Mack RWS Back Up Light Bulb Size

The Mack RWS back up light bulb uses a 1156 size bulb for this model. The replacement chart covers vehicle production years from 1981 to 1985 with corresponding bulb size specifications.

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
1981-1985	1156

Bulb Specifications

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

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Additional Notes

The Mack RWS uses the 1156 bulb for its back up light across the 1981-1985 model years.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American standard shared across the US 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 bulb uses a BA15s bayonet base, a single-contact mount that locks into the socket with a twist, distinguishing it from dual-contact bayonet variants such as the BA15d used in other automotive applications.
Because the BA15s base is a single-contact design, it is not interchangeable with physically similar bulbs that use a dual-contact base, even when the outer housing size is comparable.