Mack DMM Back Up Light Bulb Size

The Mack DMM back up light utilizes 1156 and P21W bulb sizes according to the provided technical data. Information for the specified components covers the range from 1981 through 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
1981-1997	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 Mack DMM uses the 1156 bulb for the back up light across all listed model years, spanning 1981-1997.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American standard applicable in both the United States and Canada.
This bulb operates on a 12-volt electrical system and draws 21 watts, which is typical of incandescent single-filament automotive bulbs used in signaling and reversing applications.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width. The single-contact design corresponds directly to the bulb's single-filament construction.
Because the 1156 is a single-filament bulb, it serves one dedicated lighting function per socket, which in this case is the back up light.
The BA15s base is not interchangeable with the BA15d base. The BA15d is a dual-contact bayonet fitting designed for dual-filament bulbs, and substituting one for the other is not compatible with the socket design.