The Mercedes-Benz 300SEL uses a P21/5W bulb size for the front turn signal and a P21W bulb size for the rear turn signal. The replacement chart covers vehicle production years spanning 1969 to 1991.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz 300SEL uses two distinct bulb types across its turn signal positions: the dual-filament P21/5W at the front and the single-filament P21W at the rear.
The P21/5W operates at 12V and draws 21W on its primary filament and 5W on its secondary filament, making it suitable for combined signalling and position light functions within a single housing.
The P21W operates at 12V with a single 21W filament, serving a dedicated single-function signalling role at the rear.
Both bulb types conform to ECE standards, meaning they are rated for use in European and worldwide markets rather than SAE-regulated North American specifications.
The P21/5W uses a BAY15d bayonet base, which features offset pins to prevent incorrect insertion, a design that distinguishes it physically from the P21W and prevents the two types from being interchanged.
The P21W uses a BA15s bayonet base, a single-contact base with symmetrically placed pins, which is not compatible with the BAY15d socket used by the P21W/5W front bulb.
Because the BAY15d and BA15s bases are mechanically different, the front and rear turn signal bulbs of the 300SEL are not interchangeable with each other despite sharing the same 12V operating voltage and the same 21W primary wattage.
The front turn signal position specifies the P21/5W for the 1988-1991 model years.
The rear turn signal position specifies the P21W across both the 1969-1973 and 1988-1991 model years.