Blown Fuse

[Bronco Maniac]

I pulled my light switch the other night, the lights came on and then immediately blew the fuse. I put another fuse in the other day and it blew as soon as I pulled the switch on. Where do you start and how? Wondering if the switch may have gone bad? Any assistance would be appreciated.

 

[blade]

Does it have original style bulbs for the headlights. If they are a different type and they are creating too much draw you could wire in some relays.

 

[Eoth]

I had that problem and it was driving me crazy. Turned out to be some old spliced wiring from a PO that added plow lights. When he removed the plow he just cut the wires and left them. I finally discovered the source of the issue on the side of a dark road with my hood up. I saw the blue arcing of the wire that was shorting out on the body.
I would start by just following the wires from the headlight back to the fuses. Look/feel for any bare spots that might have rubbed through over the years.

 

[Bronco Maniac]

I wired the truck with a Painless Harness about 6-7 years ago. Original style bulbs.

 

[4xfun]

Don't cheat and use something to bypass the fuse. That is an easy way to burn your truck up. Time to get familiar with a wiring diagram, volt meter, a jumper wire with a fuse, and a short circuit finder.

Actually, I would suggest first checking your light switch and the floor dimmer switch to see if something has created a short across the wires and cleaning the ground wire connections to the grill. Check the easy items first.

With a short circuit finder, you can send a tone down a wire to see if there is continuity all the way to the next switch, light , junction, etc.. It is also a good way to follow the wiring and look for burned wiring, cuts, or squirrel damage.

With a 10 gauge positive jumper wire (that includes a fuse), you can bypass the light switch and see if the issue is before the floor switch, between the floor switch and the bulbs, after the floor switch. or with the bulbs. Have plenty of spare fuses and don't use a fuse larger that the one in the circuit.

With a voltmeter, you can check the switches and wires for resistance/continuity or power. If you have not used a voltmeter, I am sure someone will chime in with a good videos and writeup. A battery, a piece of wire, a bad fuse, and an unplugged electric heater are good items to practice on and see the difference between how you check for volts and resistance and how a switch stops the circuit.

 

[Bronco Maniac]

Don't cheat and use something to bypass the fuse. That is an easy way to burn your truck up. Time to get familiar with a wiring diagram, volt meter, a jumper wire with a fuse, and a short circuit finder.

Actually, I would suggest first checking your light switch and the floor dimmer switch to see if something has created a short across the wires and cleaning the ground wire connections to the grill. Check the easy items first.

With a short circuit finder, you can send a tone down a wire to see if there is continuity all the way to the next switch, light , junction, etc.. It is also a good way to follow the wiring and look for burned wiring, cuts, or squirrel damage.

With a 10 gauge positive jumper wire (that includes a fuse), you can bypass the light switch and see if the issue is before the floor switch, between the floor switch and the bulbs, after the floor switch. or with the bulbs. Have plenty of spare fuses and don't use a fuse larger that the one in the circuit.

With a voltmeter, you can check the switches and wires for resistance/continuity or power. If you have not used a voltmeter, I am sure someone will chime in with a good videos and writeup. A battery, a piece of wire, a bad fuse, and an unplugged electric heater are good items to practice on and see the difference between how you check for volts and resistance and how a switch stops the circuit.

Thanks for the suggestions. I am a firm beleiver if you have electrical problems on a Bronco to check your grounds. If the problem was with the "gound wire connections to the grill" would this cause a blown fuse or the lights would just not come on?

I will try to check the dimmer switch to see if it is possibly causing my problem as it is easy to get to. Then maybe replace the light switch I guess. That's going to be a pain. All the wires under the hood are wrapped with the braided wire loom. No evidence of any kind of damage.

 

[Jdgephar]

Bad grounds would cause things to either not work, or just be wierd, but not blow fuses.

The wire loom could be hiding something, maybe a splice has overheated and melted to another wire. Double check anywhere the wires pass through metal panels for chafing. On the headlight side of the harness, you can use a meter to measure resistance to ground - leave the fuse out and disconnect the headlights. You can start to narrow down where the short could be.

Sent from my SM-G981U using Tapatalk

 

[904Bronco]

Another thought...
The quality of replacement switches is not the same anymore. I have had bad replacement switches brand new in the new replacement wiring harnesses I have used. Some have failed shortly after being installed.

 

[DirtDonk]

Have you tried turning the headlight switch on just to the first notch to see if anything happens? Or does it only blow a fuse when you pull it out all the way?
Another test to see if it’s before the dimmer switch would be simply to unplug the dimmer switch.
You can also do that at the headlights themselves if the lights are going out only after you turn the switch to its second position.
If you unplug the lights and it still continues to blow the fuse, then you know it’s further up line somewhere.
Which is the assumption at this point anyway, but it doesn’t hurt to narrow things down as you go.

I didn’t know that the painless harness had an actual headlight fuse, so I’ll have to go back and review the diagrams. Usually they rely on the circuit breaker built-in to the headlights switch.
But if there is a headlight fuse, it’ll just require more in depth searching.

