Mercury Monarch front and rear turn signal bulbs utilize 1157, 1034NA, and 1157NA sizes for the front position and 1157 size for the rear position. The replacement chart covers 1975 through 1980 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 1157 is the most frequently specified bulb across the Mercury Monarch's turn signal positions. It is a dual-filament bulb rated at 12V with a 21W high filament and a 5W low filament, mounted on a BAY15d bayonet base, and it meets ANSI standards for use in the United States and Canada.
The BAY15d bayonet base is shared by all three bulb types listed in these specifications, the 1157, 1157NA, and 1034NA. This base style uses two offset pins to ensure correct orientation during installation.
The 1157NA and 1034NA are amber-glass variants. Both carry the same 12V and 21W/5W ratings as the standard 1157, with the distinction being that their natural amber glass envelope produces a colored output without relying on a colored lens.
The 1034NA is described as a variant of the 1157, sharing its electrical ratings and base type while adding the natural amber glass construction.
The rear turn signal position on the Mercury Monarch uses the 1157 without variation across all model years from 1975-1980.
The front turn signal position shows variation across model years. The 1975-1976 and 1978 model years specify the 1157NA, the 1977 and 1979 model years specify the 1034NA, and the 1980 model year specifies the standard 1157.
Because the 1157, 1157NA, and 1034NA share the same BAY15d base and identical voltage and wattage ratings, physical fitment across these positions is consistent, though the glass color differs between the standard and NA variants.