My 20 amp brake light fuse blew today when I hooked up to one of my trailers.

The trailer lights are good, no shorts or anything because they work fine on our other truck. My brake lights always work fine too by themselves. But I don't quite understand why only 4 brake lights blew a 20 amp fuse.

All the other trAiler functions worked fine, running lights, turn signals. But when I hit the brakes It blew the fuse. Did the old pickups really draw that much amperage for the brake lights???

I want to be able to pull this trailer. Is it safe for me to bump up to a 25? I've had to bump up a couple other fuses already.

 

Put some LED lights on the trailer; they draw much less current.

 

Too much work for what it's worth. They work fine on every other truck.

 

With all 4 brake lights on it's using too much current. Your fix will probably fix it. 25A fuse. The newer trucks use separate circuits for the trailer

 

I just don't see how 4 little brake light bulbs can draw that much current. Unless there's a ton of resistance in the truck wires.

 

You must have a problem some where. With just adding the 4 extra lights. Hell when, I had a over head camper on it, it had 8 running lights not counting the trucks brake & running lights.

Don't you have a high out put head lights that may add to your extra pull of amperage. But if it's only with this one trailer then I'd trouble shoot it. Remove all the bulbs then put one in at a time until it pops the fuse. some people only ground through the ball not using a ground wire.. orich

 

I stuck a 25 in. They work.

 

Just for giggles 351, if ya get a chance ya might clean and scrape all grounds. Even under the hood. Just a precaution. Zillions of people have hooked up a trailer to our Bumps and didn't have to go up in fuse. Ja know what I mean Mon?

 

I installed a 12v/120/ pro-gas refer in my camper a few months before this and in doing so, must have run a screw through the left side of the O/H camper running lights. Which, I did not know at that time.

So like always checked out everything a week before heading out on a camping trip. While the motor was running hood up checking belts, hoses and so on. I thought to check all lights blinker. So pulled the lights on and they kept going off and on every few sec. with the clicking noise of a relay. I'm think what in hell now is going on.

Well it turned out the be the light switch. Wow some over load going on.
So, I thought, l'll try unplugging the camper and recheck. "Bingo" Now I'm testing each O/H running light for a shorted hot with to ground. Aw turned out to be the one near the refer vent door.

This post has gotten longer the, I though it would cut the grounded wire and bypassed and added a new wire.

Okay my point is my shorted running light Only tripped the light switch built in relay. Never blowing the Main 20 amp fuse.

A number of yrs later after selling the camper, this happen again early one morning after turning on the high beams going to Vegas to install the new motor in 2006 that my son & I built a few months early in the yr .

A new light switch fixed that! orich

 

Bad grounds do not cause fuses to blow.
Tons of resistance in wires does not cause fuses to blow.
You have a short to ground. The way to deal with your problem is to disconnect stuff in sequence and look downstream from the fusebox- with the fuse out- with an ohmeter.

 

351, some of the comments above confuse me and can lead you to a dangerous situation.
Fuses do not protect the stuff that you hang on the end of the circuit; they protect the wire. "Bumping up" to a larger fuse will not solve the underlying problem and may cause a more serious problem.
You may have had a tired 20-amp fuse. Replace it with a new 20-amp and scrub the contacts. Do not go to a higher fuse rating and think that you have solved the problem.
Wires rot and leak to ground. You may have to parallel and replace the wire running along the frame.
Do you remember the "pennies behind the fuse" solution that our oldies used to do to solve blown fuses? You are in the same situation.

Semper Fi

 

Not meaning to be a smarty pants here, just a little humor.
Well oh Cougar, I understand what you are saying but I would say it this way. Normally grounds do not blow fuses. High ampherage OR a surge of it causes fuses to blow. Now to un-correct myself I will put on the table this scenario. How 'bout if a ground on a curcuit with extra lamps on a possible power dragging trailor suddenly re-connects. Via a spark. I put to you that surge COULD be enough to blow a fuse.

 

I hooked it up again and this time measured the current flowing through the fuse. With just the truck brake lights I was reading a flow of 13 amps. With the trailer I was reading 16-17. So I'm not sure why a 20 blew. But I'm leaving the 25 in there anyway just for that much more cushion room.