Taillight Electrical Issue

The other day I discovered an issue with one of the two lights in the drivers side taillight of my ‘74. Judging from how the passenger taillight functions, both individual lights perform the same actions: come on with headlights, blink with signal, glow brighter with brake. One of my driver side lights works as described. The other one comes on with headlights, but goes out with both signal and brake.

The bulb is an 1157 double filament. During the malfunction the first filament glows with headlights but then goes out with signal/brake instead of the second/brighter filament glowing.

Before I dive into the service manual my question to anyone who can help is, am I looking for a bad ground or something else? My electrical capabilities are pretty basic. Thanks!

 

Start with a clean ground. Be sure to clean the socket(inside edges & contact points). Also try a bulb you know is good. Sometimes the filament looks good but isn't. Use a 12 volt tester & test both contacts.

 

Have you checked to be sure a 1157 is the correct bulb? You should be able to tell by looking at the empty socket.

 

Second on this. I've always found bad filaments touching cause this and a few other problems. I've also found backward-installed dual filament bulbs (remember, nothing is foolproof because fools are ingenious) causing similar problems.

 

Thanks for the thoughts. I’ll let you know what I find. It might be a couple of days.

 

Got the time this afternoon to look closer and the “fix” was embarrassingly simple. The malfunctioning bulb gets its ground from the light bezel. I was checking them hanging loose. *slaps forehead*

 

Thanks, Jeff - I needed that today.

 

Yeah, I felt foolish as I'm looking at the factory sockets and saying, "Where the Hell are the ground wires on these two?" then realizing the housing was the common denominator. But, just for my own reasons, I still ran a wire between all three sockets to ensure a good ground, since one of the non-wired sockets kept losing ground.

 

Glad it was a simple fix. I installed a bulb wrong and it did goofy things. Easy to fix once I calmed down. From there I started cleaning bulb sockets and grounds whenever I have something apart. On these cars, if Ford could have built them with one strand of wire less, they would have. We are challenged right from the start.