Mack WS Back Up Light Bulb Size

The Mack WS back up light requires bulb size P21W or 1156 depending on the specific configuration of the lighting assembly. Data for the lighting components covers the range from 1981 through 1984.

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
1981-1984	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 WS uses the 1156 bulb for its back up light across the 1981-1984 model years.
The 1156 is an ANSI-standardized bulb, meaning its physical and electrical specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is typical of incandescent automotive bulbs used in signaling and reversing light applications.
The 1156 is a single-filament bulb, meaning it serves one lighting function only, which is consistent with its dedicated role as a back up light.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet cap with a 15mm base width and a single contact pin, distinguishing it from dual-contact bayonet variants.
The BA15s single-contact base of the 1156 is not interchangeable with visually similar bulbs that use a BA15d dual-contact base, as the pin configuration differs between the two.