Mack WS front and rear side marker bulbs utilize 1891 size bulbs for both locations. The replacement chart covers 1981 to 1984 vehicle production years 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.
The Mack WS uses the 1891 bulb for both front and rear side marker positions across the 1981-1984 model years.
The 1891 is an ANSI-standardized bulb, meaning its physical and electrical specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1891 belongs to the T5 bulb family, which refers to a tubular glass envelope with a 5/8-inch width, housed in a miniature wedge base rather than a traditional bayonet or screw base.
A wedge base bulb is held in its socket by the tension of the base itself, with no separate metal contact pins, which is a common construction in low-wattage automotive marker lighting.
The 1891 operates at 12 volts, consistent with the standard electrical system voltage found in most commercial vehicles of that era.
At a rated output of 2 watts, the 1891 is a low-power bulb, typical for side marker applications where continuous illumination rather than high brightness is the functional requirement.
The identical bulb specification across both the front and rear side marker positions means a single bulb type services all four marker sockets on the vehicle.
Because the 1891 uses a wedge base, any replacement bulb must also use a wedge base construction to seat correctly in the socket, as wedge and pin-base bulbs are not interchangeable.