New marketplace truck, most of the electric does not work.

[Additivewalnut]

I just got a surprisingly nice 1990 2500 350 TBI off marketplace for the low low price of free. Runs and drives better than it should but effectively nothing in the interior works. This thing has some pretty hardcore electrical gremlins, the previous owner did some real mickey mouse bull**** on this thing. To start, the radio has to be turned on with a toggle switch because according to him, "It stopped turning on with the key one day." The gauges all work, but I don't think they read correctly, except for the speedometer which works great. When you hit the turn signals it makes all the gauges peg out, same thing when you turn the headlights on, only difference is with the headlights on, the signals stop working entirely and the fasten seatbelt light comes on, but only sometimes???. None of the backlights in the dash work. The brake warning light is on, so that light does work and it's correctly on as the rear brakes basically don't exist right now. The blower motor blows it's fuse sometimes. If you have the CTSY fuse in, the dash stays on even without the key in. The driver's window doesn't go down but it does draw power, could be motor but who knows. Passenger window still works but you have to help it up and down. Every once in a while, you'll be driving and it'll start bogging unless you give it more gas and it downshifts and it clears up. I've also got a 3 pin connector at the base of the TBI that just... isn't plugged in and I don't see anything it would plug into.

Overall, I'm sure the answer is grounds. I had an 84 el camino that had a lot of these same symptoms and cleaning my grounds + adding a few seemed to help a lot. If that's what everyone here thinks too, is there a diagram of both the engine AND interior grounds? I've looked at a few in the engine bay and some are pretty rusty and don't really look amazing so I'm sure those need to be cleaned, specifically the two on the thermostat housing. Can I add any?

 

[Caman96]

Definitely start by checking all grounds, if they even look a little dicey, I’d replace. Calling @Road Trip for that great picture you posted recently showing all the ground “zones”.
This will be really useful for your situation.

 

[Additivewalnut]

Definitely start by checking all grounds, if they even look a little dicey, I’d replace. Calling @Road Trip for that great picture you posted recently showing all the ground “zones”.
This will be really useful for your situation.

For the grounds that are built into the harness, should I just cut the old ring connectors off and put new ones on?

 

[Caman96]

How bad is the overall condition of harness itself?

 

[Schurkey]

Wild Guess: Grounds on the instrument cluster, failed ignition switch. Potential wiring problems due to prior owner needing his finger broken with his own wrenches. Dirty connections or perhaps popped fusible links at the power-supply junction which--maybe--is on the firewall, engine compartment side, near the hood-hinge area on the right. At least, that's where it is on my '88, and the '89 I looked-after.

 

[Komet]

First thing I do with a basket case is find all aftermarket electrical components and remove them with extreme prejudice. Then I start looking for the hackjob splices, burnt wires, missing stuff, and I return all circuits to factory spec. Sometimes after I'm done with those two things there aren't many problems left.

 

[Scooterwrench]

First go to the sticky on this forum and download the manuals for your truck,you're gonna need 'em.

 

[Road Trip]

Definitely start by checking all grounds, if they even look a little dicey, I’d replace. Calling @Road Trip for that great picture you posted recently showing all the ground “zones”.
This will be really useful for your situation.

No problem Caman96, can do!

Greetings AW, sounds like you have a real opportunity to take a little sweat equity and turn a 'given up on' vehicle
back into a reliable ride. Fortunately, GM did a good job of documenting the wiring harness in your new (to you)
GMT400...so the very first thing to do is to take advantage of that resource by downloading the C/K Service Manual
for your year as recommended above by @Scooterwrench.

Then, in order to make sense of the Ground nomenclature you will see in the manual, refer to this page in order
to get the big picture:

(From the '99 C/K Service Manual -- Other years? Your Service Manual may vary...)

You must be registered for see images attach

More to follow. Between the hard-won experience of the contributors in this forum plus your ability to
navigate around in the Service Manual schematics, I'm confident that you will end up sorting out the
electrical gremlins in your new ride.

Congrats on your good fortune --

 

[Road Trip]

Where the entire GMT400 wiring harness is organized by each individual, numbered, Ground (!)

("Ground Distribution Schematics", p. 8-377 thru 8-395, Wiring Systems, Vol. 4/4 of '99 C/K Service Manual)

For troubleshooters using SDD (Symptom Directed Diagnosis) *these* pages
are where you take all of the flaky electrical behavior where 2-3 or more or more functionally unrelated
subsystems seemingly affect each other?

And once you have all the failures in hand, you check out these pages in order to see if
they share a common ground? If so, then your troubleshooting theory has validity, and instead of
loading the parts cannon and replacing each suspect part, you instead first check/renew the ground
and the associated wiring that all these electrical subsystems share.

****

To illustrate my point, here's a hypothetical scenario. During cool weather, the headlights are fine, but
during hard use or hot weather, the passenger side headlights are noticeably dimmer? Or that sometimes
you notice that the right turn signals are fine during the day, but don't work right at night? To top it all off,
you happened to blow the horn at night & noticed that the passenger side headlights dimmed in time? Or
when the high beams are on, the horn sounds weak/gargly? Meanwhile, if I run the windshield washer motor,
the driver's side headlights dim? WTF? Is this thing haunted? Can I trust it?

Would it make more sense to just trade in my old GMT400 in on a new '23 model?

From my perspective, before we make that kind of financial commitment, let's first take a look at p. 8-377
in the Ground Distribution Schematics section:

G112 -- Strong like bull = a happy front end

You must be registered for see images attach

Is it possible that a dirty/corroded G112 could explain the unwanted interaction between these logically unrelated subsystems?

As Deputy Dog used to say, "It's possi-bull. It's possi-bull."

