The Volvo 780 uses the 9004 bulb for both high beam and low beam headlights across all model years from 1987-1991.
The 9004 is a dual-filament halogen bulb, meaning a single bulb contains two separate filaments: one rated at 65 watts for high beam output and one rated at 45 watts for low beam output.
Because the 9004 serves both beam functions in the Volvo 780, each headlight assembly requires only one bulb rather than separate high and low beam units.
The 9004 operates on a 12-volt electrical system, which is standard for passenger vehicles of this type.
The 9004 uses a P29t base, a specific connector type that is not interchangeable with other common halogen bases such as H4 or 9007, despite the 9007 also being a dual-filament halogen bulb with a visually similar form.
The 9004 is an ANSI-standardized bulb approved for use in the United States and Canada, meaning it meets the photometric and safety requirements defined by North American lighting regulations.