3157 automotive bulb has two filaments inside one bulb. One filament is taillight, the other is brake light. They should be able to work independently or together.
Daughter’s brand new used car brake/tail light, on one side only, will not work together. Taillight works alone and brake signal works alone. No light comes from this bulb when both circuits are active.
There is one ground wire leading away from the bulb housing and two hot wires leading in. I’ve tried three different new bulbs and same result.
Can not get brain around how both light fine alone, but nothing together… Any idea teeming masses?
Has it ever worked correctly? I would guess that the brake light switch is defective or installed incorrectly, but I’m neither a car guy nor an electrical guy, so that’s a real guess.
Ah-ha! The circuit tester makes a good solid connection at both ends. The bulb did not… I bent all the little copper contacts out a bit and we now have both brake and tail lights! The ground connection did it. Maybe someone can explain how the ground could have been good enough for brake or tail, but not both at once… Thanks
You have a newer car with an electronic light control module (instead of simple relays / switches on an older car). These modules switch the wire between ground and 12 volts, instead of off and 12 volts.
So as long as one of the elements was off (ground ) and the other was on (12 volts), you had power to light the lamp (both filaments glowing at half-brightness). If both were on, there was 12V at each of them - no difference in voltage, so no light. Connecting the ground causes it to light up in this case, as there is now a voltage differential across the filament.
This problem is often caused by using a bulb with the incorrect base material. US cars generally use bulbs with a brass base, while European cars generally use ones with a silver-colored base.
The actual ground connection was not making contact. What happened was that the brake light circuit grounded the taillight, and the taillight circuit grounded the brake light. When both brake and tail circuits were powered, however, there was no ground available.