Jaguars have a black-handled rotary switch, which controls the headlamps and sidelamps. This sort of switch was common in many cars dating from the 1930s all the way through to the 1970s. However, in the process of investigating my loose wire, I noticed on the wiring diagram that Jaguar had skillfully renamed it!

Here’s a bit of history…

In the early days of motoring, many British automobile generators did not have a regulator to regulate the amount of generated amperage output based upon the state of charge of the battery. Instead, they provided a switch that allowed you to apply a small amount of charging amperage for summertime use, and a larger amount of charging amperage for winter time use, when the cold battery needs more charging. In the center of the switch was often located a button or a key slot, which functioned to turn on the ignition (also known as the “primary” ignition circuit). And, the rotary switch also turned on the sidelamps and headlamps.

So, on these early switches, the first position to the farthest left was the summer driving position, where the generator would supply a small amount of amperage to the battery to keep it charged and to power the ignition circuits. The next position would be a winter driving position, which would provide more charging amperage to the cold battery, and similarly provide power to the ignition circuits.

The next position to the right of that would control the sidelamps. Obviously, when the sidelamps are on, you need a bit more charging amperage, so it carried over the same amount of charging as the winter time power setting in order to provide the necessary power for the lamps. One more switch position to the right controlled the headlamps, and it continued to apply the winter time amperage setting to run the headlamps.

Here are some variations on those early PLC switches:




So… Lucas, in their infinite wisdom, named the switch a “PLC Switch”, which stood for Primary, Lamps, and Charging. The switch controlled the primary ignition circuit, with the key in the center, it controlled the headlamps and sidelamps, and also controlled the charging amperage delivered by the generator.

Lucas, in their divine dedication to tradition (i.e. stubbornness), never changed the name of that switch, decades after most of the functions on it had disappeared. When voltage regulators became common in the 1930’s, the charging function of the switch disappeared, but it remained called a PLC switch. In the 1950’s the ignition switch was moved to a separate device, and that function, too, was eliminated from the PLC switch, yet Lucas doggedly retained the same name, despite the fact that it was now no longer anything more than a light switch.



I now see that on the wiring diagram, Jaguar had discretely re-named it a “Position Lighting Switch”, which was a savvy way to name the switch for its actual function, yet still keep it mostly in line with what Lucas was still calling them on their spares boxes and equipment lists, which was a “PLC Switch”!




And now you know an obscure bit of Joseph Lucas history, and why Jaguar Cars didn’t just call it a “light switch “!

[As a further bit of Lucas doggedness history, you might note that the screws on most of these switches are still Whitworth/British Standard. Lucas, again, in its divine dedication to tradition (stubbornness), never changed that either!]