on the freeway. you guys have helped me a lot and i am very grateful. i will keep you all posted as to what i find so others can learn from my experience.

Pbeanbag,

Sincere thanks for closing the loop and letting us know what you discovered. This gives your
thread additional value for those researching strange sounds in the cab. And it looks like
@Schurkey gets the win again. :0)

And the picture of what the PO left for in chef boy r dee's wiring harness is
simply breathtaking.

Safe travels --

 

[Scooterwrench]

hi guys, yes i pulled the wrong fuse. i got confused on the diagram you sent me. So...today i removed the gauges fuse and the hum was still there. next i pulled the heater fuse and it went away . So, as someone earlier replied it is probably the door actuator. also each time i removed a fuse the truck ran still. now i am not sure if i even have a hot fuel module. here is a pic of Chef Boy-R- Dee lmao. i did mention that i found a bare wire from the tranny harness and taped it up and put convulated tubing over it. it drove around the block about 3 minutes and was ok. still i will wait until my buddy is around to tow me back before venturing out on the freeway. you guys have helped me a lot and i am very grateful. i will keep you all posted as to what i find so others can learn from my experience.

Holy rat $hit Batman!!! Chef Boy-R-Dee was working overtime on your truck. Wonder what the pneumatic line and sensor was for?

 

[Pbeanbag]

Pbeanbag,

Sincere thanks for closing the loop and letting us know what you discovered. This gives your
thread additional value for those researching strange sounds in the cab. And it looks like
@Schurkey gets the win again. :0)

And the picture of what the PO left for in chef boy r dee's wiring harness is
simply breathtaking.

Safe travels --

that's only what you see . behind the center vent was (is) a cluster of wires that went to the plow which you can see where i cut that from the fuse center. also there was a strobe light control unit behind the seat with like six pigtails going everywhere. cut that sucker out too ! OK Roadtrip, let's go back to the original problem...according to the fuse box diagram the ecm fuse doesnt operate the ecm on gasoline engines right? it operates the throttle body injection.

Holy rat $hit Batman!!! Chef Boy-R-Dee was working overtime on your truck. Wonder what the pneumatic line and sensor was for?

pneumatic line is for air assist rear suspension which works (thank God). on the dash where the stock radio was is an air gauge and a switch to inflate as well as a button to deflate. fun fun fun

 

[Pbeanbag]

Pbeanbag,

Sincere thanks for closing the loop and letting us know what you discovered. This gives your
thread additional value for those researching strange sounds in the cab. And it looks like
@Schurkey gets the win again. :0)

And the picture of what the PO left for in chef boy r dee's wiring harness is
simply breathtaking.

Safe travels --

that's only what you see . behind the center vent was (is) a cluster of wires that went to the plow which you can see where i cut that from the fuse center. also there was a strobe light control unit behind the seat with like six pigtails going everywhere. cut that sucker out too ! OK Roadtrip, let's go back to the original problem...why am i blowing the ecm fuse

 

[Scooterwrench]

You're probably going to have to chase that circuit and see if they tied into it with something that is either overloading the circuit or shorting it.
If I were to have to deal with that mess I would chop out all the add ons,get the original harness back to the way it left the factory then add the extras you want to keep back on their own circuits.

 

[PlayingWithTBI]

also each time i removed a fuse the truck ran still. now i am not sure if i even have a hot fuel module.

Just an FYI - once your engine is running, and your Oil Pressure Switch (on the China rail) sees ~4 PSI, it'll keep the fuel pump running. You'll have to pull the fuse for the fuel pump circuit to kill the engine. The Hot Fuel Module is only for purging the fuel system, before starting, so you don't get vapor lock.

 

[Road Trip]

OK Roadtrip, let's go back to the original problem...according to the fuse box diagram the ecm fuse doesnt operat the ecm on gasoline engines right? it operates the throttle body injection. so what is the next step should it die again? Is the ESC failing? and the original question ( put on that thinking cap) why do i have the orange and red with black stripe at my two firewall relays grounded? are they just ground until the truck runs or is there a short i havent found.? i think i will send you guys a video of this truck so you all can see what happens when people just find power sources and tie into them. BTW you are all awesome! oh and i have a red wire on my rocker panel that is ground should i cut it and see what happens?

Hello Pbeanbag,

I like the fact that you are trying to arm yourself with the knowledge about where to look
just in case the ECM fuse blows again. Very savvy. I did some digging around, both in here and
elsewhere, and the threads requesting help for an 'ECM blowing' issue seems to be one of those
questions where the thread nearly always peters out without any answers posted? :-(

But I don't think that has to happen here. But it's gonna take some careful visual inspection
and a bit of sleuthing to put this ECM fuse blowing problem into the rearview mirror of life?

