The Audi 90 brake light requires a P21/5W bulb size for proper installation and function within the rear lighting assembly. Data for the specified component covers the range from 1988 to 1995.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Audi 90 uses the P21/5W bulb for its brake lights across the 1988-1995 model years, with 1992 absent from the listed data.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base.
The BAY15d base is an offset twin-pin bayonet fitting, which prevents incorrect single-filament bulbs from being seated in the same socket.
The dual-filament design of the P21/5W allows a single bulb to serve both brake and tail light functions simultaneously within one housing.
The Center High Mount Stop Light on the Audi 90 uses the type 70 bulb across the same 1988-1995 model years covered in the specifications.
The type 70 is an ANSI-standard miniature wedge bulb of the T5 form factor, rated at 12V and 1.4W, intended for low-intensity indicator applications.
The wedge base of the type 70 is not interchangeable with the BAY15d bayonet base of the P21/5W, as the two bulbs use entirely different mounting systems.
The P21/5W follows ECE regulations, which are the prevailing standards in Europe and much of the world, while the type 70 follows ANSI standards used in North America.
The significant wattage difference between the P21/5W at 21W and the type 70 at 1.4W reflects their distinct roles, with the higher-output bulb serving primary brake signalling and the lower-output bulb serving the supplemental high-mount position.