The BMW 318i brake component utilizes P21/5W and P21W bulb sizes for various lighting applications. Data for the specified vehicle covers the range from 1984 through 1998.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 318i brake light circuit uses two distinct bulb types across its model years: the single-filament P21W and the dual-filament P21/5W. Both operate at 12 volts and share a 21-watt primary filament rating, though they serve different electrical functions and are not interchangeable.
The P21W follows the ECE signalling standard and uses a BA15s bayonet base, which has a single locating pin. This single-filament construction means the bulb carries one circuit, dedicated solely to the brake light function.
The P21/5W also follows the ECE standard but uses a BAY15d bayonet base, which has two offset locating pins. Its dual-filament design carries a 21-watt primary filament and a 5-watt secondary filament, allowing it to serve two lighting functions within a single housing.
Because the BA15s and BAY15d bases are physically different, a P21W and a P21/5W cannot be substituted for one another. The offset pin arrangement on the BAY15d base prevents incorrect installation in a BA15s socket, and vice versa.
For the BMW 318i brake lights, the P21W is specified for the 1984-1985, 1991, 1993-1994, and 1996 model years. The P21/5W is specified for the 1995, 1997, and 1998 model years.
The Center High Mount Stop Light on the BMW 318i uses the P21W exclusively across all listed model years from 1991 through 1998, making its base and filament requirements consistent regardless of what the main brake lights specify for a given year.
In the 1997 and 1998 model years, the main brake lights call for the dual-filament P21/5W while the Center High Mount Stop Light calls for the single-filament P21W. These two positions therefore require different bulbs despite both being part of the brake lighting system.
The ECE designation on both bulb types indicates compliance with United Nations Economic Commission for Europe regulations, a standard applied across European and many international vehicle markets. The BMW 318i specifications reflect this European-origin standard throughout all listed model years.