The Merkur XR4Ti brake light utilizes 1157, 1156, and P21W bulb sizes depending on the specific version of the vehicle. Data for the lighting components covers the period from 1985 through 1989.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Merkur XR4Ti brake light circuit used two different bulb types across its production run, with the 1157 fitted for 1985-1987 and the 1156 fitted for 1988-1989.
The 1157 is a dual-filament bulb carrying an ANSI designation for the US and Canadian markets. It operates on a BAY15d bayonet base at 12V, with filaments rated at 21W and 5W respectively, allowing a single bulb to serve both brake and tail light functions.
The 1156 is a single-filament bulb also carrying an ANSI designation for the US and Canadian markets. It uses a BA15s bayonet base at 12V and 21W, meaning it handles only one lighting function rather than two.
The BA15s base of the 1156 and the BAY15d base of the 1157 are physically similar but not interchangeable. The BAY15d base features offset pins that prevent incorrect installation, so a 1156 cannot be seated correctly in a 1157 socket.
The wattage difference between the 1156 and 1157 is relevant to the brake circuit: the 1157 draws 21W on its brake filament and 5W on its tail filament, while the 1156 draws 21W on its single filament. Substituting one for the other would affect circuit behavior beyond just the physical fit.
The center high mount stop light on the XR4Ti used the 912 bulb across all model years from 1985-1989, with no change in specification during that period.
The 912 is a wedge-type bulb in the T15 form factor, operating at 12V and 12W. It carries an ANSI designation for the US and Canadian markets and is classified as a variant of the 921 bulb.
The 912 and 921 share the same wedge base and general form factor but differ in wattage. The 912 is rated at 12W while the 921 is rated at 16W, making them physically compatible but electrically distinct.