GMC C6000 front and rear turn signal bulbs utilize 1157 and 2057 sizes for replacement applications. The replacement chart covers 1979 to 1990 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 bulb is the most frequently specified turn signal bulb across the GMC C6000 range, appearing in both front and rear positions for the majority of model years covered in these specifications.
Both the 1157 and 2057 bulbs share the BAY15d bayonet base, a dual-contact base design where the two pins are offset to ensure correct orientation during installation.
The 1157 operates at 12V with a dual-filament output of 21W on the high filament and 5W on the low filament, making it suitable for combined turn signal and parking light functions.
The 2057 operates at 12V with a dual-filament output of 26.9W on the high filament and 8W on the low filament, producing greater brightness than the 1157 despite sharing the same base type.
Although the 1157 and 2057 share the BAY15d base and are physically interchangeable in many sockets, the difference in wattage means substituting one for the other may affect light output and could place different electrical loads on the circuit.
The GMC C6000 (1984) is specified with the 2057 bulb in the front turn signal position, while all surrounding model years in the same position use the 1157, indicating a year-specific fitment distinction for that position.
In the rear turn signal position, both 1983 and 1984 model years of the GMC C6000 are specified with the 2057 bulb, whereas adjacent years revert to the 1157.
Both bulb types listed in these specifications conform to ANSI standards applicable in the United States and Canada, indicating standardized electrical and photometric performance requirements.
The Medium Duty designation appears consistently across the 1981-1990 model years for both front and rear positions, distinguishing those fitments from the 1979-1980 entries, which carry no such designation.