The Stovebolt Site Forums The Parking Lot Big Bolts Electrical principle of a brake light switch

Hello all, could someone please explain the 'principle' of the brake light switch on a 58 Spartan. I removed mine (loose arm) to replace it and took it apart to inspect the innards. It appears that the contacts are normally connected, and broken when the pedal is pressed and lever moves. I notice that the 'feed' for the brake switch comes from the headlight switch which confuses me a bit more than I already am .

Am I wrong in thinking that when the brake switch contact is broken, a ground circuit is somehow completed, allowing the brake lamps to light.....or am I way off? Thanks!

Very likely you're looking at the switch in it's "relaxed" position, with the spring pushing the lever to the switch closed position.
I know that on my AD truck, the spring in the lever is compressed with the brake pedal released and when the pedal is pushed, it allows the lever to be pushed by the spring to the closed switch position, and completing the circuit to the brake lights.
I don't know of any way that opening a contact would complete a circuit, unless a normally activated relay were involved.

That switch is known as a "normally closed- - - -held open" design. With no outside force applied to the lever arm, the circuit is closed, which sends voltage to either the brake lights or the turn signal switch, depending on whether or not the vehicle has factory-installed signals. When the brake pedal is released the pedal moves the lever to the "open" position and interrupts the circuit. The voltage to the brake light switch is supplied from the same source that provides voltage to the headlights- - - -"upstream" of both switches. If you follow that wire to its source, it comes from the ammeter- - - -"hot" all the time with the ignition on or off. The ground for the tail/brake lights is at the light sockets- - - -at the end of the circuit.

The only vehicles I know of that have switches on the ground side are older British vehicles, and some of the Japanese clones of them.
Jerry

To make sure that I understand, when the brake light switch is resting (closed), it serves as a ground {interruption) to the wire which feeds the brake lights?

Reread Hotrod Lincoln's reply above.

The switch has nothing to do with ground. The brake pedal holds the switch open when the brakes are not applied and the lights will be off.

When the brake pedal is depressed to apply the brakes, the switch arm moves to the closed position and takes power from the headlight switch/ammeter and sends it to the lights.

The ground for the lights is at the bulb socket in the tail light. No switching of ground, just like he said.

Got it! All of you are saying the same thing, in different ways! Not being very familiar with this type switch, I was not thinking that, while the switch is normally closed on the workbench, it is 'normally open' (so to speak) when installed, because the brake pedal holds it that way.
Many thanks to each of you for helping to clear the fog ....