Mack RB Back Up Light Bulb Size

The Mack RB back up light bulb uses a 1156 size bulb for this model. The replacement chart covers vehicle production years spanning 1989 to 1997.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1989-1997	1156

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

The Mack RB uses the 1156 bulb for its back up light across the 1989-1997 model years.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American standard shared across the United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is typical of incandescent single-filament automotive bulbs used in directional or reverse lighting applications.
The 1156 uses a BA15s bayonet base, a single-contact mount where the "s" designation indicates a single pin, distinguishing it from the dual-contact BA15d base found on two-filament variants.
Because the 1156 is a single-filament bulb, it is dedicated to one lighting function, which in this case is the back up light.
The 1156 is not interchangeable with the 1157, a visually similar bulb that shares the BA15 bayonet base family but carries a dual-filament design and a two-contact base, making the two electrically and physically incompatible in most sockets.