The Ford LTD II front turn signal bulb size measures 1157NA or 1034NA, while the rear turn signal bulb size measures 1157 or 1156. The replacement chart covers 1977 to 1979 vehicle 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 Ford LTD II turn signal specifications reference four distinct bulb types across front and rear positions for the 1977-1979 model years: 1157NA, 1034NA, 1157, and 1156.
Three of the four bulb types, specifically the 1157NA, 1034NA, and 1157, are dual-filament designs operating at 12V with a 21W/5W power split, where the higher wattage filament serves the turn signal function and the lower wattage filament serves a secondary running light function.
The 1156 is the only single-filament bulb in these specifications, rated at 12V and 21W, with a BA15s bayonet base rather than the BAY15d base shared by the dual-filament types.
The BAY15d base found on the 1157NA, 1034NA, and 1157 features two offset locating pins, which physically prevents substitution with a BA15s-based bulb such as the 1156, making the rear turn signal base type for 1977 incompatible with the socket used in 1978-1979.
The 1034NA is described as a variant of the 1157, sharing the same BAY15d base, 12V rating, and 21W/5W filament configuration, with the distinction being the natural amber glass envelope rather than a clear one.
The 1157NA similarly carries natural amber glass, differentiating it from the clear-envelope 1157 used in the rear positions for 1978-1979. Both share identical electrical ratings and the BAY15d base.
The Ford LTD II front turn signal changed from the 1034NA in 1977-1978 to the 1157NA in 1979, representing a shift between two amber-glass dual-filament bulbs that are variants of each other but carry different ANSI designations.
All four bulb types in these specifications conform to ANSI standards recognized in both the United States and Canada.
Because the 1034NA and 1157NA are both amber-glass variants with BAY15d bases and identical 12V/21W/5W ratings, a vehicle using one may accept the other physically and electrically, though they carry separate ANSI designations.
The Ford LTD II rear turn signal for 1977 uses the 1156, a single-filament bulb, while the same position for 1978-1979 uses the dual-filament 1157, meaning the two configurations serve different circuit designs and the bulbs are not interchangeable.