The Mack FDM front and rear side marker bulb uses a 1891 size across both locations. The replacement chart covers vehicle production spanning 1990 to 1992.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1891 bulb is the sole bulb size specified for the Mack FDM across both front and rear side marker positions for the 1990-1992 model years.
The 1891 is classified as a miniature wedge bulb, designated T5 in the tubular bulb sizing system, where the number represents the bulb width in eighths of an inch.
The wedge base design means the 1891 is held in its socket by the glass envelope itself rather than a separate metal base, a construction common in low-wattage marker and indicator applications.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower end of the automotive bulb power range and consistent with side marker lighting requirements.
The 1891 carries ANSI standardization recognized in both the United States and Canada, meaning the bulb conforms to a defined set of electrical and physical tolerances under North American standards bodies.
Because the same bulb size is specified for both the front and rear side marker positions on the Mack FDM, the 1891 is interchangeable between those two locations on this vehicle.