The White/GMC WCS uses 1157 bulb size for both front and rear turn signal applications. Replacement chart data spans 1988 to 1991 model years with corresponding bulb 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 across all listed positions and model years for the White/GMC WCS, covering both the front and rear turn signal functions from 1988-1991.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions at different wattages from one socket.
It operates on a 12-volt electrical system and draws 21 watts on its primary filament and 5 watts on its secondary filament, with the higher-wattage filament typically associated with the turn signal and braking function and the lower with a running or parking light function.
The 1157 uses a BAY15d bayonet base, which is an offset dual-contact base. The offset pin configuration is a deliberate design feature that prevents the bulb from being inserted in the wrong orientation, ensuring each filament connects to its correct circuit.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, placing it within a widely adopted North American specification framework.
Because the same 1157 bulb is specified for both the front and rear turn signal positions across all four model years, the White/GMC WCS uses a single bulb type for all turn signal sockets on the vehicle.
The BAY15d base is not interchangeable with single-filament bayonet bases such as the BA15s, as the dual-contact and offset-pin design is physically distinct from single-contact configurations.