The Mack CM front and rear turn signal bulbs both utilize a 1157 size bulb for proper operation. The replacement chart covers vehicle production spanning 1988 to 1991.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1157 is the sole bulb type specified for the Mack CM across all listed positions and model years, covering both the front and rear turn signals from 1988-1991.
The 1157 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning it conforms to a defined set of electrical and physical parameters shared across manufacturers.
The 1157 operates on a 12-volt system, which is consistent with standard automotive electrical architecture.
As a dual-filament bulb, the 1157 houses two separate filaments within a single envelope, rated at 21 watts and 5 watts respectively, allowing it to serve both a high-intensity signaling function and a lower-intensity running or parking function from one unit.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design where the two pins are positioned at different heights to prevent incorrect insertion, ensuring the filaments are oriented correctly within the socket.
Because the front and rear turn signal positions on the Mack CM share the same 1157 specification across 1988-1991, a single bulb type serves both locations without substitution.
The BAY15d base is not interchangeable with single-filament bayonet bases such as the BA15s, as the dual-pin offset configuration is physically distinct and serves a different electrical purpose.