Mack CF Back Up Light Bulb Size

The Mack CF back up light utilizes 1156 and P21W bulb sizes for the lighting components. Data for the specific bulbs covers the time range from 1981 through 1991.

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
1981-1991	1156

Bulb Specifications

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

Other Locations

Additional Notes

The Mack CF uses the 1156 bulb for its back up light across all listed model years, spanning 1981-1991.
The 1156 is a single-filament bulb, meaning it serves one lighting function rather than combining two, such as stop and tail, into a single unit.
The 1156 operates at 12 volts and 21 watts, which places it in the standard range for automotive reverse lighting applications.
The 1156 uses a BA15s bayonet base, a single-contact mount where the bulb is inserted and twisted to lock into the socket.
The 1156 conforms to ANSI standards recognized in both the United States and Canada, reflecting a widely adopted specification in North American vehicle manufacturing.
Because the 1156 is a single-filament type, it is not interchangeable with dual-filament variants such as the 1157, which shares a similar bayonet base but carries a second contact pin and filament.