The Mercedes-Benz 300D uses front turn signal bulbs in sizes P21/5W and P21W, along with rear turn signal bulbs in size P21W. The replacement chart covers vehicle production spanning 1975 to 1993 with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz 300D uses two distinct bulb types across its front and rear turn signal positions: the P21/5W and the P21W, both governed by ECE standards applicable in Europe and worldwide.
The P21W is a single-filament signalling bulb rated at 12V and 21W, fitted with a BA15s bayonet base. It serves as the rear turn signal bulb across all listed model years from 1975-1993.
The P21/5W is a dual-filament signalling bulb rated at 12V with filaments at 21W and 5W, fitted with a BAY15d bayonet base. It serves as the front turn signal bulb for model years 1977-1993.
The BAY15d base of the P21/5W and the BA15s base of the P21W are physically distinct and not interchangeable. The BAY15d features offset locking pins to ensure correct filament orientation, while the BA15s has symmetrical pins suited to a single-filament design.
Because the P21/5W carries a second 5W filament, it is capable of serving a dual function, such as combining a turn signal with a parking or position light, whereas the single-filament P21W cannot replicate this behavior.
The front turn signal position on the 300D used the P21W for model years 1975-1976, then transitioned to the P21/5W from 1977 onward through 1993.
Substituting a P21W in place of a P21/5W in the front turn signal position would result in the loss of the secondary 5W filament function, as the two bulbs are not functionally equivalent despite sharing the same 21W primary rating.
The rear turn signal specification remains uniform at P21W across all listed model years from 1975-1993, indicating no change in the rear signalling requirement throughout the production span covered by these specifications.