The Ford C8000 turn signal system relies on two bulb types across its model years: the 1157 and the 1156, both of which conform to ANSI standards recognized in the United States and Canada.
The 1157 is a dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. The dual-filament design allows a single bulb to serve two lighting functions simultaneously.
The 1156 is a single-filament bulb rated at 12V and 21W, mounted on a BA15s bayonet base. It serves only one lighting function per socket.
The BAY15d base of the 1157 and the BA15s base of the 1156 are physically distinct and not interchangeable, as the pin offset and filament count differ between the two base types.
The Ford C8000 front turn signal uses the 1157 for model years 1980, 1982, and 1985-1990, while the 1156 is specified for 1981, 1983, and 1984.
The rear turn signal of the Ford C8000 uses the 1157 for model years 1982-1986, 1988, and 1990, while the 1156 is specified for 1980, 1987, and 1989.
Several model years show different bulb types between the front and rear turn signal positions, meaning the front and rear sockets are not necessarily fitted with the same bulb within a given year.
Because the 1156 and 1157 share a similar physical profile but differ in base configuration and filament count, substituting one for the other in a given socket is not supported by these specifications.