Electrical issue, brake lights stay on

My D90 Td5 has brake lights stuck on since my last drive. They remain on even when switching the ignition off. When I step on the brake pedal with the keys out of the ignition it starts glowing the glow plugs.

I removed the battery leads for now to avoid bigger problems, but does anyone have an idea where to start searching? My idea is that there's more involved than just the brake switch, because it should go off when the ignition is turned off, and it certainly shouldn't turn on the glow plugs.

An earth fault of some kind.

Or a shorting in the wires behind the dash.

Break out the Multimeter and start digging!!

I did some testing that revealed that the brake light switch is working properly:

cables disconnected: brake light on, ignition off
cables connected: brake light on, ignition on.

Problem remains that the brake light is always on, and the brake switch controls ignition.

I'm not quite sure what to start looking for, or what to measure from this point on . Tips very welcome...

srijken said:

I did some testing that revealed that the brake light switch is working properly:

cables disconnected: brake light on, ignition off
cables connected: brake light on, ignition on.

Problem remains that the brake light is always on, and the brake switch controls ignition.

I'm not quite sure what to start looking for, or what to measure from this point on . Tips very welcome...

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sounds like a job for an auto spark you could be going round in circles with that one

An auto spark?

srijken said:

An auto spark?

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Mr google is your friend :doh:

car electrician basically

A place thats always worth checking is where the main harness passes down the left side of the engine/gearbox it can sometimes chaff on one of the tunnel screws. You'll have to get underneath to check.

Thanks for that suggestion. I checked there and everything looks fine.

The problem is now gone by the way, magically solved itself. I think I should be checking all places that have permanent power, and are somehow close to the brake light circuitry. The only place I can think of right now is where the socket for the trailer connection is hooked up to the wiring.

Was it by any chance wet/damp when you had the problem and now it's warmer and gone away?

If so, earth fault very high on the list

It started after driving through some heavy showers, went away over night. I don't think it's *just* an earth problem though, because there was enough juice in it to run the fuel pump and wipers etc through the brake lights. IE there has to be some sort of power supply through the brakelight circuit.

There are separate earths all over the vehicle - so perfectly normal to have most stuff working and other bits playing up.

Unusual for the brake lights to stay on, rather than not light at all I'll admit, but the link to water is almost certainly more than coincidence.

Were the side lights on by any chance when the brake lights stayed on, or were they on with no other lights turned on at all? Just trying to figure where/how they might have drawn power.

Might be worth unscrewing the brake light lens, removing the buld, looking at the contact condition, spraying with silicon spray (or WD40) and reassembling.

If you're lucky it won't recur, but...

Sorry to revive an old thread but did you get to the bottom of this?

UnderwaterStan said:

Sorry to revive an old thread but did you get to the bottom of this?

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Like so many forum users they cone here to pick brains and not to contribute. They haven't been on the site for 5 years

Marmaduke said:

Like so many forum users they cone here to pick brains and not to contribute. They haven't been on the site for 5 years

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Worth a try haha.
Thanks for the reply man, merry christmas.