****

The first step is to identify every nook and cranny of the wiring circuit connected to the ECM fuse
at the factory -- and if you can find *anything additional* added to that circuit, you want to remove
it chop chop! (And if you need that aftermarket load still working, then we figure out what's the
best place to reconnect it elsewhere.)

To lessen the confusion factor on the factory wiring diagram for the ECM fuse, I have taken the
liberty to expunge all the don't care diesel stuff and the other circuit too. Here's what I ended
up with:

You must be registered for see images attach

(origin: '88 FSM Electrical & Diagnosis)

STEP 1: -- Careful visual inspection of every linear inch of wire fed by the ECM fuse:

The short answer that you need to run down the Pink/Black +12v power wire from
the ECM fuse to every one of the following components fed by circuit #439:

* Brake Switch
* EGR Valve (NOTE: Only if truck was originally VIN H = 305)
* Air Divert Valve
* ECM (!)
* EVRV (Electronic Vacuum Regulator Valve - VIN K = 350)
* ESC (Electronic Spark Control module - VIN K = 350)

If you find any of the following on circuit 439, it is an illegal condition and must be fixed immediately:

* Anything connected to this circuit that's not on this diagram. (!)
* Any chafing exposing bare copper between the pink/black wiring and the chassis or other wires.
* Any signs of loose connection/heat generated.

EDIT: Although the following video is for the '96+ GMT400 owners with the ECM fuse blowing, it's
well worth the <4 minute viewing time, for it shows you the kind of wiring harness issues that our
old trucks can suffer from: (ECM fuse blowing due to underhood wiring harness meltdown.)

STEP 2: -- Pay careful attention to exactly what is going on IF the ECM fuse were to blow again. And also
take inventory of your fuse block to see if any other fuses also blew, for this may give us an
important clue.

For example, you didn't tell us if you had an automatic? ~90% of production says it is, so I'm going
to go with the odds. There was an 'ECM blowing' thread in here where fuse blowing seemed random
at first, but after paying close attention @haroldwca deduced that when it was time for the TCC
(Torque Converter Clutch) to engage the fuse blowing would occur? It's a good read, with all the
usual suspects in to assist: (Gent buys GMT400 with electrical connector hanging in the breeze with surprise ending)

Also, that Brake Switch gets involved with enabling the RWAL. (Rear Wheel AntiLock brakes)
More specifically, a signal from the brake switch is mixed in with 4WD circuit - short answer is
RWAL allowed in 2WD, disallowed in 4WD mode? (See attached)

STEP 3 - Still blowing fuses? Try to remove as many electrical loads as possible yet still be
able to test drive the truck. For example, temporarily disconnect the TCC connector to the
transmission, especially if the ECM fuse blows, but only when you get above the normal
engagement speed? Only problem is that some suspects (like the ESC module) have to
remain in circuit in order to drive the truck? (Which reminds me, have you been able to
extract any codes from the ECM? Including any that don't set the SES/CEL light?)

STEP 4 - Something has failed inside the ECM, causing a short when the right sequence of
bumps are hit the driver. Please notice that I put this at the very last step, after first attempting
to prove that the problem is elsewhere. (Steps #1-#3, ECM is victim of power loss caused
elsewhere. Step #4 is ECM is the perpetrator of the fuse blowing.)

EDIT: I found a thread where the ECM fuse was blowing on an '88, and in reply #5
@1989GMCSIERRA shared his experience of the ECM being the root cause, and even
better, how he discovered the internal short: (ECM blowing fuse)

****

The point I'm trying to make is that in the easy peasy world you discover that Chef Boy R Dee
plugged in the rear air bag suspension right into the nearest wire, which just happened to be
a Pink/Black wire, and now we have an illegal wiring diagram condition. Removing said illegal
connection solves problem. Life is good.

In the real world, at some point in the 36 year history of your truck a weird intermittent problem
cropped up in your truck, the root cause was never discovered, the poor thing was troubleshot
& given up on a few times, and it bounced from one frustrated PO to the next frustrated PO. No
doubt that by now this truck is worried about the next stop being the crusher?

****

OK, I'm going to stop here and await your next status report. Please note that IF you can
return the wiring harness to factory condition, then between the all of us the answer will
eventually be found. And you can leave in the aftermarket stuff, but ONLY if you sequester
those ancillary loads to their own dedicated circuits.

Hope you find the above helpful. If I managed to obfuscate the issue, don't
hesitate to reply back and ask for clarification.

Happy Hunting!

Cheers --

 

[Road Trip]

@Pbeanbag said:

"OK Roadtrip, let's go back to the original problem...according to the fuse box diagram the ecm fuse doesnt operat the ecm on gasoline engines right? it operates the throttle body injection."

You know, after I read what you wrote I went back to the factory drawing of the '88 Fuse block,
and boy howdy the way they drew this, it's all too easy to associate the wrong fuse with the wrong circuit,
for the names are *not* closest to the associated fuses?

