I was driving and suddenly my engine died. I found the ECM fuse blown. I replaced it and as soons as I opened the ignition it would blow again. I replaced the ignition module and the fuel pump. It still blows the fuse. I'm having no luck finding a wiring diagram for it. Help please
Greetings tuhijo_2000,
Judging from the number of times that a 'ECM fuse blowing' thread is started, only for the
original poster to disappear, this can be a tough problem to track down and repair? From
looking at the FSM (Factory Service Manual) this circuit goes several places, so this kind of
explains why we hear back from so few folks that experience this?
****
But challenging does not mean impossible. But you may want to grab a notepad and
write everything down as you work your way through your 3500 so that you don't get lost
in the details. I'll do my best to make this as clear & concise as possible.
* In the manual, the ECM fuse is identified as the 10A "ECM IGN" fuse. And this fuse provides
the power for Circuit #
439. And thanks to the Stop Lamp/TCC switch, it is also going to provide
power to the entire Circuit #
420. (!) I tell you this because I want you to be looking for all
instances of wiring & connections that implement #
439 & #
420.
Electrical Power circuit fanout for the
ECM IGN fuse:
You must be registered for see images attach
(taken from '88 FSM - omitted Diesel ECM IGN & unrelated B/U light fuse diagrams for clarity)
Obviously this fuse sends power up to 6 different places. (5 engine + 1 brake switch)
What isn't so obvious is the color code at the fuse is Pink w/white stripe, but at
Connector C203 it switches to Pink w/black stripe. But note that both colors are
still identified as part of Circuit #439.
By the way, the '88 C/K Electrical manual uses a more architectural/functional drawing organization.
Personally I find this to be more helpful to get a better feel for the big picture. However, since you
are driving an '89 I am going to include that as well. They changed to drawing the diagrams in
more of a 'physical implementation' style, which (to me) is a bit busier, and harder to follow.
But we are all different, so I wanted to include a taste of both, and let you decide which works
best for you.
So here's how the '89 C/K Electrical Manual illustrates the start of circuit #439:
(Detail of page 82 from the '89 FSM - lower RH corner focusing on Fuse Block)
You must be registered for see images attach
To orient yourself, again look for Pink/white stripe wire and '439'.
On this next page, look for the Stop Lamp/TCC switch in the upper RH corner:
(
Entire page of the previous - p. 82 {.pdf} = p. 78 {paper} )
You must be registered for see images attach
Note that the 12v power from Circuit #439 is connected to Circuit #420 via the Stop Lamp/TCC switch. (!)
And one more page to complete the picture. This details the Circuit #420 fanout:
You must be registered for see images attach
(taken from '88 C/K Electrical supplement)
If you follow Circuit #420 all the way to the end, we are looking looking at the RWAL, Turbo 400
downshift relay (assuming your 454 has an automatic) and a front axle switch if you are 4WD.
And this is on top of the aforementioned 6 different places that Circuit #439 went. There's a
lot of places where a potential chafe could have developed, or for that matter, a failed device.
All of the above is just a basic roadmap for the problem you have shared.
In order to give yourself all the details necessary to maximize the chance of finding this electrical
fault and fixing it, head over to the forum's cache of GMT400 FSM's and at least get the '89
manuals that best match your truck. (
Factory Service Manuals)
And if I were you I'd consider also downloading the '88 manuals. I find that using both together seems
to answer more questions than just using one or the other.
****
One last thing. Most ECM fuse blowing threads end up being a dead end, but if you haven't already
seen it this thread came to a successful conclusion AND the OP came back and identified what fixed
his truck: ("
Another ECM issue")
That's all I got for now. You potentially have a lot of electrical real estate to cover, difficult but certainly
not impossible.
If you have any questions about the above don't hesitate to ask. And when you discover the root
cause and get your truck back up & running, please be sure to come back and document what
you discovered. And we like clear pics, especially of failed bits that others can learn from.
Happy Hunting --
