GMC Topkick front and rear turn signal bulbs use 1157 size bulbs for replacement applications. The replacement chart covers vehicle production spanning 1990 to 2001.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC Topkick uses the 1157 bulb for both the front and rear turn signals across the 1990-2001 model years.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single housing, allowing it to serve more than one lighting function simultaneously.
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 the bulb is inserted in the correct orientation, preventing the filaments from being wired to the wrong circuit.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, reflecting a standardized specification shared across North American automotive applications.
Because the same bulb size is specified for both the front and rear turn signal positions on the GMC Topkick, a single bulb type serves all four turn signal housings on the vehicle.
The BAY15d base of the 1157 is not interchangeable with single-pin bayonet bases such as the BA15s, which is used on single-filament bulbs. Substituting a single-filament bulb in a dual-filament socket would result in an incomplete circuit for one of the two functions.