The Mack RW front and rear side marker bulb size is 1891 for this truck model. A replacement chart covering 1985 to 1993 vehicle production years contains the bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RW uses the 1891 bulb for both its front and rear side marker positions across the 1985-1993 model years.
The 1891 is an ANSI-standardized bulb, meaning its 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, a designation that refers to a tubular glass envelope with a wedge-style base rather than a traditional pin or bayonet fitting.
A wedge base bulb is retained by the tension of the socket itself, with no separate locking mechanism, which is a common construction in low-wattage marker lighting applications.
The 1891 operates at 12 volts, consistent with the standard electrical system voltage found in commercial vehicles of this era.
At 2 watts, the 1891 draws minimal current, a characteristic typical of side marker bulbs, which are required to remain illuminated for extended periods without placing significant load on the vehicle's electrical system.
Because the same bulb number applies to both the front and rear side marker positions on the Mack RW, a single bulb type serves the full side marker lighting complement of the vehicle.
The T5 wedge form factor is not interchangeable with bayonet-base or indexed-pin bulbs, so socket compatibility must be confirmed before substituting any alternative bulb, even one with matching voltage and wattage ratings.