The Mack RS front and rear turn signal bulb size specifications indicate a 1157 bulb for both applications. The replacement chart covers vehicle production data spanning 1981 to 1989.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RS uses the 1157 bulb for both its front and rear turn signals across the 1981-1989 model years.
The 1157 is an ANSI-standardized bulb type, meaning its specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1157 is a dual-filament bulb, which means it contains two separate filaments operating at different wattages within a single housing, allowing it to serve more than one lighting function.
The two filaments in the 1157 operate at 21W and 5W respectively, with the higher-wattage filament typically handling the turn signal or brake function and the lower-wattage filament handling a secondary function such as running light duty.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that physically prevents incorrect insertion, as the two pins are positioned at different heights to ensure proper filament orientation.
The 1157 is rated at 12 volts, consistent with the standard electrical system voltage found in most commercial vehicles of the RS series era.
Because the same bulb number applies to both the front and rear turn signal positions across all listed model years, a single 1157 bulb serves as a direct replacement for either location on the RS.