The Jaguar XJ-S has bulb failure sensors that detect when a lighting circuit is not drawing as much current as it should and turns on a light on the dash telling the driver that a light bulb is burned out.  There are actually two different types of sensors:  a single sensor in the console that monitors the brake lights, and four sensors that monitor the parking lights, one for each corner of the car.  This is what the latter type look like:

This particular one is for the right rear of Kirby Palm's '83, and it mounts along the right side of the trunk opening.

Supposedly, some earlier or European models had only three sensors instead of four, using a single sensor to monitor all the parking lights on the front of the car.

These sensors are thermal in nature.  Inside each sensor is a resistance that heats up due to the current flowing through on its way to the lights.  If there is enough current, the heater gets hot enough to cause a bimetal strip to open a pair of contacts to extinguish the warning light on the dash.  Of course, this heating takes time; that's why the warning light is on for about the first twenty seconds after you turn on the lights.

There is an adjustment on these sensors; a tiny screw that moves the contact that the bimetal strip closes on.  It's visible in this picture at the upper left, but the screw head has been dyked off on this one.  It can still be turned with a pair of pliers.  Using this adjuster, the sensors can be tuned to respond at exactly the right current level -- but it appears Jaguar didn't want you fiddling with it, and instead sells these units pre-adjusted.

Since they work by heating, these sensors take power.  In fact, these bulb failure sensors seem to soak up about two volts of the power to the lights.  So, the lights that should be operating at full system voltage (13.8 - 14.4V) are actually operating at somewhere around 11-12V.  This makes the lights look anemic, since the brilliance of incandescent lights is very sensitive to voltage.

Palm is working on a replacement sensor design that uses a magnetically-operated switch.  The current to the lights goes through a coil, and the magnetism generated by the current through the coil opens the contacts in the switch to extinguish the warning light on the dash.  By wrapping the coil with thick wire, the voltage loss can be reduced to insignificance and the lights will operate at full brightness.  Since the switch will operate instantly, the warning light won't be on for twenty seconds any more.

A suitable magnetic switch has been found.  It is made by GC, part number 35-752, "Normally Open/Normally Closed Magnetic Reed Switch", available at better electronics supply stores.  The package includes the switch itself, a spool suitable for winding the coil, and a permanent magnet.  Here's the switch and spool:

No need to show the magnet, since it isn't used in this project.

Yeah, the thing is small.  It's sensitive, too; you get a magnet anywhere near this thing and it switches.  The contacts are rated at 0.5A, which is enough to operate that warning light.

I used a green background to make the white spool show up in the picture, but the glass tube surrounding the switch also has a green tint.

The plan is to insert the switch into the center of that spool and then wrap 16 gauge wire around just until the switch opens with the lighting current, and then perhaps one or two more turns.  There is no convenient way to adjust such a sensor other than by varying the number of turns.

The project isn't done yet; once done, this page will be updated with more pictures.