Original 1972 configuration
The Jaguar XJ6 lighting design borrows from earlier saloons. Standard Jaguar wiring links fog lamps to both running lights and headlights in that if fog lights are switched on, headlights automatically switch off, and vice versa. Running lights always turn on with either fog lights or headlights. Two front and back running lights (our model has front and rear side marker running lights, too) can be turned on independently from either headlights or fog lights.
Fog lights could be wired to turn on independently, but Jaguar standard wiring has them activated only with the running lights (or so the Service Handbook states, but see wiring discussion below).
Jaguar has two pairs of front headlamps; high beams illuminate all four bulbs.
The reverse light is placed on two horizontal bars on either side of the license plate.
Turn signal lights use the same side of the front and rear enclosures for left and right indicators, and hazard warning flashes all four lights.
Brake lights are housed in the rear enclosures.
Brake, reverse, and turn lights are automatically controlled when driving functions dictate (like stepping on the brake pedal), but the running, headlight, and fog lights must be activated by switches.
Rocker switch controls
The XJ6 rocker switches carry on the tradition of the Mark II saloon toggle switches, but Jaguar elected to dispense with the rotary light switch. The rotary switch proved to be logically convenient as it moved from S (running lights) to H (headlights) and F (fog) since the running lights were turned on first, and then either the headlights or fogs where switched on next.
But the linear XJ6 rockers remove the traditional rotary logic, and the result is somewhat awkward and confusing (especially since the fog lights have a separate rocker switch away from the main panel).
How do the fog and headlights work in tandem with the side lights? A slight-of-hand trick actually; the two center rockers are ganged together asymmetrically so that the headlight rocker automatically moves the side light rocker, but the side light rocker can also work independently.
When the headlights are on, the fog lights won’t work at all; confusingly the fog lights are controlled by a separate rocker, leaving the driver to fiddle around with various rocker switch combinations depending on what’s turned on and off.
This solution is too clever by half since the driver must remember which rocker combinations trigger which lighting configurations. Alas, if only we had the old rotary light switch back!
Rocker switch wiring
The Jaguar XJ6 Service Handbook says:
They [fog lamps] will not operate until the side lamps are switched ON and will be extinguished when the head lamps are switched ON.
But this logic is not supported by the official wiring diagram (C494925) for our 1972 XJ6 (chassis numbers 1L.53097 onward).
Only when the side light rocker is off does current flow to the fog light rocker switch. Jaguar could have wired the XJ6 as shown below to operate like the Service Handbook says but apparently never did.
Unless we return to the rotary light switch in the dash or somehow attach a modern steering column light switch, we must mitigate the rocker switch limitations.