Mercedes-Benz 300D Back Up Light Bulb Size

Additional Notes

• The Mercedes-Benz 300D uses the P21W bulb size for its back up light across all listed model years from 1975 to 1993.
• 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 international vehicle markets.
• P21W bulbs operate on a 12-volt electrical system and draw 21 watts, which is a standard output level for single-filament signalling applications.
• The single-filament design of the P21W means it serves one lighting function only, distinguishing it from dual-filament bulbs that handle two functions such as braking and running light duties simultaneously.
• The P21W uses a BA15s bayonet base, where "BA15" refers to the 15mm pin-collar width and "s" denotes a single-contact base with one pin on the collar.
• The BA15s base is not interchangeable with the BA15d base, which carries two offset pins for dual-filament bulbs. Substituting a BA15d bulb in a BA15s socket is not possible without modification.
• The 1975-1976 and 1979-1993 model years all share the same P21W specification, indicating a consistent socket and electrical requirement across those production periods.
• Because the P21W is an ECE-regulated type rather than an SAE-regulated type, its photometric and dimensional standards differ from North American equivalents such as the 1156, even though both are single-filament 21-watt bayonet bulbs. Direct substitution between the two types warrants verification of base pin alignment and light output compliance.