Kenworth K100 Back Up Light Bulb Size

The Kenworth K100 back up light uses 1156 and P21W bulb sizes according to the provided data. The information covers the range from 1981 through 1985 for the specified vehicle.

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
1981-1985	1156

Bulb Specifications

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

Other Locations

Additional Notes

The Kenworth K100 uses the 1156 bulb for the back up light across the 1981-1985 model years.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American 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 incandescent backup lighting.
The 1156 is a single-filament bulb, meaning it serves one lighting function only, as opposed to dual-filament types that can handle two functions such as braking and running light simultaneously.
The bulb uses a BA15s bayonet base, where "BA15" refers to the 15mm base width and "s" denotes a single contact pin, distinguishing it from the BA15d, which carries two offset pins for dual-filament applications.
The BA15s base type is a common fitment in automotive applications, though substitution with visually similar bases such as BA15d is not appropriate due to the differing pin configuration and filament count.