The Mack Mid-Liner MS Series uses the 1157 bulb for both its front and rear turn signals across all model years from 1981-1997.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope, each rated for a different wattage.
The two filaments in the 1157 are rated at 21W and 5W respectively, with the higher-wattage filament typically serving the turn signal function and the lower-wattage filament serving a secondary role such as running light duty.
The 1157 operates on a 12-volt electrical system, which is standard for automotive and commercial vehicle applications.
The 1157 uses a BAY15d bayonet base, which is an offset twin-pin mount. The two pins are positioned at different heights on the base, a design that prevents incorrect insertion by ensuring the bulb can only be seated in one orientation.
The 1157 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning its electrical and physical specifications conform to a defined industry standard rather than a proprietary format.
Because the BAY15d base is offset, it is not interchangeable with the BAY15s single-filament bayonet base, even though the two bases share a similar physical profile.
The uniform use of the 1157 across both front and rear positions means a single bulb type serves all turn signal sockets on the vehicle.