Mack RM front and rear turn signal bulbs utilize a 1157 size across both locations. The replacement chart covers 1981 to 1993 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RM uses the 1157 bulb for both its front and rear turn signals across the 1981-1993 model years.
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 all compliant manufacturers.
The 1157 is a dual-filament bulb, which means it houses two separate filaments within a single glass envelope, allowing it to serve two distinct lighting functions from one unit.
The bulb 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 assigned to the active turn signal function and the lower-wattage filament to a secondary role such as a parking or position light.
The 1157 uses a BAY15d bayonet base, a two-pin base type where the two pins are offset at different heights along the base, a physical design that prevents incorrect filament-to-circuit orientation during installation.
Because the front and rear turn signal positions share the same bulb type on the Mack RM, a single 1157 bulb serves as a direct replacement for either location without requiring a separate part.
The BAY15d base is not interchangeable with single-pin bayonet bases such as the BA15s, as the dual-pin offset configuration is specific to dual-filament applications and will not seat correctly in a single-filament socket.