The Audi 100 uses the 9004 bulb across all listed model years, spanning 1989-1994, for both high beam and low beam headlight positions.
The 9004 is a dual-filament halogen bulb, meaning a single bulb unit contains two separate filaments to serve both the high and low beam functions within one housing.
Because the 9004 handles both beam functions simultaneously, the Audi 100 does not require separate bulb types for high and low beam operation.
The 9004 operates at 12 volts and carries a wattage rating of 65W on the high beam filament and 45W on the low beam filament, reflecting the higher light output demanded of the high beam circuit.
The 9004 uses a P29t base, a three-pronged connector designed to prevent incorrect installation by allowing only one orientation when seating the bulb into its socket.
The 9004 conforms to ANSI standards applicable in the United States and Canada, which defines its electrical and photometric performance requirements.
The 9004 and 9007 bulbs share a physically similar appearance but are not interchangeable; the 9007 reverses the wattage arrangement between filaments and uses a keying configuration that differs from the P29t base of the 9004.