Mack CH Back Up Light Bulb Size

The Mack CH back up light uses 1156 and P21W bulb sizes for the rear illumination components. Data for the specified vehicle covers the period from 1989 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
1989-1997	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 CH uses the 1156 bulb for its back up light across all listed model years, spanning 1989-1997.
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 United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is typical for incandescent automotive signal and reverse lighting applications.
The bulb uses a BA15s bayonet base, a single-contact mount where the "s" designation indicates a single pin, distinguishing it from dual-contact bayonet bases used in other bulb types.
The single-filament design of the 1156 means it serves only one lighting function, unlike dual-filament bulbs that handle two functions such as braking and running light simultaneously.
Because the BA15s base is a single-contact type, substituting a dual-contact bayonet bulb such as the 1157 would result in an incompatible fit, as the base geometry differs between the two.