Pontiac Tempest Back Up Light Bulb Size

Additional Notes

• The 1156 is the sole back up light bulb size specified for the Pontiac Tempest across all listed model years from 1961 to 1970.
• The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical characteristics conform to a defined North American standard recognized in both the United States and Canada.
• The 1156 operates on a 12-volt electrical system, consistent with the 12V automotive standard that became universal in passenger vehicles during this era.
• The bulb carries a 21-watt rating, which places it in the mid-range of incandescent automotive bulbs and reflects a balance between adequate reverse lighting output and electrical load.
• The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width. The single-contact design corresponds directly to the bulb's single-filament construction.
• Because the 1156 is a single-filament bulb, it serves a dedicated switching function. In a back up light application, the filament is active only when the transmission is engaged in reverse.
• The BA15s base is not interchangeable with the BA15d base. The BA15d is a dual-contact bayonet fitting used on dual-filament bulbs such as the 1157, and substituting one for the other is not physically compatible.
• The 1156 and 1157 share the same 15mm bayonet base width, which can cause confusion during replacement. The critical distinction is that the 1157 has two contact pins offset at different heights, while the 1156 has a single centered contact pin.