Tail light fuse keeps blowing
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7th Gear
Joined: Nov 2022
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Tail light fuse keeps blowing
Hi guys I have a 73 f100 4x4 auto and the brake lights stop working every time i drive it around the corner. If its parked it doesn't blow the fuse. My dash lights work, turn signals and lamps work, i replaced both flashers, i replaced both bulbs. Any idea what im missing any suggestions what i should try i appreciate anything thanks
Moderator & parts slinger
Joined: Feb 2007
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From: S/W Missouri, Polk county
You have a hot wire in the tail light wiring harness with a worn/burnt/mice chewed spot that moves when you drove it and it grounds out and pops the fuse. Insp the wiring harness...
Lead Driver
Joined: Jul 2017
Posts: 9,603
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From: San Jose, CA
What happens when you turn on the four-way flashers? What about the turn signals?
What about the flashers and the brakes?
All of those use the same filament in the lightbulb, so if it’s anything between the firewall and the tail lights, it should show up using those as well.
If they work just fine while sitting, you could actually drive it a little bit with those activated and see if it still has the problem.
For example, drive down the street and if possible before the next time you hit the brakes activate the turn signal or flashers.
What about the flashers and the brakes?
All of those use the same filament in the lightbulb, so if it’s anything between the firewall and the tail lights, it should show up using those as well.
If they work just fine while sitting, you could actually drive it a little bit with those activated and see if it still has the problem.
For example, drive down the street and if possible before the next time you hit the brakes activate the turn signal or flashers.
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7th Gear
Joined: Nov 2022
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Thanks i will look into that today, is there any chance that the turn signal switch could be the problem? The lever is kinda loose and i've heard that could be an issue possibly interfering with brake light fuse?
Thread Starter
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7th Gear
Joined: Nov 2022
Posts: 7
Likes: 0
What happens when you turn on the four-way flashers? What about the turn signals?
What about the flashers and the brakes?
All of those use the same filament in the lightbulb, so if it’s anything between the firewall and the tail lights, it should show up using those as well.
If they work just fine while sitting, you could actually drive it a little bit with those activated and see if it still has the problem.
For example, drive down the street and if possible before the next time you hit the brakes activate the turn signal or flashers.
What about the flashers and the brakes?
All of those use the same filament in the lightbulb, so if it’s anything between the firewall and the tail lights, it should show up using those as well.
If they work just fine while sitting, you could actually drive it a little bit with those activated and see if it still has the problem.
For example, drive down the street and if possible before the next time you hit the brakes activate the turn signal or flashers.
I drove it last night using hazards and turn signals and is only when in motion because it can be parked and with all hazards and hitting the brake and it doesn't get blown only when driving it so i was thinking maybe the turn signal switch maybe when i move the steering wheel certain direction that something might be loose inside
FTE Legend
Joined: Nov 2015
Posts: 30,919
Likes: 4,123
From: Angier, NC
If you want to pull the wheel and check go ahead but I dont think that is the issue.
With the truck sitting still and the lights on the blow the fuse and a helper checking when the lights go out get under the truck and start at 1 end and start moving the wire harness working your way to the other end.
Dont over look the tail light harness when moving wires.
When the lights go out & fuse blows that is the area to look for the bare wire that is shorting out when the truck is moving.
Dave ----
With the truck sitting still and the lights on the blow the fuse and a helper checking when the lights go out get under the truck and start at 1 end and start moving the wire harness working your way to the other end.
Dont over look the tail light harness when moving wires.
When the lights go out & fuse blows that is the area to look for the bare wire that is shorting out when the truck is moving.
Dave ----
Thread Starter
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7th Gear
Joined: Nov 2022
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If you want to pull the wheel and check go ahead but I dont think that is the issue.
With the truck sitting still and the lights on the blow the fuse and a helper checking when the lights go out get under the truck and start at 1 end and start moving the wire harness working your way to the other end.
Dont over look the tail light harness when moving wires.
When the lights go out & fuse blows that is the area to look for the bare wire that is shorting out when the truck is moving.
Dave ----
With the truck sitting still and the lights on the blow the fuse and a helper checking when the lights go out get under the truck and start at 1 end and start moving the wire harness working your way to the other end.
Dont over look the tail light harness when moving wires.
When the lights go out & fuse blows that is the area to look for the bare wire that is shorting out when the truck is moving.
