The Mack U 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 physical and electrical characteristics conform to a defined North American specification 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 more than one lighting function from a single socket.
The 1157 operates on a 12-volt electrical system, consistent with the standard automotive and commercial vehicle electrical architecture of that era.
Its two filaments are rated at 21 watts and 5 watts respectively, with the higher-wattage filament typically associated with the turn signal or stop function and the lower-wattage filament associated with a running or parking light function.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that physically prevents incorrect insertion by positioning the two pins at different heights along the base.
Because the front and rear turn signal positions on the Mack U share the same bulb type, a single 1157 bulb serves as a direct fitment for all four turn signal sockets on the vehicle.
The BAY15d base is not interchangeable with single-pin bayonet bases such as the BA15s, as the offset dual-pin configuration is specific to dual-filament applications and will not seat correctly in a single-filament socket.