****

Seriously, if you are having power issues with ONLY 1 electrical subsystem, then as
troubleshooters we first look at the associated fuse(s), and if nothing is blown, then we look for a
grounding issue (broken wire) that only affects that one system.

On the other hand, the hallmark of a shared ground issue is when multiple related or unrelated
systems are adversely affecting each other. As a matter of fact, for confirmation you can
try and force the failure by turning on as many electrical loads as possible for a single ground
and see if you really force the malfunction.

NOTE: All electrical devices work off the difference between the positive voltage and whatever
ground is being provided. So, with a solid ground on one side and the alternator sitting at
14 volts, the headlights are going to be nice & bright.

But if the current drawn by the Auxillary Cooling Fan motor causes the corroded G112 ground to lift +4 volts
higher than true ground, *now* the passenger side headlights are only going to have a 10 volt net difference
across them...and they will be visibly dimmer. (!)

But what if ALL of the failures aren't covered by G112? Now what? Hint, on the very next page there is
a G113. At first I wasn't reading between the lines all that well, but now I see that G112 is shared
by the stuff primarily on the passenger side...while G113 is shared by the electrical loads on the
driver's side? Duh! Looks like I should go ahead and clean both grounds up front!

And that's the hidden troubleshooting beauty of these ground schematics, arranged ground by ground.
Don't believe me? No problem, go check out those 18 pages for yourself, and watch as the mystery of the
GMT400 grounding distribution system disappears right in front of your eyes. No kidding, few things are
better on a cold morning than to test various electrical troubleshooting theories out with a cup of hot coffee
and these pages at the kitchen table before I head outside. (The colder it is, the more of a step-by-step
results-oriented action plan this old dog wants in order to fix it asap! :0)

And to tie this to the previous Truck Zoning post, a ground named G112 or G113 should be located roughly
in what area on a GMT400 vehicle?

****

In my next post, I am going to share where the Service Manual gives me a little verbiage about where
to actually find a particular ground. And we'll even trip over an omission in the book, and how we
overcome it. (Hint: I personally prefer my hardcopy for normal use (just the way I was raised) ...but when
I hit a snag, I don't hesitate to use my laptop to power search thousand of pages quickly via the
downloaded '99 C/K Service Manual .pdf file.

Sometimes modern technology works to the caveman's advantage.

Cheers --

 

[Road Trip]

Hey FSM (Factory Service Manual) ...now that I know which ground I'm interested in, where is it physically located?

("Component Locator", p. 8-402 thru 8-444, Wiring Systems, Vol 4/4 of '99 C/K Service Manual)

For some folks, the "Truck Zoning" picture is all the hint they need in order to successfully find
the actual physical location of a numbered Ground, Splice, or Connector.

For others learning the ropes of the GTM400 wiring, it would be helpful to have a more detailed hint.
Well, today is your lucky day, for in the 42 pages above there's an alphabetized list of electrical-related
stuff, and more importantly, where it physically resides. (Yes!)

By the way, the Alphabetized listing is further divided into the following categories, in this order:

* PARTS
* CONNECTORS
* DIODES
* GROUNDS <--------
* SPLICES
* "Component Views" (hand drawn pictures)

****

For example, let's say that you decide that the highest priority fix is figure out why the driver's
side headlight dims when you run the windshield washer motor. According to the previous post
in this thread, G113 is the ground I want to find, inspect/disassemble/clean/reassemble/verify.

Scanning through the Component Locator section, sure enough I eventually find G113 on p.8-418.
Looked just like this:

You must be registered for see images attach

How cool is that? "On the radiator support, near the left side headlamp."

But wait, for anyone still geeking out on this, there's more:

A little further down the alphabetized list, into the 'Component Views' section:

You must be registered for see images attach

SUMMARY:

So if you are still with me, we've covered a lot of 'ground' this evening. Therefore, let's summarize what we've
accomplished.

* Our original problem description included 2+ intermittent electrical malfunctions affecting logically-unrelated subsystems.

* We theorized that this could be related to a common (shared) marginal ground that is getting lifted above
true ground during periods of higher current flow, adversely impacting subsystem performance by starving them
via a loss of total 'net voltage' drop across the affected subsystem.

* Using the "Ground Distribution Schematics" we discovered that Grounds named G112 & G113 were involved
with the intermittent/flaky subsystems.

* We chose to repair the driver's side headlight/windshield wiper issue first, so we looked up the verbiage for the
physical location of G113 in the alphabetized "Component Locator" listing.

* And finally, a few pages later, we had the line drawing of G113 to give us the no-kidding physical location of the
ground we want to clean/restore to brand new electrical functionality.

And if you've done this in anger in exchange for a paycheck for a good chunk of your career?
Then there's no way you are still reading this, so please disregard all of the above. :0)

But IF you are new to the world of making automotive electrical systems work when *you* need them to, whether in
your driveway or a 1,000 miles away while visiting the WWV towers in Fort Collins, then may I suggest that you go back to
Post #8 and read through this string one more time. (And if you have any questions don't hesitate to ask them.)

Or, after familiarizing yourself with this, do what I was told by a respected elder a long time ago: "If you really want to
learn something, try to teach it to someone else." Father/son, parent/child, mentor/apprentice, doesn't matter, go for it.

Why? Well, it's way more fun to come up with a premeditated fix for a tough intermittent, rather than to curse the electrical
darkness and pleading with the sparky deities to throw you a bone. Or worse, load the parts blunderbuss with increasing
frustration. I've done it all possible ways, and over time learned to prefer the assistance of the General's Service Manual as
opposed to troubleshooting solo. Cheaper too -- have you seen the going shop rates for troubleshooting?

Hope this helps those in the target audience...including the OP?