Kenworth C540 Back Up Light Bulb Size

The Kenworth C540 back up light requires bulb size 1156 or P21W depending on the specific configuration of the vehicle. Data for the bulb sizes covers the range from 1985 to 1993.

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
1992-1993	1156
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 1156 is the sole bulb type listed across all model years in the Kenworth C540 back up light specifications.
The 1156 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning it conforms to a defined set of electrical and physical standards shared across those markets.
It is a single-filament bulb, which is typical for back up lights, as this application requires only one lighting function rather than the dual functions found in combination stop/tail bulbs.
The 1156 operates on a 12-volt electrical system at 21 watts, both of which are standard figures for automotive auxiliary lighting in vehicles of this era.
The bulb uses a BA15s bayonet base, where "BA15s" denotes a bayonet fitting with a 15mm collar and a single contact pin, distinguishing it from the BA15d double-contact variant used in dual-filament applications.
The BA15s base type is a widely used automotive standard, meaning the 1156 has broad cross-compatibility with other vehicles and aftermarket sources that share this base specification.
The Kenworth C540 back up light uses the 1156 bulb across the 1985, 1992, and 1993 model years, indicating a consistent socket and base specification throughout those years.
Because 1985 and the 1992-1993 entries share the same bulb specification, a single replacement part number satisfies the back up light requirement for all listed model years of the C540.