The Mack CM front and rear side marker bulbs both utilize a 1891 bulb size specification. The replacement chart covers vehicle production spanning 1988 to 1991.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack CM uses the 1891 bulb for both its front and rear side marker positions across the 1988-1991 model years.
The 1891 is classified as a T5 miniature wedge bulb, a designation that refers to its tubular glass envelope measuring 5/8 of an inch in width.
The wedge base design means the bulb is held in place by the socket gripping its base directly, with no separate metal cap or pins involved.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-wattage bulbs used in automotive marker lighting applications.
The 1891 carries ANSI standardization recognized in both the United States and Canada, meaning its electrical and physical specifications conform to a defined industry standard in both markets.
Because the same bulb size applies to all four side marker positions on the Mack CM, front and rear sockets accept the same 1891 unit without substitution or modification.