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Old October 12th, 2019, 02:17 PM   #1
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Default Brake light fuses keeps blowing - SOLVED

So, my problem is that I do not have brake lights. The fuse blows almost as soon as you hit the pedal, the lights come on for a second, then the fuse blows.

Here is what i've done/checked so far:

1) Replaced the fuse (dozens of times) blows no matter what. 20, 25, and 30 all blow in about the same time. The 30 took a second or two more.

2) unplugged wiring harness at the connector in the back for the lights (under rear bumper). Fuse still blows.

3) unplugged the harness connector under the hood right when it comes out of the firewall, fuse doesn't blow.

3a) checked the wires going from the fire wall to the back of the truck
short, wires all look great, no corrosion, no cracked wire insulation.
3b) disconnected the 3rd brake light wire at the light, fuse still blew.

4) disconnected the brake light switch on the foot pedal, fuse didn't blow. Then replaced the brake light switch, and fuse still blew. unplugged the brake light switch, and fuse didn't blow.

5) unplugged the multi-function switch from the harness under the dash, and the fuse still blew.



So, to summarize, unplugging the wiring harness at the firewall doesn't blow the fuse. Unplugging the brake light switch doesn't blow the fuse. Brake light switch is new.

This sounds like it's a simple short to ground, since all the fuses blow, and then don't blow with certain wires disconnected. Yet, nothing I see shows a short. I Found a wiring diagram but it doesn't show the brake light switch side of it.

I am at a loss, and ready to crash this f-ing thing into a very large ditch.


I've been trying different combinations of unplugging and plugging in wiring, and so far, that's the only thing I found that makes the fuse not blow, is unplugging the brake light switch, and the harness at the fire wall.
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Last edited by wave_crusher; October 15th, 2019 at 08:27 AM.
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Old October 12th, 2019, 03:07 PM   #2
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Quit using your brakes. Duh
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Old October 12th, 2019, 05:15 PM   #3
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short somewhere wire to metal
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Old October 12th, 2019, 06:50 PM   #4
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I know you mentioned you unplugged the wires at the back and it still blew. Would this be before the hitch wiring or after? The few times I had a problem with wiring on a truck it had something to do with the hitch wiring.
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Old October 12th, 2019, 08:30 PM   #5
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Quote:
Originally Posted by capone View Post
I know you mentioned you unplugged the wires at the back and it still blew. Would this be before the hitch wiring or after? The few times I had a problem with wiring on a truck it had something to do with the hitch wiring.
I unplugged the harness before the hitch wiring. Took everything out, unplugged the trailer lights and taillights and the fuse still blew.

Then I moved to the connector next to the firewall under the hood. If I unplug that (the gray connector that has the taillight wires and the purple fuel pump wire) then the fuse doesn't blow. I pulled all the wires out of the loom going back and they all look good, clean, no weird corrosion or anything that indicates they'd be bad.

So thinking on it, the brake light switch being unplugged and not blowing the fuse can probably be thrown out, since all that means is theres no power going to the short. So it doesn't really tell me anything, since if that was the problem, I'd think the fuse would blow with the taillight wires unplugged.

So, if the fuse doesn't blow from under the dash to the firewall, but does blow fro the firewall back....then theres some sort of short, right? Either in the wires that you cant see or even the connector it self.
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Old October 14th, 2019, 10:04 AM   #6
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I have had a similar issue in the past with full size GM cars and square body trucks. The first time it happened it drove me crazy trying to find it, was told to check the connection on the steering column. Sure enough there was a bad connection where is plugged into the column (would have never thought of that on my own). When it happened in other vehicles, that became the fist place I looked, and usually was where I found the bad wire.
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Old October 14th, 2019, 10:20 AM   #7
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Quote:
Originally Posted by Iggy View Post
I have had a similar issue in the past with full size GM cars and square body trucks. The first time it happened it drove me crazy trying to find it, was told to check the connection on the steering column. Sure enough there was a bad connection where is plugged into the column (would have never thought of that on my own). When it happened in other vehicles, that became the fist place I looked, and usually was where I found the bad wire.
After work today I plan to check all the wiring at the connectors, I scoured up to page 3 of google search and finally find some more useful info other than 100 people saying "your taillight bulb/panel is bad". Seen a few people on GMT400 that had a similar issue and it turned out to be burned wires at or near the column, like yours.

Just weird that if I unplug that connector at the firewall under the hood, it doesn't blow the fuse. Though I have to keep reminding myself that shit rarely makes sense with auto electrical faults.
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Old October 15th, 2019, 08:27 AM   #8
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Found the issue, and it is both a stupid simple problem and one that I caused-ish.

Long story short, I found out that those trucks (at least some of them) are pre-wired with an spare aux power in the back for something like a truck topper that has a brake light on it. Well, me, thinking everything for the lights is a white ground, grounded it. Yes, I had a brake power wire directly bolted to a nice solid ground.

The kicker is, it has been like that for a long time. Something shook loose or shook tight, depending on how you look at it. I took a 10x12 shed out to our hunting camp and after that I was informed that my brake lights didn't work...then it turned into this whole process. So, the only that I can think is that it was a shitty ground, that shook and cleaned itself up?

I don't know...at this point, not going to look a gift horse in the mouth and just leave it be.
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Old October 15th, 2019, 09:29 AM   #9
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At least you figured it out.
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