Tail Lights Blow Fuse: Please Help!

You've probably got insulation scraped off one of the taillight circuit wires where it goes through a hole in the body, trunk lid, etc. What I like to do is connect an old headlight bulb in place of the fuse. Make a connector out of a blown fuse and wire up to that, and plug the modified fuse into the taillight fuse location.

The headlight bulb will glow brightly while the short is present- wiggle the taillight circuit wires until the bulb goes out or glows dimly, and you know you've found the offending spot.

Note that you have to use a bulb that draws more than the taillights do in order for this to work- a #194 instrument lamp bulb won't tell you anything since it's small compared to the load drawn by one or more taillight bulbs.

You have a short circuit somewhere. It's not horribly bad or the lights wouldn't have come on at all -- you'd just see the fuse blink and that would be it.

If you're lucky one of the bulbs has burnt out in such a way that it's shorted. If you're not, then the most likely culprit is that one of the tail light wires has rubbed off some insulation and is touching metal (although this isn't terribly consistent with the "goes on for a moment" bit).

If you have a meter, turn off the lights, connect the meter between the tail light power and ground, and check the resistance. Then mess with things.

If you don't have a meter, get some fuses and mess with things.

Remove all the bulbs.

Check the meter and note the resistance to ground. Barring oddball wiring, it should be very high (certainly over 100 ohms, probably in the tens or 100's of thousands). If you don't have a meter, plug in a fuse, turn on the lights, see if the fuse survives. If it does, get new bulbs.

If that doesn't fix it, inspect the wires from the tail lights _carefully_. Look for worn insulation somewhere -- without looking all I can say is to see if there's a projecting metal bracket that may have scrubbed off some insulation. With the meter you can wiggle/tug/pull on the wires while watching the reading -- if the resistance to ground suddenly gets high then you're hanging onto the wire with a problem. If it gets high and _stays_ high you're not done -- you need to find the short and fix it, 'cause you don't want your tail lights crapping out at rush hour during a rain storm.

Once you've found the worn spot, wrap it with electrical tape, and if it seems indicated maybe try to hold the wire down so it won't rub on whatever rubbed it raw in the first place.

Good luck -- finding short circuits is a PITA.

That is an excellent idea! Thanks.

Trailer hitch?

Something is shorted to ground somewhere. Disconnect the tail light wiring and remove the bulbs. Put a new fuse in and hit the brakes. Does the fuse blow?

Somewhere, either before or after the tail light switch, the tail light wire is touching the car chassis. First find out if it's before or after the switch, then find out if it's in the lamp holders itself.

--scott

I like this approach.. It is safe, and will give you a simple and positive indication to help you troubleshoot the system.

If a beginner tries to use a voltmeter or ohmmeter, he or she can get confused easily. Using this approach, you cant blow the system or misinterpret what is happening.