 

[Bronco Maniac]

Bad grounds would cause things to either not work, or just be wierd, but not blow fuses.

The wire loom could be hiding something, maybe a splice has overheated and melted to another wire. Double check anywhere the wires pass through metal panels for chafing. On the headlight side of the harness, you can use a meter to measure resistance to ground - leave the fuse out and disconnect the headlights. You can start to narrow down where the short could be.

Sent from my SM-G981U using Tapatalk

Thanks. That's what I thought on the ground. I hope you are wrong on the loom. Lol Good info on measuring resistance. I had started with unplugging the headlights today. I was going to determine that the bulbs were not causing the problem first. That's about as far as I got.

 

[Bronco Maniac]

Have you tried turning the headlight switch on just to the first notch to see if anything happens? Or does it only blow a fuse when you pull it out all the way?
Another test to see if it’s before the dimmer switch would be simply to unplug the dimmer switch.
You can also do that at the headlights themselves if the lights are going out only after you turn the switch to its second position.
If you unplug the lights and it still continues to blow the fuse, then you know it’s further up line somewhere.
Which is the assumption at this point anyway, but it doesn’t hurt to narrow things down as you go.

I didn’t know that the painless harness had an actual headlight fuse, so I’ll have to go back and review the diagrams. Usually they rely on the circuit breaker built-in to the headlights switch.
But if there is a headlight fuse, it’ll just require more in depth searching.

30 A fuse I have pulled the switch all the way out. I also like the idea with the dimmer switch. As mentioned above, I had started with unplugging the headlights today. I was going to determine that the bulbs were not causing the problem first. That's about as far as I got. Thanks for the info.

 

[DirtDonk]

Wow, 30 amps eh? That’s a lot of fuse for headlights. I wonder what else is on that circuit.
Even using one hundred watt high beams and adding another fifty watts for good measure is only pushing 20a or so.
But blowing a thirty amper like that probably indicates a dead short somewhere along the line.

But definitely test with the switch only pulled out to the first notch. Just to make sure that the fault lies in the headlight circuit only and not with the running light circuit.

Speaking of overloads…is there anything else you have installed that might be putting too much load on the lighting circuits?
A long shot maybe, but worth asking.

 

[Bronco Maniac]

Wow, 30 amps eh? That’s a lot of fuse for headlights. I wonder what else is on that circuit.
Even using one hundred watt high beams and adding another fifty watts for good measure is only pushing 20a or so.
But blowing a thirty amper like that probably indicates a dead short somewhere along the line.

But definitely test with the switch only pulled out to the first notch. Just to make sure that the fault lies in the headlight circuit only and not with the running light circuit.

Speaking of overloads…is there anything else you have installed that might be putting too much load on the lighting circuits?
A long shot maybe, but worth asking.

Nothing added to the circuit. Thanks for the suggestion.

 

[jamesroney]

Nothing added to the circuit. Thanks for the suggestion.

Do what @DirtDonk recommended. Install a 20 amp fuse, and then pull the headlight switch 1/2 way to the parking lamp position. Check for illumination of the dash lights, tail lights, parking lights, and marker lights. If they are all OK and they all work...then move on to headlights. If the fuse blows, then STOP working on the headlight circuit, and start working on the parking light circuit. Be cautious of gear shift indicator illumination lamps, if so equipped.

If the fuse does not fail at the 1/2 way position, and everything looks good, then move on to the headlights. If the fuse blows in the headlamp position only...go unplug the dimmer switch at the floor. 50% chance that the problem is down there...

 

[Bronco Maniac]

OK Sorry for the delay. The holidays and Bronchitis for the last month. I had unplugged the headlights. Pulled the switch halfway and no problems with parking lights. Pulled all the way out and did not blow the fuse. Put some dielectric grease on headlight plugs (mainly to lube the connections). Plugged one side in and all good. Plugged the other side in and no problem. At some point I hit the dimmer switch and no problem.

So I didn't find the problem. But it doesn't seem to be a problem now?????

Thanks for all the input.

 

[DirtDonk]

Well, a gone problem is a good problem.
At least temporarily!

You may have cured a buildup of resistance, but that doesn’t really sound like what your problem is. However, if it works for now, then great.
You’d probably have a hard time tracking it down now anyway, just looking for something out of place, rather than being able to test it with the symptoms.
Good luck and have fun!

And thanks for the follow up. We will be waiting to hear any future issues, but hoping there are none.

 

[Bronco Maniac]

Well, a gone problem is a good problem.
At least temporarily!

You may have cured a buildup of resistance, but that doesn’t really sound like what your problem is. However, if it works for now, then great.
You’d probably have a hard time tracking it down now anyway, just looking for something out of place, rather than being able to test it with the symptoms.
Good luck and have fun!

And thanks for the follow up. We will be waiting to hear any future issues, but hoping there are none.

Thanks. Hoping for none.