frustrated, fuses blowing

[white72c10]

All lights on the truck work fine until I hit the brakes then I lose tail and brake lights and the dash lights. It blows the 20a fuse every time I hit the brakes. I've removed the rear bulbs and it still blows. I replaced the brake light switch about 2 mts ago and no problems till this week. I've checked the harness in the rear and it looks fine. Any suggestions?

[white72c10]

Update - I unplugged the harness at the rear crossmember and then when I hit the brake the headlights dim and the dash lights go out but didnt' blow the fuse with a 30 amp but smoked the 20 amp. I'm sure this means the problem is in front of the rear crossmember. what should I check next?

[Green Machine]

Unplug the tailight harness at the firewall connector to see if the long wires to the back are bad. Harness may look good, but worth a try. Several things it could be, but try that next.

[sjarrett71]

Bad ground?

[Longhorn321]

My guess is that if it's not the brake light switch and its not in the rear harness, it's in the wiring from the brake light switch to the rear harness. I don't see how it could be a bad ground. You might have better luck tho if you pose your question in the electrical section. Those guys are pretty familiar with these trucks and their wiring.

[kamikazekyle05]

Do you have upgraded lights, maybe its drawing alot of power?

[fixit-p]

you have a short to ground if you unplug the firewall harness as green machine says you could narrow it down to either in the cab or between the cab and rear cross member also if you've done any work on the chassis lately you might check there first.

[white72c10]

Can anybody show me a pic of were the harness is at the firewall? I'm not sure what I'm looking for.

[68gmsee]

Check next to the left of the brake master cylinder looking from the front of the truck.

[68gmsee]

Here's a better one.

[Green Machine]

It hooks up under the dash near the heater, might be easier to unplug from inside the cab.

[white72c10]

Does that harness actually unplug? I found it on both sides of the firewall but dont' see that it comes apart.

[Longhorn Man]

yes it does. 2 sperate harnesses

[white72c10]

did something maybe not so smart. I can't figure this out so I wired in a switch in the panel bypassing the fuse. Held the brake pedal hoping that because there is no fuse to blow something would eventually start smoking. I held the brake for about 5 min and nothing and the switch didn't get hot. by doing this shouldn't I make the short obvious by burning something up?

[Hottrucks]

ummm did anyone ever think that all the brake lights also go thru the turn signals.... try leaving a turn signal or em flashers on ...and keep narrowing it done

[68gmsee]

Originally Posted by white72c10

did something maybe not so smart. I can't figure this out so I wired in a switch in the panel bypassing the fuse. Held the brake pedal hoping that because there is no fuse to blow something would eventually start smoking. I held the brake for about 5 min and nothing and the switch didn't get hot. by doing this shouldn't I make the short obvious by burning something up?

Nooooooooooooooo!!!!! Good way to burn up your whole wiring harness, start a fire or worse lose your truck...

The connector will come apart. I used silcone spray after I used some Armorall to soften the rubber up. It's a tight fit and I had to use a small screwdriver. It will come off.

[Camper Power]

Originally Posted by Hottrucks

ummm did anyone ever think that all the brake lights also go thru the turn signals.... try leaving a turn signal or em flashers on ...and keep narrowing it done

the signal switch is between the brake switch and the rear harness so it could be shorting out.

[fixit-p]

True the short could be in the cab...that why I think it's a good idea to disconnect the brakelight harness at the cab to narrow it down to being in the cab or between the cab and rear harness.

[Longhorn Man]

bypassing the fuse to trouble shoot is a bad idea. i know I'd get fired for doing it.
Have you unhooked the plug on the firewall yet? We are pretty much giving you step by step directions on how to fix your truck.

[Longhorn321]

Originally Posted by Hottrucks

ummm did anyone ever think that all the brake lights also go thru the turn signals.... try leaving a turn signal or em flashers on ...and keep narrowing it done

How about disconnecting the switch at the column? I'm sure I have heard LonghornMan or somebody saying that that is easy enough. And I think I remember some saying that the turn signal switch is a trouble spot. To me, that would be better than seeing if the turn signals blow the fuse....see if it does NOT blow when you disconnect the switch and step on the brakes.

[fixit-p]

If you disconnect at the colum you've pretty much eliminated every thing except for the brake light sw and the wire runnig from there to the plug.

[white72c10]

I disconnected the signal switch under the dash, doesn't blow a fuse but then I don't get brake lights. I still haven't got the harness apart at the firewall but I"m working on it.

[Longhorn Man]

brakes and turn signals are not on the same fuse anyways.

[white72c10]

Ok, finally got the harness at the firewall disconnected and then no blowing fuses. It's better to unhook the harness from under the dash because it wouldn't come out on the hood side. So I guess its back under the truck I go to try to find a chaffed wire or something touching the frame. Brake lights are brown wire correct?

[white72c10]

plugged it back in and now I'm not blowing fuses anymore. Guess I must have moved what ever was shorting out. Seems like the troubleshooting job just got harder cause now I'm not sure its under the truck. THANKS FOR ALL HELP THUS FAR.