Check this out -- I used different colored boxes to show which tags go with which fuses:

You must be registered for see images attach

NOTE: The factory manual missed marking the "Stop/Haz" fuse, so I corrected that as well.

****


Anyway, in order to give us the best chance at making this long-distance troubleshooting effort
work, I want to be absolutely sure that we are both referring to the same thing. If you could
pull the fuse in the green box and then verify that the ECM is now dark with the key on, this
would be a good thing. (The quickest way to verify this is that your SES (Check Engine Light)
is off. (Or your scan tool cannot establish communication with the ECM.)


Still can't find the problem? Factory Shop Manual shows how to cheat to win with 'Short Finder'

While poking around in the '88 FSM, I came across a little nugget of electrical troubleshooting gold.
While I've used these at the day job, I didn't realize that at one time the General had their talent
using Short Finders on vehicles. Check this out:

You must be registered for see images attach

I won't belabor the point unless we simply can't find the problem the normal way,
but suffice it to say that these short finders really do work. (FWIW I've attached
a couple of related pages if this test tool piques your interest.)

Sure Road Trip, maybe this technique was in vogue 36 years ago. But is anybody
doing something similar in today's repair environment?

A: Yes. Check out this short & sweet *80 second* video of how these kinds of troubleshooting
tools work to find a real world wiring harness problem:

xc_hide_links_from_guests_guests_error_hide_media

Disclaimer: I know that he was looking for an open instead of a fuse-blowing short, but
in reality both failures are more similar than different - both represent a showstopping
wiring harness failure. And believe me, vintage wiring harnesses instigate more parts cannon
loading/firing than just about anything else. Train yourself to become proficient at
chasing down electrical faults, and you will beat the system -- new vehicle reliability sans
the new vehicle price of admission.

Alright, enough of the chit-chat. Go out and find whatever Hector left behind.

 

[Pbeanbag]

hi guys, yes i pulled the wrong fuse. i got confused on the diagram you sent me. So...today i removed the gauges fuse and the hum was still there. next i pulled the heater fuse and it went away . So, as someone earlier replied it is probably the door actuator. also each time i removed a fuse the truck ran still. now i am not sure if i even have a hot fuel module. here is a pic of Chef Boy-R- Dee lmao. i did mention that i found a bare wire from the tranny harness and taped it up and put convulated tubing over it. it drove around the block about 3 minutes and was ok. still i will wait until my buddy is around to tow me back before venturing out on the freeway. you guys have helped me a lot and i am very grateful. i will keep you all posted as to what i find so others can learn from my experience.

@Pbeanbag said:

"OK Roadtrip, let's go back to the original problem...according to the fuse box diagram the ecm fuse doesnt operat the ecm on gasoline engines right? it operates the throttle body injection."

You know, after I read what you wrote I went back to the factory drawing of the '88 Fuse block,
and boy howdy the way they drew this, it's all too easy to associate the wrong fuse with the wrong circuit,
for the names are *not* closest to the associated fuses?

Check this out -- I used different colored boxes to show which tags go with which fuses:

You must be registered for see images attach

NOTE: The factory manual missed marking the "Stop/Haz" fuse, so I corrected that as well.

****


Anyway, in order to give us the best chance at making this long-distance troubleshooting effort
work, I want to be absolutely sure that we are both referring to the same thing. If you could
pull the fuse in the green box and then verify that the ECM is now dark with the key on, this
would be a good thing. (The quickest way to verify this is that your SES (Check Engine Light)
is off. (Or your scan tool cannot establish communication with the ECM.)


Still can't find the problem? Factory Shop Manual shows how to cheat to win with 'Short Finder'

While poking around in the '88 FSM, I came across a little nugget of electrical troubleshooting gold.
While I've used these at the day job, I didn't realize that at one time the General had their talent
using Short Finders on vehicles. Check this out:

You must be registered for see images attach

I won't belabor the point unless we simply can't find the problem the normal way,
but suffice it to say that these short finders really do work. (FWIW I've attached
a couple of related pages if this test tool piques your interest.)

Sure Road Trip, maybe this technique was in vogue 36 years ago. But is anybody
doing something similar in today's repair environment?

A: Yes. Check out this short & sweet *80 second* video of how these kinds of troubleshooting
tools work to find a real world wiring harness problem:

xc_hide_links_from_guests_guests_error_hide_media

Disclaimer: I know that he was looking for an open instead of a fuse-blowing short, but
in reality both failures are more similar than different - both represent a showstopping
wiring harness failure. And believe me, vintage wiring harnesses instigate more parts cannon
loading/firing than just about anything else. Train yourself to become proficient at
chasing down electrical faults, and you will beat the system -- new vehicle reliability sans
the new vehicle price of admission.

Alright, enough of the chit-chat. Go out and find whatever Hector left behind.