The Audi Super 90 front turn signal bulb size measures 1157 or P21W, while the rear turn signal bulb size measures P21W. The replacement chart covers 1969 through 1972 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1157 is an ANSI-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, fitted with a BAY15d bayonet base. It is the North American standard for automotive signalling applications.
The P21W is an ECE-standard single-filament bulb rated at 12V, 21W, fitted with a BA15s bayonet base. It is the European and international standard for dedicated turn signal and signalling functions.
Although both the 1157 and P21W share a 12V, 21W operating rating for their primary filament, they are not interchangeable. The BAY15d base of the 1157 has two offset pins at different heights, while the BA15s base of the P21W has a single pin, making physical substitution between the two impossible without a socket adapter.
The Audi Super 90 front turn signal used the 1157 bulb for the 1970-1972 model years.
The 1969 Audi Super 90 front turn signal used the P21W rather than the 1157, representing a different base type and filament configuration from the later model years.
The rear turn signal of the Audi Super 90 used the P21W across all four model years covered, from 1969-1972.
Because the 1157 is a dual-filament bulb, it is capable of serving both a high-intensity turn signal function and a lower-intensity parking or running light function from a single unit. The P21W, being single-filament, serves only one lighting function per socket.
The front and rear turn signal positions on the 1969 Audi Super 90 both used the P21W, meaning the same bulb type was shared across both positions in that model year.
For the 1970-1972 Audi Super 90, the front and rear turn signals used different bulb types, with the front requiring the 1157 and the rear requiring the P21W. These two positions are therefore not served by a common replacement bulb.