Brake light grimlins

[Dan Tweed grey]

My brake lights only work when the lights are turned on. I've had the engine out so may have disturbed somthing?
Any ideas?
cheers

[LarryLebel]

You don't say what Mini you have but most brake lights get their power by way of a green wire directly from the fuse box. Somehow in your case the wire has been changed to come from the light switch.

[Dan Tweed grey]

I've bridged the two wires from the switch and the brake lights come on , so I've just ordered a new switch .
I'ts a hydraulic switch 'probably from 1965 so it's done it's duty I suppose. I don't want to take it off untill the new one arrives .

[111Robin]

That doesn't explain why the brake lights are coming on with the tail lights though. There is no common connection between the brake and tail light circuits, the feeds come from two separate sources ie. brake light switch and light switch. Are the brake lights still coming on when you switch on the tail lights ?.

[woodypup59]

There is no common connection between the brake and tail light circuits, the feeds come from two separate sources ie. brake light switch and light switch.

But they both use the same earth. Check the earthing of the rear lights.

[111Robin]

There is no common connection between the brake and tail light circuits, the feeds come from two separate sources ie. brake light switch and light switch.

But they both use the same earth. Check the earthing of the rear lights.

The earth is common to all circuits on the car. The brake lights are working so there is a circuit to earth through the bulb. The issue is that the feed to the bulb appears to be coming from the main light circuit. What doesn't make sense is that bridging the wires at the brake light switch proves that the circuit is correct.
A question for the OP : When you say the brake lights only work with the lights on, do you mean they are on as soon as you switch the lights on, or do they work with the pedal, but only while the lights are on ?.

[finchy533]

The earth is common to all circuits on the car. The brake lights are working so there is a circuit to earth through the bulb. The issue is that the feed to the bulb appears to be coming from the main light circuit. What doesn't make sense is that bridging the wires at the brake light switch proves that the circuit is correct.
A question for the OP : When you say the brake lights only work with the lights on, do you mean they are on as soon as you switch the lights on, or do they work with the pedal, but only while the lights are on ?.

Trying to think this through with some dodgy application of ohm's law, apologies if I'm talking nonsense...

The two filaments in the bulb share a path to earth via the base of the bulb. If there is current flowing through the tail light filament to earth, could that effectively reduce the resistance of the (shared) path to earth via the brake light filament, just enough to overcome a higher-than-normal resistance being experienced at the brake light switch?

(That is assuming that the switch is failing and therefore offering some resistance when closed - or that could also be caused by a poor connection to the switch, perhaps as a result of a wire being disturbed during the engine removal - might be worth re-checking).

[woodypup59]

The earth is common to all circuits on the car. The brake lights are working so there is a circuit to earth through the bulb. The issue is that the feed to the bulb appears to be coming from the main light circuit. What doesn't make sense is that bridging the wires at the brake light switch proves that the circuit is correct.
A question for the OP : When you say the brake lights only work with the lights on, do you mean they are on as soon as you switch the lights on, or do they work with the pedal, but only while the lights are on ?.

Trying to think this through with some dodgy application of ohm's law, apologies if I'm talking nonsense...

The two filaments in the bulb share a path to earth via the base of the bulb. If there is current flowing through the tail light filament to earth, could that effectively reduce the resistance of the (shared) path to earth via the brake light filament, just enough to overcome a higher-than-normal resistance being experienced at the brake light switch?

(That is assuming that the switch is failing and therefore offering some resistance when closed - or that could also be caused by a poor connection to the switch, perhaps as a result of a wire being disturbed during the engine removal - might be worth re-checking).

Thats what I was driving at when I said check the earthing of the rear lights.

[Dan Tweed grey]

Well after some fault finding ......
The rear lights are l.e.d's so very little amperage is needed so yes the brake lights worked when the lights were on (pushing pedal) .
I put standard bulbs in and the brake lights did not work at all but did when the switch was bridged.
Anyway,,,,,,,,,,,,, fitted a new hydraulic switch and all is sorted.
Cheers