The Jaguar XJ12 brake light utilizes P21/5W, P21W, and 1157 bulb sizes depending on the specific production version. Available information covers the span from 1973 through 1996.
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 Jaguar XJ12 brake light data, covering model years 1973-1977 and 1994-1996.
The 1157 is a dual-filament bulb operating at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base under the ANSI standard used in the United States and Canada.
The dual-filament design of the 1157 allows a single bulb to serve both brake and tail light functions, with each filament handling a separate circuit.
The Jaguar XJ12 for model years 1978-1979 uses the P21W in place of the 1157 for the brake light position.
The P21W is a single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base under the ECE standard common in European and international markets.
The BA15s base of the P21W and the BAY15d base of the 1157 are not interchangeable despite their similar physical appearance, as the BAY15d has offset locating pins to prevent incorrect installation.
The 1994-1996 Jaguar XJ12 includes a Center High Mount Stop Light position, which takes the 12961 bulb rather than the 1157 used in the standard brake light position for those same years.
The 12961 is a Philips part number designating a miniature wedge-type bulb in the T10 form factor, using a W2.1x9.5d wedge base and rated at 12V and 5W.
The wedge base of the 12961 is mechanically distinct from the bayonet bases found on both the 1157 and P21W, making it specific to sockets designed for wedge-type fitment.
At 5W, the 12961 operates at a significantly lower wattage than either the 1157 or P21W, reflecting its role as a supplemental high-mount stop light rather than a primary brake light source.