Mack FDM Turn Signal Bulb Size

The Mack FDM uses 1157 bulb size for both front and rear turn signal applications. Replacement chart data covers 1990 to 1992 production years.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Front Turn Signal

Years	Bulb Size
1990-1992	1157

Years	Bulb Size
1990-1992	1157

Description	Bulb Size
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157

The 1157 is the sole bulb type across all listed positions and years in the Mack FDM turn signal specifications.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions at different wattages.
The 1157 operates on a 12-volt electrical system and draws 21 watts on its primary filament and 5 watts on its secondary filament.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that prevents incorrect insertion by positioning the two pins at different heights along the base.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, placing it within a widely standardized North American automotive lighting classification.
The Mack FDM (1990-1992) uses the 1157 for both the front and rear turn signal positions, indicating a uniform bulb specification across those two circuits.
Because the BAY15d base uses offset pins, it is not interchangeable with single-pin bayonet bases or with the similarly sized 1156, which uses a BAY15s single-pin base despite sharing a comparable physical form.