The White/GMC WHS uses 1157 bulb size for both front and rear turn signal applications. The replacement chart covers 1988 to 1991 vehicle production 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 for the White/GMC WHS across both front and rear turn signal positions for the 1988-1991 model years.
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.
The 1157 operates on a 12-volt electrical system and draws 21 watts on its high-intensity filament and 5 watts on its low-intensity filament.
The 1157 uses a BAY15d bayonet base, which is an offset twin-pin mount. The two pins are positioned at different heights to ensure correct polarity orientation during installation.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, placing it within a widely standardized specification framework for North American vehicles.
Because the same bulb number is specified for both the front and rear turn signal positions, the WHS uses a single bulb type across all four turn signal sockets for the covered model years.
The BAY15d base of the 1157 is not interchangeable with single-pin bayonet bases such as the BA15s, which is used by single-filament bulbs like the 1156. Substituting a single-filament bulb in a socket designed for the 1157 would result in incorrect function.