Dave ----
so i went through the wiring today and it looks like everything is in place today i only pulled forward like 6ft then reversed it and it blew the fuse again, i noticed the reverse lights arent working at all i get power on the fuse box but nothing on the sockets and i spliced them a little to test em before the socket and no signal at all. I will try a new turn signal/hazard switch and possibly the neutral safety switch maybe??
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Hotshot
Joined: Feb 2004
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From: The Shenandoah Valley
How did you manage to check the 4 wire harness from just above the rear axle back to the rear cross member behind the gas tank. Where my problem was.
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Mountain Pass
Joined: May 2018
Posts: 133
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I believe that there is a bad ground feeding power into that system. Clean the bulb sockets, I like using a plumbers wire brush for cleaning 3/4" copper pipe, and replace them. Cheap to do and good maintenance. I've had the filament break and fall on the outside brass causing shorts.
. When I hear about electrical problems that act weird it is usually ground problems. And bad ground is the reason for most electrical problems. BTW there is a ground strap from body to frame under the driver seat. Check it. Good luck n stay positive
. When I hear about electrical problems that act weird it is usually ground problems. And bad ground is the reason for most electrical problems. BTW there is a ground strap from body to frame under the driver seat. Check it. Good luck n stay positive
FTE Legend
Joined: Nov 2015
Posts: 30,919
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From: Angier, NC
so i went through the wiring today and it looks like everything is in place today i only pulled forward like 6ft then reversed it and it blew the fuse again, i noticed the reverse lights arent working at all i get power on the fuse box but nothing on the sockets and i spliced them a little to test em before the socket and no signal at all. I will try a new turn signal/hazard switch and possibly the neutral safety switch maybe??
If you were moving the wires (on the frame?) and it popped the fuse why would you want to change out the turn signal switch?
If the frame harness has a plug for the tail lights unplug the tail lights and do the pull test again.
If it popps it is not the tail light harness and you need to look over the wires that moved a lot closer.
For the back up lights do you have power to the switch?
If not you need to find out why and fix.
If you have power into the switch but not out then I would say the back up light switch is bad.
I have never seen a bad ground blow a fuse but the power wire to ground will blow it every time!
Dave ----
Mountain Pass
Joined: May 2018
Posts: 133
Likes: 21
Ok you have 2 things going on here so lets look at the blown fuse one first.
If you were moving the wires (on the frame?) and it popped the fuse why would you want to change out the turn signal switch?
If the frame harness has a plug for the tail lights unplug the tail lights and do the pull test again.
If it popps it is not the tail light harness and you need to look over the wires that moved a lot closer.
For the back up lights do you have power to the switch?
If not you need to find out why and fix.
If you have power into the switch but not out then I would say the back up light switch is bad.
I have never seen a bad ground blow a fuse but the power wire to ground will blow it every time!
Dave ----
If you were moving the wires (on the frame?) and it popped the fuse why would you want to change out the turn signal switch?
If the frame harness has a plug for the tail lights unplug the tail lights and do the pull test again.
If it popps it is not the tail light harness and you need to look over the wires that moved a lot closer.
For the back up lights do you have power to the switch?
If not you need to find out why and fix.
If you have power into the switch but not out then I would say the back up light switch is bad.
I have never seen a bad ground blow a fuse but the power wire to ground will blow it every time!
Dave ----
FTE Legend
Joined: Nov 2015
Posts: 30,919
Likes: 4,123
From: Angier, NC
Power to 1 side of filament and the other side to ground?
I have seen the filament fall across the other filament and cause a feed back problem is that want you meant?
I dont think I have seen that blow a fuse just pull your hair out as other things light up when they should not almost like a bad ground.
I think he said he replaced the bulbs or was that a different post from someone as I read a lot of posts.
Dave ----
Thread Starter
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7th Gear
Joined: Nov 2022
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I could be wrong but I thought that is how bulbs worked?
Power to 1 side of filament and the other side to ground?
I have seen the filament fall across the other filament and cause a feed back problem is that want you meant?
I dont think I have seen that blow a fuse just pull your hair out as other things light up when they should not almost like a bad ground.
I think he said he replaced the bulbs or was that a different post from someone as I read a lot of posts.
Dave ----
Power to 1 side of filament and the other side to ground?
I have seen the filament fall across the other filament and cause a feed back problem is that want you meant?
I dont think I have seen that blow a fuse just pull your hair out as other things light up when they should not almost like a bad ground.
I think he said he replaced the bulbs or was that a different post from someone as I read a lot of posts.
Dave ----