GMC K15 Suburban Back Up Light Bulb Size

The GMC K15 Suburban back up light uses a 1156 bulb size for this model. The replacement chart covers vehicle production years spanning 1973 to 1978 with corresponding bulb specifications.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1973-1978	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

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Additional Notes

The GMC K15 Suburban uses the 1156 bulb across all listed model years, spanning 1973-1978.
The 1156 is an ANSI-standardized bulb type, meaning its specifications are governed by a recognized industry standard shared across the United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard automotive backup light applications.
As a single-filament bulb, the 1156 serves one dedicated function per socket, distinguishing it from dual-filament types that handle two functions such as braking and running light simultaneously.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width, a common base type in automotive lighting.
The BA15s base requires a twist-lock installation rather than a screw-in method, which is a defining fitment characteristic when sourcing a replacement.
Because the 1156 is a single-contact bayonet type, it is not interchangeable with the 1157, which shares the same 15mm bayonet base width but carries two contacts for dual-filament operation.