Nonsensical taillight problem

[audiomagnate]

I'm measuring 12 volts at three sockets, but only one works. It makes no sense. I know how to troubleshoot audio gear and this is extremely simple in comparison. I've wire brushed all the contacts and pried up the positive contact. I know the bulb is good, I know it's making contact with the 12 volt center tab because I feel the spring resistance. This makes no sense! My meter's ground lead is probing the actual socket ground, not the chassis. This one's a real head scratcher.

 

[boonies]

I would test the grounds for each of those sockets using a multi-meter set to ohms. One lead on the negative of the battery and one on the negative of the socket and look for high resistance. If you find high resistance then begin with cleaning the ground screw, check the wire connectors, etc.

 

[jmackro]

I would test the grounds for each of those sockets using a multi-meter set to ohms. One lead on the negative of the battery and one on the negative of the socket and look for high resistance. If you find high resistance then begin with cleaning the ground screw, check the wire connectors, etc.

Excellent suggestion. I presume you are using incandescent bulbs and not LED's. A multimeter draws only a few milliamps, so if there is resistance somewhere in the circuit, there won't be much voltage drop. E.g., your multimeter will still show ~ 12V when measuring an empty socket. But an 1156 bulbs draws ~2 amps, resulting in enough voltage drop that the bulb remains dark if there is resistance in the circuit.

Hint: look at the rivets that secure those brass bus bars that run across the backside of the taillight housings. If the brass has corroded, the resistance could be there.

 

[audiomagnate]

Excellent suggestion. I presume you are using incandescent bulbs and not LED's. A multimeter draws only a few milliamps, so if there is resistance somewhere in the circuit, there won't be much voltage drop. E.g., your multimeter will still show ~ 12V when measuring an empty socket. But an 1156 bulbs draws ~2 amps, resulting in enough voltage drop that the bulb remains dark if there is resistance in the circuit.

Hint: look at the rivets that secure those brass bus bars that run across the backside of the taillight housings. If the brass has corroded, the resistance could be there.

I've already checked the grounds but I'm going back this morning with my own meter. That bus bar caught my attention yesterday. I'm handy with a soldering iron. Maybe soldering the rivets to the bus bar would do it.