| PM | Quote | Reply | Full Topic |
Gunslinger
Joined: Sat Nov 4th, 2017
Posts: 426
Status:
Offline
Reputation: 
Reputation Points: $user_rep
The 3157 is a wedge base dual filament type bulb, one being brighter than the other. For example, on my 1999, the 3157 is used in the brake light socket for both running lights at night and as brake lights. Designers usually use these so they don't have to add 194s or the like in the housing as well, driving up the cost and complexity.
3156 bulbs on the other hand is still a wedge base bulb but with only the one filament. These are used when only a signalling or reversing lamp is needed.
3156 on top, 3157 on bottom.
The reason why the 3157 can be used in place of a 3156 is because they both share the same contacts for the lower, brighter, filament. When used this way, the top filament is simply left unused.
____________________
1999 Ford Ranger XLT - 3.0 V6 4x4 - Loaded (Totaled) 143k. Rest in Peace, Gold Dust.