Mercedes-Benz 240D Back Up Light Bulb Size

The Mercedes-Benz 240D back up light utilizes a P21W bulb size for illumination. Data for the specific component covers the range from 1974 through 1983.

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
1974-1983	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The Mercedes-Benz 240D uses the P21W bulb size for the back up light across all listed model years, spanning 1974-1983.
The P21W is an ECE-standard bulb, meaning it conforms to the Economic Commission for Europe's regulations, which govern lighting equipment used in European and many worldwide automotive markets.
The P21W operates at 12 volts with a 21-watt output, placing it in the standard single-filament signalling category rather than the higher-wattage range associated with driving or fog lamps.
The bulb uses a BA15s bayonet base, where "BA" denotes a bayonet cap fitting, "15" refers to the 15mm base width, and the lowercase "s" indicates a single contact at the base, consistent with its single-filament design.
Because the P21W carries only one filament, it is dedicated to a single lighting function, which in this application is the reverse or back up light.
The BA15s base is not interchangeable with the BA15d base, which features two contacts and is designed for dual-filament bulbs. Substituting a BA15d bulb in a BA15s socket is not compatible.
The ten-year consistency of the P21W fitment across the 240D's listed production span indicates a stable electrical and socket specification throughout that period.