The BMW 733i uses a P21/5W bulb size for front turn signal applications and a P21W bulb size for rear turn signal applications. Replacement chart data covers 1978 to 1984 model production years 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 BMW 733i uses two distinct bulb types across its turn signal positions: the P21/5W for the front and the P21W for the rear, both governed by ECE standards recognized in Europe and worldwide.
Both bulb types operate on a 12-volt electrical system, which was the standard automotive voltage architecture for the 733i throughout its production run.
The P21/5W is a dual-filament bulb rated at 21W and 5W, with the lower-wattage filament typically serving a secondary function such as a parking or position light alongside the primary turn signal filament.
The P21W is a single-filament bulb rated at 21W, dedicated solely to the turn signal function at the rear of the vehicle.
The front turn signal of the BMW 733i uses a BAY15d bayonet base, which is an offset twin-pin fitting. This base design physically prevents incorrect single-filament bulbs from being seated in a dual-filament socket.
The rear turn signal uses a BA15s bayonet base, which is a single-pin fitting. The BA15s and BAY15d bases are not interchangeable, meaning a P21W cannot be substituted into a front socket designed for the P21/5W.
The P21/5W specification remained consistent across all front turn signal positions for the 733i from 1978-1984, indicating no mid-production change to the front lighting circuit during that period.
The P21W specification was equally consistent across all rear turn signal positions for the 733i from 1978-1984, reflecting the same stability in the rear lighting configuration.