#3157 rear light

[Donald]

As I look at a #3157 bulb it has two filaments, not longer and one shorter. So I assume one will be brighter than the other. But it does not appear the bulb base is keyed, so I can push it in or flip 180 degrees and push in. There must be something I am missing. Maybe both filaments are always used together for turn signal. There is another bulb in the tail light assembly.

 

[HerrStig]

The socket is wired so the terminals are in proper configuration either way. However, if you look at the metal based version of the bulb the bayonet pins are offset to permit insertion only one way because the two bottom contacts - the base is the third- are for a high power and low power filament.
I have LED versions in my turn signals and while they can be inserted either way they only work one way.

 

[Donald]

Originally Posted By: HerrStig
The socket is wired so the terminals are in proper configuration either way. However, if you look at the metal based version of the bulb the bayonet pins are offset to permit insertion only one way because the two bottom contacts - the base is the third- are for a high power and low power filament.
I have LED versions in my turn signals and while they can be inserted either way they only work one way.


It's not a bayonet base. Plastic base you push in

 

[SwampSurvivor]

My Cherokee has 3 3157 bulbs in each tail light. One for stop/tail, one for turn and one for reverse.

The only one that has 4 wires going to it is the stop/tail. I'm not sure putting it in backwards causes any issues with brightness. At least not that I could tell.

 

[mk378]

The spacing of the pins in the slots means that it works the same either way. It's based on the distance from the center not which side. The small filament (only) is used for the tail lights. The larger filament (only) is used for the brake lights. For turn or reverse usually the small filament isn't hooked up at all. It saves having to stock two kinds of bulbs at the factory.

 

[exranger06]

One filament is for stop and turn, the other is for the tail (running) light. Sometimes one filament burns out while the other keeps working. That's why you sometimes see a car with a burnt out brake light, yet the tail/running light still works and it's the same bulb (or vice versa).

 

[UNC017]

Originally Posted By: Donald
As I look at a #3157 bulb it has two filaments, not longer and one shorter. So I assume one will be brighter than the other. But it does not appear the bulb base is keyed, so I can push it in or flip 180 degrees and push in. There must be something I am missing. Maybe both filaments are always used together for turn signal. There is another bulb in the tail light assembly.


The thicker filament is stop/turn, the finer one is parking lamp. The pin locations and spacing are such that its perfectly mirrored. The same filaments are connected to the same contacts in the socket either way you install the bulb. LED bulbs may or may not work only one way, depending on whether the bulb has a rectifier built into it.

 

[splinter]

I swapped out my 3157s to 3357s for a little more candlepower btw.
If you're into this kind of stuff, some good bulb intel from Don...

www.donsbulbs.com/cgi-bin/r/t.pl?searchb=3157&bclick.x=0&bclick.y=0

 

[cjcride]

If I remember correctly they push in one way and lock into place.