GMC C5000 front and rear turn signal bulbs both utilize the 1157 bulb size for proper operation. The replacement chart covers vehicle production spanning 1979 to 1990.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC C5000 uses the 1157 bulb for both front and rear turn signals across the 1979-1990 model years, with all entries carrying a Medium Duty classification.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, 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 features two offset pins at different heights on the base to ensure correct orientation during installation and prevent incorrect filament activation.
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 front and rear turn signal positions on the GMC C5000 share the same bulb type, the 1157 is interchangeable between those two positions on this vehicle.
The BAY15d base is specific to dual-filament bayonet bulbs and is not interchangeable with the single-filament BA15s base, which shares a similar physical form but lacks the second offset pin.
The Medium Duty designation associated with each entry reflects the vehicle's classification within GMC's commercial truck lineup, rather than a characteristic of the bulb itself.