Mack FDM License Plate Bulb Size

The Mack FDM license plate light requires a bulb size of 1891 for proper installation and function. Data for the specified component covers the range from 1990 to 1992.

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
1990-1992	1891

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Miniature wedge bulb (T5). 12V, 2W.	1891

Other Locations

Additional Notes

The 1891 bulb is the sole license plate bulb type listed for the Mack FDM across the 1990-1992 model years.
The 1891 is classified as an ANSI-standard miniature wedge bulb, meaning it follows a standardized North American specification recognized in both the United States and Canada.
The T5 designation refers to the bulb's tubular envelope, with a size expressed in eighths of an inch, making the T5 a relatively small tubular form factor suited to compact lighting housings such as license plate lamps.
The 1891 operates at 12 volts and 2 watts, placing it among the lower-wattage miniature bulbs used in automotive applications where minimal power draw is sufficient.
The wedge base design of the 1891 means the bulb is held in its socket by the tension of the base contacts alone, with no separate metal bayonet pins or screw threads involved in retention.