Gauge lights, lights left on warning chime, and climate control back lights stop working with headlights on…

[That1994wt]

This has been a mess for about a year, gauge lights worked fine then suddenly not.
Under further investigation my lights left on warning (when you turn the truck off and leave the lights on it screams at you) stopped working, along with my climate control lights when you turn the headlights on, but when they’re off it illuminates.
I’ve replaced the headlight switch and dimmer, I have the cluster out with a couple burnt bulbs but figured that doesn’t matter as it’s probably been burnt out since I got the truck. Sounds like a ground issue but where could said grounds be?

It has an aftermarket radio could that maybe be the culprit?
Third brake light have anything to do with it? Leds?
I’m not a mechanic by any means but I can figure stuff out after a while but figured I’d hit the forum and hope someone has had this issue and fixed it before I pull my hair out over it


You don’t realize how bad it is until you drive without gauge lights at night, it sux.

 

[Road Trip]

Greetings That1994wt,

This has been a mess for about a year, gauge lights worked fine then suddenly not.
Under further investigation my lights left on warning (when you turn the truck off and leave the lights on it screams at you) stopped working, along with my climate control lights when you turn the headlights on, but when they’re off it illuminates.
I’ve replaced the headlight switch and dimmer, I have the cluster out with a couple burnt bulbs but figured that doesn’t matter as it’s probably been burnt out since I got the truck. Sounds like a ground issue but where could said grounds be?

It has an aftermarket radio could that maybe be the culprit?
Third brake light have anything to do with it? Leds?
I’m not a mechanic by any means but I can figure stuff out after a while but figured I’d hit the forum and hope someone has had this issue and fixed it before I pull my hair out over it


You don’t realize how bad it is until you drive without gauge lights at night, it sux.

Greetings That1994wt,

Welcome to the GMT400 forum! You're in good company here, lot's of like-minded
folks hang out in this forum who still insist on driving these old gems around.

By the way, your detailed problem description really matches up to the following page
out of the '95 FSM. Without further ado:

Instrument Panel Fuse 14 -- (ILLUM)

You must be registered for see images attach

Note the following items you are having issues with are included in the electrical fan out:

* Audio Alarm module
* Instrument Cluster Printed Circuit Section (illumination bulbs)
* Heater and A/C control lamp section
* Cargo Lamp Switch
* Radio Section (illumination circuit inside radio)

NOTE: This is just the big picture overview of the circuit. When you go to troubleshoot this,
just go to each page documented for each subsection in order to get to pinout details, etc.

Unless you've lurked here for awhile before joining (like I did) you may not be aware
that this forum has a cache of the best documentation there is for keeping a truck
like yours on the road 30 years after it rolled off the production line: The Factory Service Manuals

If you don't already have the full set on hand, head over here and hook yourself up: '88+ GMT400 FSMs

To troubleshoot this, I would simplify this circuit down until it started working properly,
even if you only end up with the Heater & A/C control lamps as your canary. And then
methodically add in the remaining loads, one at a time, until the fault returns. And you'll
know what to do at that point.

By the way, we recently had a member of the forum experience fuse 14 blowing after a skating
rink/fender bender jolt. Pretty interesting read, showing how a small detail can create a bit
of havoc. (Fuse 14 blows every time parking lights turned on after minor incident.)

More in a bit having to do with your request for ground information. Hint: G202 is going to be
your best bet/focus on finding/cleaning/verifying.

Hope you find this helpful. Once you track down the root cause of your problem please be sure
to return and share what you discovered.

Cheers --

 

[Road Trip]

This has been a mess for about a year, gauge lights worked fine then suddenly not.
Under further investigation my lights left on warning (when you turn the truck off and leave the lights on it screams at you) stopped working, along with my climate control lights when you turn the headlights on, but when they’re off it illuminates.
I’ve replaced the headlight switch and dimmer, I have the cluster out with a couple burnt bulbs but figured that doesn’t matter as it’s probably been burnt out since I got the truck. Sounds like a ground issue but where could said grounds be?

Ok, to prove/disprove that the cab interior ground is contributing to the
symptoms you describe, follow this link to where the location of G202 is
identified: (G202 info)

And since we're on the subject of grounds, the quickest/easiest way to
get your truck to act more like it's 30 weeks old instead of 30 years old
is to methodically work through every ground on your ride. It's not
nearly as crazy as it sounds.

I've attached the General's 'big picture' of the ground naming scheme for
the GMT400. And here's one more link, this time to the thread where we
dove down this particular rabbit hole & picked through all the goodies
we unearthed:
(Getting really grounded in order to fix a free truck that was given up on.)

****

If you have any questions about all this, don't hesitate to post them.

And as always, when you solve the puzzle please be sure and reply here
with what was the solution for the benefit of others working similar issues.

Best of luck in your hunt.

Cheers --