Low beams only stay on for 20 seconds

[GrimsterGMC]

Any idea where the thermal circuit breakers are?

There are 2 in the fuse box, they have aluminium cases and plug in like fuses but they reset after cooling down. I can't be sure that they supply power to the headlights but the symptoms sure point that way.

 

[Road Trip]

Hey guys, having an issue with the 94 Yukon. I have turn signals and high beams, but when I turn the truck on the low beams only stay on for around 10 seconds then shut off and won't come back on.

I've checked the fuses and also replaced the headlight switch, which didn't fix the issue.

Anyone had this problem?

Greetings jfletch,

I washed your problem description against the headlight circuit theory description in the '94 Electrical Diagnosis FSM,
and I think I have an idea of what's going on. But before I write that, first here's what I read:

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(source: 1994_NATP-9442_DRIVEABILITY_EMISSIONS_ELECTRICAL_DIAGNOSIS_MANUAL)

So from here it looks like the problem definition is as follows:

* No problem observed with the headlights off.
* No problem observed with the High Beams on.
* Replacing the headlight switch did *not* solve problem. (ie: verifying that self-resetting circuit breaker
in original headlight switch was not at fault via tripping too soon. NOTE: Correlates with high beams not tripping.)

Given the above, the problem description is that somewhere in the low beam circuit (only)
excess current is being drawn. A close inspection of every inch of the low beam wiring between
the headlight switch and both low beam bulbs should uncover the source of the fault.

Any idea where the thermal circuit breakers are?

A: In the headlight switch. But thanks to the headlight switch replacement, instead of the
circuit breaker being the perpetrator of the fault, an excess current draw on low beams only
is the root cause, and both thermal circuit breakers in both headlight switches were working
as designed.

 

[jfletch]

Greetings jfletch,

I washed your problem description against the headlight circuit theory description in the '94 Electrical Diagnosis FSM,
and I think I have an idea of what's going on. But before I write that, first here's what I read:

You must be registered for see images attach

So from here it looks like the problem definition is as follows:

* No problem observed with the headlights off.
* No problem observed with the High Beams on.
* Replacing the headlight switch did *not* solve problem. (ie: verifying that self-resetting circuit breaker
in original headlight switch was not tripping too soon. NOTE: Correlates with high beams not blowing.)

Given the above, the problem description is that somewhere in the low beam circuit (only)
excess current is being drawn. A close inspection of every inch of the low beam wiring between
the headlight switch and both low beam bulbs should uncover the source of the fault.



A: In the headlight switch. But thanks to the headlight switch replacement, instead of the
circuit breaker being the perpetrator of the fault, an excess current draw on low beams only
is the root cause, and both thermal circuit breakers in both headlight switches were working
as designed.

This is a hell of an answer. Thanks so much.

I'll trace all of the wiring down and see if I can find the short before I try replacing the dimmer switch.

 

[Road Trip]

Saw another thread here that said you can pull the jumper connecting to the

This is a hell of an answer. Thanks so much.

I'll trace all of the wiring down and see if I can find the short before I try replacing the dimmer switch.

I agree with your proposed sequencing of repairs. It *could* definitely be the dimmer switch,
but personally I would want to prove to myself that there isn't an issue with the rest of the
low beam wiring harness first.

And if I don't find anything in the wiring harness? Then I would definitely go after the dimmer
switch with intent.

That sounds like a thermal circuit breaker doing it's job. If there is a higher than expected load the bi-metal strip contact will heat up and pull away from the fixed contact and open the circuit breaker. Once it has time to cool down it will work again, unless the reason for the high current draw is still there and it will heat and break contact again. I would suggest looking for a short in your low beam wiring that would cause a higher than usual current draw.

*Reply collision*

You posted this while I was poking around in the FSM. Looks we independently came to the
same conclusion. We're definitely on the same page on this.

 

[Road Trip]

I agree with your proposed sequencing of repairs. It *could* definitely be the dimmer switch,
but personally I would want to prove to myself that there isn't an issue with the rest of the
low beam wiring harness first.

And if I don't find anything in the wiring harness? Then I would definitely go after the dimmer
switch with intent.

@jfletch, I dug a little further, and thankfully the General was able to put the *entire*
headlight wiring harness onto a single page. Give this a gander, and afterwards I'll explain how
we can decide if the problem is the switch or wiring harness easily:

You must be registered for see images attach

If we were to disconnect C203 from the Headlamp Dimmer Switch, and then jumper from Pin D
(from the headlight switch) directly to Pin A (wire to low beam headlamps) ...IF the low beam headlights stay
on for more than twice the observed failure interval as long as you want them to then we now *know* it's the
'Headlamp Dimmer Switch'.

And conversely, IF the low beam headlights still cause the circuit breaker in the headlight switch to trip,
then we now know that the problem is located somewhere in the harness between the headlamp dimmer switch on the
steering column and the low beams on the front of the truck.

****

Believe me, I am most familiar with checking a wiring harness as a hunch and no joy. Also messing with
a replacing a fiddly switch on a hunch and no joy afterwards. Both scenarios are frustrating & time consuming.

On the other hand, once I've proven to myself that it's the harness or the switch, then it's not nearly
as frustrating to find/fix the problem. Instead of wondering if/when are we ever going to be able to fix the
problem, now it's just a matter of time, for we've regained control of the circuit, figuratively speaking. (!)

When you solve the problem, please be sure to report back with what it took to fix it so that others
suffering a similar issue can benefit from your efforts. (And if it's a short-circuit in the switch or a chafed wire
in the harness somewhere, please try to get a sharp photo of when you caught it red-handed & post that as well. :0)

Happy Hunting --

 

[AuroraGirl]

That sounds like a thermal circuit breaker doing it's job. If there is a higher than expected load the bi-metal strip contact will heat up and pull away from the fixed contact and open the circuit breaker. Once it has time to cool down it will work again, unless the reason for the high current draw is still there and it will heat and break contact again. I would suggest looking for a short in your low beam wiring that would cause a higher than usual current draw.

the headlight circuit uses a CB?

 

[AuroraGirl]

Does OP have fog lamps?

You must be registered for see images attach

 

[AK49BWL]

the headlight circuit uses a CB?

It's built into the headlight switch itself. All GMT400s.

 

[AuroraGirl]

It's built into the headlight switch itself. All GMT400s.

Did not know that, my suggestion now turns to being there is a short to ground somewhere on the low beam wiring which is overloading the CB

 

[jfletch]

Got it figured out!

It was a bad dimmer switch

Went and looked over everything and the switch itself and the connector were all burnt up and melted looking.

Got everything replaced and headlights are working!

What I didn't realize is that the dimmer switch wasn't actually making the click sound when I tried to switch between high beams and low beams, so that was probably a warning sign to begin with.

Thanks for all the help guys you're the best!

On to the next thing trying to get this old girl back to running like new!