The Jaguar XJS brake light utilizes P21/5W, P21W, and 1157 bulb sizes across various production versions. Available data covers the specific span from 1976 through 1996 for the mentioned vehicle.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Jaguar XJS brake light circuit uses two distinct bulb standards across its production run: the 1157, which follows ANSI specifications common to North American markets, and the P21W, which follows ECE standards used in European and worldwide applications.
The 1157 is a dual-filament bulb rated at 12V with a 21W high filament and a 5W low filament, mounted on a BAY15d bayonet base. Its dual-filament design allows a single bulb to serve both brake and tail light functions simultaneously.
The P21W is a single-filament bulb rated at 12V, 21W, mounted on a BA15s bayonet base. Because it carries only one filament, it is dedicated to a single signalling function, unlike the dual-filament 1157.
Although the 1157 and P21W share the same 21W rating on their primary filament and both use bayonet-style bases, they are not interchangeable. The BAY15d base of the 1157 has two offset pins, while the BA15s base of the P21W has a single pin, making physical fitment incompatible between the two types.
The Jaguar XJS used the 1157 for brake lights in 1976-1979, then transitioned to the P21W from 1980-1991, before returning to the 1157 for 1992-1996.
The Center High Mount Stop Light on the Jaguar XJS uses the 12961 bulb across all listed years, 1984-1996, without variation.
The 12961 is a Philips part number designating a T10 miniature wedge bulb with a W2.1x9.5d base, rated at 12V and 5W. Its low wattage reflects its role as a supplemental stop light rather than a primary brake signal source.
The wedge base of the 12961 is physically and electrically distinct from the bayonet bases used by both the 1157 and P21W, meaning no substitution is possible between the Center High Mount Stop Light socket and the standard brake light sockets.