Fuse blows when I hit the brake pedal

[jgmc]

What do I check for.

[FamilyTruckster65]

Is the fuse the proper size? Otherwise you are usually looking for a short to ground. Unless it is shorting to another circuit. Is anything else powering on when you activate the brakes?

Check the bulb sockets or the brake switch first, as they are the most accessible. If you see nothing obvious such as melted or chafed insulation start at one brake light and disconnect it from the circuit. Do only one light at a time. with a good fuse or circuit breaker installed activate the brake lights. If the fuse keeps blowing trace your way to the switch looking for a bad spot in the wire.

Carefully inspect where the wires pass corners, clips, holddowns, etc.

[jumpsoffrock]

I had a light bulb filament blow one time, in a way where the remaining piece of filament miraculously shorted out to a metal piece of the bulb base. so whenever that circuit was activated the fuse blew.

Because that rotten bulb would blow the fuse whenever the circuit was active, I couldn't just see which bulb was "out"; I had to remove every bulb and inspect closely one at a time.



RANT:
Of course I discovered this little issue at 5 a.m. in the winter time at a gas station when leaving town for a special weekend getaway with my lady sitting patiently in the passenger seat insisting she wasn't mad, meanwhile, I was blowing every extra fuse I had to try and resolve the issue, because it was still pitch black outside and we apparently were driving with no running/brake lights in the back.
Thanks for listening lol

[RustyPile]

During my years as a mechanic, I "majored" in electrical repairs. I bought this tool off the Matco truck in the 80s.

https://www.toolaid.com/product/25100

It's very easy to use and will take you by the hand and lead you to the short. I never failed to locate a short using it.

[MARKDTN]

Originally Posted by jumpsoffrock

I had a light bulb filament blow one time, in a way where the remaining piece of filament miraculously shorted out to a metal piece of the bulb base. so whenever that circuit was activated the fuse blew.

Because that rotten bulb would blow the fuse whenever the circuit was active, I couldn't just see which bulb was "out"; I had to remove every bulb and inspect closely one at a time.......

Had something very similar in a backup light once. Drove me nuts trying to find it.

[kwmech]

I think I told my cruise control story on here regarding a brake light bulb on a newer vehicle

[HotWheelsFan]

If you have rusty fuse terminals, that can cause fuses to blow.

Fuse heats up due to excessive resistance and breaks the contact where the fuse strip is soldered to the terminal (outside metal ends of glass type of fuses).

Just something to look at during the diagnosis.


Bob

[mikeshome]

I would unplug the plug at the back of the frame and see if it still blows. Then you have it narrowed down to in the truck or the back lights.

[jgmc]

The wiring for the tail lights to the cab is all new. I unplugged it at the rear of the truck and hit the brake pedal and the fuse blew. The left turn signal works but when I turn on the right one it makes a weird noise and burns out the flasher. Ugh �� this is driving me crazy ��

[kwmech]

Now you can unplug at the firewall and try again. Short may be in the column or the turn sig switch. Orange is power to the BL switch and the white wire goes into the column up to the switch and back down both turn sig wires to the tail lamps only to be cancelled on either side depending on which way you activate the signals. So your right turn is probably the culprit

[jgmc]

Originally Posted by kwmech

Now you can unplug at the firewall and try again. Short may be in the column or the turn sig switch. Orange is power to the BL switch and the white wire goes into the column up to the switch and back down both turn sig wires to the tail lamps only to be cancelled on either side depending on which way you activate the signals. So your right turn is probably the culprit

I unplugged at the firewall and pushed the brake pedal and it blew the fuse. I pulled out the turn signal switch and the green wire had been pinched. I wrapped some tape around and the turn signals work both left and right but when I push on the brake pedal it still blows the fuse.
Does it matter how the brake light switch is plugged in, Orange and white wire on either side of the switch?
I’m going to pick up a new turn signal switch and maybe a brake light switch tomorrow and try that. Just don’t know what else to try.

[RustyPile]

Originally Posted by jgmc

I unplugged at the firewall and pushed the brake pedal and it blew the fuse. I pulled out the turn signal switch and the green wire had been pinched. I wrapped some tape around and the turn signals work both left and right but when I push on the brake pedal it still blows the fuse.
Does it matter how the brake light switch is plugged in, Orange and white wire on either side of the switch?
I’m going to pick up a new turn signal switch and maybe a brake light switch tomorrow and try that. Just don’t know what else to try.

The brake light switch is a simple SPST (single pole single throw) switch, and it's only purpose is to interrupt current flow. Doesn't matter which wire is on what terminal as long as both wires are connected. W.A.G. approaches usually result in unnecessary expense. Did you follow the advice given in post #4??

[jgmc]

Originally Posted by RustyPile

The brake light switch is a simple SPST (single pole single throw) switch, and it's only purpose is to interrupt current flow. Doesn't matter which wire is on what terminal as long as both wires are connected. W.A.G. approaches usually result in unnecessary expense. Did you follow the advice given in post #4??

I just ordered one.

[RustyPile]

Originally Posted by jgmc

I just ordered one.

I sent you a PM.

[jgmc]

Originally Posted by RustyPile

The brake light switch is a simple SPST (single pole single throw) switch, and it's only purpose is to interrupt current flow. Doesn't matter which wire is on what terminal as long as both wires are connected. W.A.G. approaches usually result in unnecessary expense. Did you follow the advice given in post #4??

How does it detect a short when the fuse blows as soon as a push the pedal?

[RustyPile]

Originally Posted by jgmc

How does it detect a short when the fuse blows as soon as a push the pedal?

Inside that red cylindrical device is an automatic reset circuit breaker.. Remove the fuse that's blowing and connect the leads of the finder to the fuse terminals in the fuse box. In your case, also install a jumper across the brake light switch.. The circuit breaker will continually make and break causing the current to flow intermittently. Starting at the fuse box, the meter is passed along the wiring harness, and as the circuit breaker makes and breaks the circuit, the meter's needle will deflect in the direction of the shorted circuit.. If the meter passes the shorted branch of the harness, the meter will reverse it's direction of deflection, or in some cases, cease deflecting.. When the meter reaches the point of the short, the meter will stop deflecting in either direction..

Go to the link I posted and download the instructions. Also included in the instructions is a detailed description of how the device works.

[jgmc]

Thanks Rusty Pile for the help! Finally good to go

[RustyPile]

Originally Posted by jgmc

Thanks Rusty Pile for the help! Finally good to go

I'm glad it worked for you.. I came across that Short Finder on a Matco Tool truck some 40 + years ago. I have never failed to locate a short using it..