The White/GMC J9C0 uses a 1157 bulb size for both front and rear turn signal applications. The replacement chart covers 1988 to 1989 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 1157 is the sole bulb type specified for the White/GMC J9C0 across all listed turn signal positions and model years.
The 1157 is an ANSI-standard bulb recognized in both the United States and Canada, making it a widely standardized part in North American automotive applications.
As a dual-filament bulb, the 1157 operates two separate filaments within a single envelope, one rated at 21W for the higher-intensity turn signal function and one at 5W for lower-intensity running light use.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that prevents incorrect insertion by positioning the two pins at different heights along the base.
The bulb operates on a 12V electrical system, consistent with standard automotive voltage in passenger and commercial vehicles of this era.
The 1157 specification applies to both the front and rear turn signal positions on the J9C0 for the 1988-1989 model years, indicating a uniform bulb requirement across all four turn signal sockets.
Because the BAY15d base uses offset pins, it is not interchangeable with single-pin bayonet bases such as the BA15s, despite a similar physical profile.
The dual-filament nature of the 1157 means a single-filament substitute of the same base type, such as the 1156, would not replicate the full electrical behavior of the original specification.