The White/GMC WIS high beam and low beam headlight bulbs utilize a 9004 or H6024 size. The replacement chart covers 1988 to 1991 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The White/GMC WIS across the 1988-1991 model years accepted two distinct headlamp systems: composite headlamps using the 9004 bulb and sealed beam headlamps using the H6024 unit. The correct replacement depends entirely on which headlamp system the vehicle is equipped with.
The 9004 is a dual-filament halogen bulb rated at 12V with a wattage split of 65W for high beam and 45W for low beam. It uses a P29t base and meets ANSI standards applicable in both the United States and Canada.
Because the 9004 carries both high beam and low beam filaments within a single bulb, it serves both the High Beam Headlight and Low Beam Headlight positions on composite-equipped WIS vehicles. A single bulb type handles both functions.
The H6024 is a 7-inch round sealed beam unit operating at 12V. It also conforms to ANSI standards for the United States and Canada. Unlike the 9004, the H6024 is not a replaceable bulb in the conventional sense; the entire sealed unit is replaced as one assembly.
The H6024 similarly covers both the high beam and low beam headlight positions on sealed beam-equipped WIS vehicles across the full 1988-1991 range, mirroring the dual-position role of the 9004 on composite-equipped variants.
The 9004 and H6024 are not interchangeable. One is designed for composite headlamp housings that accept a removable bulb, while the other is a self-contained sealed beam assembly. Substituting one for the other is not supported by the specifications.