I am planning to change out all the bulbs in my XJ with LED bulbs. In all the documentation that I can find they call for 3157 bulbs for the back-up lights, turn signal lights and the brake/run lights. I can see the use of the dual filament bulb in the brake/run bulb but not in the back-up and turn signal lights. The back-up and turn signal bulbs should be a single filament bulb.

So, my question is,,,why put a dual filament bulb where a single filament bulb is needed?

I know that with the old bases, 1156/1157, the bulbs are not interchangeable. It looks like with the new style 3156/3157 base that it would be the way that the socket is wired that would make a difference.

 

Check and see if both filaments light with the reverse and brake lights with the 3157. If not 3156 should be fine. It’s been a while since I changed one of those bulbs but I always assumed they would be 3156.

 

I wish I had though before I posted. The brake lights should be dual because the running lights are the same red lights. It’s the signals that may be only 3156 as they are not on with Park lights, only when signals are on.

 

Wish that I had thought about that earlier today. Had the housing off to check to see if the sockets were standard or "CK", they are standard. Oh well,,,its only three bolts,,,

I think that maybe the factory installed 3157 bulbs in all of the sockets to keep the wrong bulb from being put into the wrong socket and to reduce inventory.