Brake light fuse blows, steps to diagnose?


Old 01-30-10, 10:46 PM
Thread Starter
Join Date: Jan 2010
Location: new mexico
Posts: 5
Brake light fuse blows, steps to diagnose?

I have an 88 F150 with Uhaul-installed trailer wiring connector. My brake light fuse blows immediately or soon.

I'm comfortable with a multimeter and basic electrical theory.

How do I search for the problem?

Sponsored Links
Old 01-31-10, 01:03 AM
Join Date: Jul 2009
Location: Washington State
Posts: 312
I'd start at the connector first you have a direct short to ground some where.I take it this happens without a trailer hooked up. Sometimes Uhaul uses a bulb that has the "pigtail" to connect to the connector on them as well but if this is a round connector I bet a wire has come loose or is touching the body or ground connection inside of it. Either way start at the connector and work your way up to the pedal.It will involve a lot of elect. tape removal and I use a "bed o nails" clip to test wires as not to strip wires or poke probe holes in them since that leads to more troubles later.Good Luck
Old 01-31-10, 09:38 AM
Join Date: Oct 2008
Posts: 1,312
I assume that the fuse is blowing without a trailer connected?

If the trailer wiring is just a connector which goes between an existing connector, try disconnecting the add-on trailer connector. Then reconnect the factory connectors. Place a plastic bag over the disconnected trailer connector and tie wrap it so it will not dangle. Then try driving around for awhile and see if the problem goes away.

If yes, then one of the wires associated with the add-on trailer wiring is shorting to ground. Make it look like "factory" wiring. Electrical tape around wires, plastic tubing over the wires, then tie wrap in place so the wires can't move when you hit bumps. Wires which can move can rub against sharp metal objects. The metal object cuts into the insulation and causes a short to ground.

If you still have the fuse blowing problem after removing the trailer wiring connector, inspect every inch of wire from each tail light all the way under the vehicle to the engine area.

For wiring running under the vehicle, you would be laying on your back looking up. So you could not see the top of the wire. This of course is where a "cut" in the wire would be! So use a mirror held above the wire so you can see the top side. Also use a flashlight. And use your hand to feel behind wires.

There could be a slim slit cut into the wire like this...

Very hard to find! And this is where a sharp piece of metal is cutting into the wire, then shorting it to ground. This may happen when you hit a bump in the road.

Another way to find these problems is to connect a vehicle horn or 12 volt dc sound making device to battery +. And the other connection to the brake + wire running under the vehicle. Remove the bulbs (path to ground via the element in the bulb). Then go around and "giggle" wires. If that wire contacts a ground, it will make the horn (or noise making device) sound.*

*You would need to be DARN sure of the electrical wiring for the vehicle and TOTALLY understand all wiring before doing anything like this. Some modern vehicles have up to 24 computers and 3 different computer networks. Connecting a noise making device to the wrong wire could cause all sorts of problems and might "fry" an electronic device. So best to have the factory electrical wiring diagrams and fully understand the electrical system before doing anything like this.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes