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 t
he 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 --