The Mack WS uses the 1157 bulb for both its front and rear turn signals across the 1981-1984 model years.
The 1157 is an ANSI-standardized bulb type, meaning its physical and electrical specifications conform to a defined North American standard recognized in both the United States and Canada.
The 1157 is a dual-filament bulb, which means it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit.
The 1157 operates on a 12-volt electrical system and carries a wattage rating of 21W on its primary filament and 5W on its secondary filament, reflecting the higher power draw of the turn signal function versus a lower-intensity parking or running light function.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that physically prevents incorrect insertion, since the two pins are positioned at different heights to ensure proper filament alignment.
Because the front and rear turn signal positions on the Mack WS share the same 1157 specification, a single bulb type serves all four turn signal sockets on the vehicle.
The BAY15d base of the 1157 is not interchangeable with single-filament bayonet bases such as the BA15s, as the dual-pin offset configuration is specific to dual-filament applications.