Mack CM Back Up Light Bulb Size

The Mack CM back up light utilizes P21W and 1156 bulb sizes for the rear illumination system. Data for the specific components covers the period from 1988 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
1988-1991	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 1156 is the sole bulb type listed across all model years covered in these specifications for the Mack CM backup light position.
The 1156 bulb follows the ANSI standard, which is the American National Standards Institute designation used in the United States and Canada to define consistent electrical and physical characteristics for automotive lighting components.
All four model years covered span 1988-1991, each sharing an identical bulb specification with no variation across that range.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is typical of incandescent single-filament automotive bulbs used in backup light applications.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet-style mount with a 15mm base width and a single contact pin, distinguishing it from double-contact variants such as the BA15d.
Because the 1156 is single-filament, it serves only one lighting function per socket, which is consistent with its use in a dedicated backup light rather than a combined stop and tail light assembly.
The BA15s bayonet base is not interchangeable with screw-base or wedge-base bulbs, even when wattage and voltage ratings are otherwise similar.
The 1156 should not be substituted with the visually similar 1157, as the 1157 uses a BA15d double-contact base with two filaments and a different pin configuration that is physically incompatible with a BA15s socket.