MGB Brake light switch

together)

pressed.

without

straight

It is possible to have voltage available but no amperage, so although a meter will read volts when you try to take a load like a bulb the voltage drops to nothing.

So the way to test is with both wires connected to the switch, check that you have 12 volts or nearly at the supply terminal, then apply brake pressure and check each lead, if both read 12 volt then the lights will be working, if the supply side still says 12 but the bulb side is very low then the switch is the fault.

Replace the switch quickly without trapping any air and there will be no need to bleed the brakes.

MrCheerful

Presumably this is measured with the feed to the lights disconnected? If so, the reason is simple. The DVM has a very high impedance input, so draws almost no current. A 'dirty' switch will have a high resistance which prevents it passing enough current for the lights but enough for the meter.

You could measure the voltage drop across the switch with everything wired as normal - you'll find it will be nearly 12 volts if it's faulty. Simply probe both contacts at get someone to operate the brake. It

If you really don't want to replace it, you could add a transistor to act as the switch - it will draw little current like the DVM so should switch ok. But it's probably easier to simply change it.