The Mack CL uses the 1157 bulb for both its front and rear turn signals across all listed model years, 1991-1997.
The 1157 is an ANSI-standardized bulb type recognized across the United States and Canada, meaning it conforms to a defined set of electrical and physical specifications shared by multiple manufacturers.
The 1157 is a dual-filament bulb, which means it contains two separate filaments within a single housing, allowing it to serve more than one lighting function from a single socket.
The bulb operates on a 12-volt electrical system and carries a wattage rating of 21W for the high-intensity filament and 5W for the low-intensity filament, reflecting its dual-function design.
The 1157 uses a BAY15d bayonet base, a two-pin offset base type that physically prevents incorrect insertion, as the two pins are positioned at different heights along the base.
Because the front and rear turn signal positions on the Mack CL share the same bulb type, a single 1157 bulb serves as a direct replacement for either location throughout the 1991-1997 model range.
The BAY15d base is not interchangeable with single-pin bayonet bases such as the BA15s, which is used in single-filament bulbs, so substitution with physically similar but electrically different base types would result in an improper fit.