Mack CL Back Up Light Bulb Size

The Mack CL back up light utilizes 1156 and P21W bulb sizes for illumination across various production versions. Data for the specified lighting components covers the period from 1991 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
1991-1997	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 1156 is the sole back up light bulb size listed for the Mack CL across all model years covered in the specifications, spanning 1991-1997.
The 1156 is a single-filament bulb, meaning it serves one lighting function rather than sharing a housing with a dual-purpose filament for combined stop and tail duties.
The 1156 operates at 12 volts and draws 21 watts, which is consistent with standard automotive electrical systems of that era.
The 1156 uses a BA15s bayonet base, a single-contact mount where the bulb is inserted and rotated to lock into place.
The BA15s designation indicates a single-contact bayonet fitting with a 15mm base width, distinguishing it from the dual-contact BA15d base used on some visually similar bulbs.
The 1156 conforms to ANSI standards recognized in both the United States and Canada, reflecting a standardized specification rather than a proprietary or regional-only fitment.
Because the 1156 uses a single-contact BA15s base, it is not interchangeable with the 1157, which shares a similar physical profile but uses a dual-contact BA15d base and carries two filaments.