The Mack RL front and rear turn signal bulbs both utilize a 1157 size bulb for proper operation. The replacement chart covers vehicle production spanning 1981 to 1987.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RL uses the 1157 bulb for both its front and rear turn signals across the 1981-1987 model years.
The 1157 is an ANSI-standardized bulb type recognized across the United States and Canada.
It is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope, allowing it to serve two distinct lighting functions at different wattages.
The 1157 operates on a 12-volt electrical system and draws 21 watts on its high-intensity filament and 5 watts on its low-intensity filament.
The bulb uses a BAY15d bayonet base, which features two offset pins at different heights to ensure correct orientation during installation and prevent incorrect seating.
Because the same 1157 bulb is specified for both the front and rear turn signal positions, the two locations share a common replacement part.
The BAY15d base type is distinct from single-filament bayonet bases, so single-filament alternatives with a similar base profile are not interchangeable without risking incorrect function.