If a brake light switch is bad, will it blow fuse? Is there any way to test this?
The switch makes and breaks the connection to the lights from the ungrounded (usually positive) side of the electrical system. If the switch has broken down internally, the current could find a path to the chassis of the vehicle which is the grounded side (usually negative). Disconnect the brake light switch and put an ohmeter between each terminal and the chassis of the car (a bolt going into the frame,etc.) While someone steps on the brake, see if the resistance goes to zero. If it does, there is problem in the switch and it needs to be replaced.
Just curious. Are all brake light switches in the hot side? Why? I have the impression that a lot of similar functions (e.g., door jamb switch) are in the ground side of the circuit, in which case the failed switch would not blow the fuse.
I think most stoplights are gounded to the chassis. If the circuit was completed through the ground when the brake was depressed, an additional wire would have to be run back to the brake light switch from the rear of the car.
You could also have the possibility of a sagging bulb filament in a dual filament bulb. (brake/tail) One filament hits the other and it’s quite possible to have a direct short.
Very poor contacts on the bulb sockets can also cause high resistance in the electrical circuit and this could also cause a popped fuse; especially with those old ceramic fuses.
I thought of that. But if you run a +12V bus to the back for all the various things back there, and then run separate ground-side leads back to the front for the control switches, you have only one add’l wire (the +12V). I was wondering if there were some safety or reliability concern that I had not thought of. Not important enough to worry about any more, and if we had Private Message available I would not have used up the Forum’s space.
Is this a hydraulic switch screwed into the master cylinder? If so, determine the “hot” wire to the switch, disconnect the OTHER wire that goes to the brake lights, and step on the brakes. If the fuse blows, the switch is shorted internally…