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2008 crew cab long bed 3500 sw
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Discussion Starter #1
Checked all fuses and bulbs...

2008 LMM gmc crew cab 3500 long bed
Purchased this truck about eight months ago with 140kmi and I’m beginning to wonder if I bought the wrong one.... left blinker flashes in hyper speed. Burns out the fuse inside the dash in a couple minutes if put your foot on the break and use the left turn signal.
Brake light and tail lights work the just flash super fast.. The front stays solid on bright so it lights up both filaments...

Thought it was a wiring harness in the front of the truck but new headlights with wiring harnesses didn’t matter didn’t change a thing disconnected the front headlight assembly still has a super fast flash ....

Disconnected the left tail light from the distribution block didn’t matter same result

disconnected the seven way from the back bumper didn’t change anything

next step I believe it’s possible that there’s a short in the rear distribution block connection for the whole tail light assembly all the connections are clean

anyone got any ideas Beyond what I’ve looked at or anyone else that has had that issue?
I’ve read up on almost everything it’s in here don’t see anything that relates 100% to what this truck is doing

Thanks in advance From a Flustered owner. Lol
 

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Did you test for any hard ground shorts? That's where i'd start. and any grounds for the harness as well to frame/body. Bad grounds can cause some funky stuff, but shouldnt take out a fuse unless its dead shorted.

I feel you. Electrical is tedious and NOT fun. I havent even tackled why my passenger side windows work, but lock, Mirror blinker, Mirror heater, and seat heat is out. Maybe unplugged but i know once i get into it i better have an afternoon planned. Good Luck! Joys of a used vehicle though not knowing who's had their paws into it.
 

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2008 crew cab long bed 3500 sw
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Discussion Starter #3
Did you test for any hard ground shorts? That's where i'd start. and any grounds for the harness as well to frame/body. Bad grounds can cause some funky stuff, but shouldnt take out a fuse unless its dead shorted.

I feel you. Electrical is tedious and NOT fun. I havent even tackled why my passenger side windows work, but lock, Mirror blinker, Mirror heater, and seat heat is out. Maybe unplugged but i know once i get into it i better have an afternoon planned. Good Luck! Joys of a used vehicle though not knowing who's had their paws into it.
Thanks

Thanks It’s on my list of things to check as well . Just spent 3 hours on what I did already I just hate waisted time n was possibly over thinking something.

elect issues are evil.....! Lol
 

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Which fuse is being blown? What specific number? With that information and a wiring diagram from here GM UPFITTER - Body Builder Manuals, you should be able to see what's in the circuit where the fuse is being blown. Are ANY aftermarket electrical devices wired into the truck? Remove anything plugged into the OBD port, cigarette lighter. Eliminate any piece of the circuit to get to the problem area.
 

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2008 crew cab long bed 3500 sw
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10 Posts
Discussion Starter #5
Which fuse is being blown? What specific number? With that information and a wiring diagram from here GM UPFITTER - Body Builder Manuals, you should be able to see what's in the circuit where the fuse is being blown. Are ANY aftermarket electrical devices wired into the truck? Remove anything plugged into the OBD port, cigarette lighter. Eliminate any piece of the circuit to get to the problem area.
All OE stuff
no led lights
Nothing aftermarket
In the dash fuse #6 (LT stop turn)
 

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2021 Denali HD
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I had a 03 2500hd 6.0 years ago, I had weird problems like you describe. I tore the whole truck apart, and it turned out being that distribution box in the rear. I would start there, I believe it wasn’t too expensive
 

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2008 crew cab long bed 3500 sw
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Discussion Starter #7
I had a 03 2500hd 6.0 years ago, I had weird problems like you describe. I tore the whole truck apart, and it turned out being that distribution box in the rear. I would start there, I believe it wasn’t too expensive
I’ve priced it out at the dealership looking like I’m leaning that way $48
 

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2008 crew cab long bed 3500 sw
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Discussion Starter #8
Another 2 hours in da wind today
Cleaning ground on the frame hooking n disconnected cleaned rear distribution block in back
 

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2008 crew cab long bed 3500 sw
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10 Posts
Discussion Starter #9
Which fuse is being blown? What specific number? With that information and a wiring diagram from here GM UPFITTER - Body Builder Manuals, you should be able to see what's in the circuit where the fuse is being blown. Are ANY aftermarket electrical devices wired into the truck? Remove anything plugged into the OBD port, cigarette lighter. Eliminate any piece of the circuit to get to the problem area.
Not sure how to completely understand that wiring diagram lol

I’ve never studied those symbols
 

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If you unplug the rear turn lamp, and you still blow a fuse, the problem is NOT in that part of the circuit. If you unplug the front turn lamp and you still blow a fuse, the problem is NOT in that part of the circuit. What's left? The BCM sends a signal (power) to the relay which closes and then sends power to the fuse to turn on the lamp. Somewhere between the fuse and the lt turn stop junction, there is a short. That's what causes the fuse to blow.
 
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