The Kenworth C550 front and rear turn signal bulbs utilize 1157 and 1156 sizes, with the front requiring the 1157 bulb size exclusively. The replacement chart covers vehicle production spanning 1986 to 1994, detailing bulb specifications across the model's manufacturing range.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Kenworth C550 uses the 1157 bulb as the dominant turn signal type, with it appearing across both front and rear positions throughout the 1986-1994 model range.
The 1157 is a dual-filament bulb operating at 12 volts with a 21W/5W output rating, meaning it carries one filament for the higher-intensity turn signal function and a second for a lower-intensity running light function.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that physically prevents single-filament bulbs from being seated in its place.
The 1156 bulb is a single-filament type rated at 12 volts and 21 watts, using a BA15s bayonet base with a single locating pin.
Because the 1156 and 1157 share a similar physical profile but differ in base pin configuration, the two types are not interchangeable despite their close appearance.
Both the 1157 and 1156 conform to ANSI standards recognized in the United States and Canada, placing them within a standardized classification system used across a wide range of North American vehicle applications.
The front turn signal position on the Kenworth C550 uses the 1157 consistently across all listed model years from 1986-1994, with no variation in that position.
The rear turn signal position shows variation between the 1156 and 1157 across different model years, with the 1156 specified for 1987-1988 and 1990-1991, while the 1157 is specified for 1986, 1989, and 1992-1994.
The rear position difference between model years carries a practical fitment consequence, as installing a dual-filament 1157 in a socket wired for a single-filament 1156 application, or vice versa, risks incorrect electrical behavior due to the differing base pin configurations.
Both bulb types are classified under ANSI standards, which means their electrical and physical specifications are governed by a defined industry standard rather than being proprietary to any single manufacturer.