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Discussion Starter · #1 ·
Hello everybody,

I have a SIII 109” 6cyl (well…. It used to be but that’s another story)
So I have a query about a light which is mounted in my centre console with the word ‘test’ on it. It is red and rectangular.

I believe that it is a brake failure warning light – but how does it work? It has four wires connected to the back – 1 white, 1 black and 2 black and white.

The black goes to earth, the white and one black and white are live when the ignition is on and one black and white is entirely isolated (ie not live and not earthed).

The light has never lit in the 14 years I have owned my SIII except when I wiggle the ‘test’ part of the light.

Is this how it should be? I cannot logically work out how these electrical connections lead to any sort of warning light….

Thanks for your help in advance


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222 Posts
I have a similar brake warning light on my SIII diesel, but it doesn't have the test button- it self tests when you crank the starter.

The light will come on when:

1) A difference in brake pressure is detected between the front and rear brake systems (on dual-circuit brakes). In other words, when one system has a major leak.

2) The vacuum in the servo drops.

3) The system detects excessive pedal travel.

The switch for the pressure sensor is on a nasty bit of engineering called the Pressure Differential Warning Actuator (PDWA). This is a 4-way switch mounted on the chassis rail under the master cylinder. It has a shuttle-valve in it, the idea being that if the pressure in one system fails, the valve will seal it off and activate the warning light.

The vacuum pressure sensor is simply screwed into the front of the servo unit, tucked behind the master cylinder.

The excessive travel switch (if you have it, not many do) is just behind the trim panel in the driver's footwell that the pedals poke out of.

That's what should work your warning light. Quite why LR decided that the Series III needed such a complex brake warning system, I don't know.

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