The White/GMC J9C0 headlight bulb uses a 9004 or H6024 size for both high and low beam functions. The replacement chart covers 1988 to 1989 vehicle 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 J9C0 for model years 1988-1989 accommodates two distinct headlamp systems: composite headlamps using the 9004 bulb and sealed beam headlamps using the H6024 unit. The applicable bulb size is determined entirely by which headlamp system is installed on the vehicle.
The 9004 is a dual-filament halogen bulb operating at 12V with a wattage rating of 65W on the high beam filament and 45W on the low beam filament. It uses a P29t base and conforms to ANSI standards applicable in both the United States and Canada.
Because the 9004 carries both high and low beam filaments within a single housing, it serves both the high beam and low beam functions on J9C0 vehicles equipped with composite headlamps. A single bulb type therefore covers both lighting positions in that configuration.
The H6024 is a 7-inch round sealed beam unit rated at 12V and conforming to ANSI standards for the United States and Canada. Unlike the 9004, it is not a replaceable bulb; the entire sealed unit is replaced when the lamp fails.
The H6024 also serves both the high beam and low beam positions on J9C0 vehicles equipped with sealed beam headlamps, mirroring the dual-position role of the 9004 in the composite headlamp configuration.
The 9004 and H6024 are not interchangeable. Each is specific to its respective headlamp system, and substituting one for the other is not supported by the fitment specifications for the 1988-1989 J9C0.