Brake Servo failure switch

TheMegaMan · Feb 28, 2020

A previous owner has fitted a brake servo to my 1979 S3. A good addition, IMHO, but I don't know its source.

It does have a switch just below the master cylinder mounting, which I assume is a failure switch. The switch for the brake lights is above the pedal box, so it's definitely not that.

Does anyone know what such a switch would be checking? Is it simply a vacuum switch, or does it check the physical integrity of the diaphragm or some other internal component?

My switch is currently closed circuit...

My engine is out still out so I can't pull a vacuum to check whether that will open the switch to remove the fail, so I'm hoping that's all it is, but wondered if anyone could warn me of another possible fault in the meantime.

Thanks,
Adam

steve2286w · Feb 28, 2020

Have a look at the diagram in your previous post
https://www.landyzone.co.uk/land-rover/brake-fail-harness.356421/#post-4782653

It sounds like either the vacuum switch or an excess pedal movement switch , can you post a pic , have you removed the switch to see what actuates it , movement/ air etc

Rodeo Joe · Feb 28, 2020

I think you're talking about the servo vacuum failure switch. Its connected in with the PDWA switch, if you look at the wiring diagram in the brake section of the workshop manual it shows the set up.
It was mentioned and somebody posted a pic of it in another thread recently but I can't remember which one.
It section 70-1 of the book, cant seem to copy and paste it at the moment.

TheMegaMan · Feb 29, 2020

Rodeo Joe said:
It was mentioned and somebody posted a pic of it in another thread recently but I can't remember which one..

Oops, I did it again. RTFM...

@steve2286w is right - it was my 'brake fail harness' thread where the page of the manual was posted. And yes, it clearly refers to it as the servo vacuum switch. I must learn to read before asking stupid questions.

So that explains why my switch is closed when I can't yet run the engine.

But also means I kind-of wasted my time hooking up the relay to the starter wire as a test for the fail lamp, as it will be naturally tested before the engine starts by the lack of vacuum.

Oh well...belt and braces...

steve2286w · Feb 29, 2020

Stupid questions lead to learning all a good thing, I wonder why yours has the switch and mine does not
Looking at the diagram the switch is open circuit and you can test it by shorting the two switch wires then the BRAKE lamp should light on centre console

TheMegaMan · Feb 29, 2020

steve2286w said:
Stupid questions lead to learning all a good thing, I wonder why yours has the switch and mine does not
Looking at the diagram the switch is open circuit and you can test it by shorting the two switch wires then the BRAKE lamp should light on centre console

As a 1979 vehicle, mine shouldn't have a servo at all, so it was an upgrade by a previous owner (along with making it dual circuit). I've no idea what vehicle it would have come from, although knowing the guy was a Ferrari fan and the carb on it was from a Fiat, I suspect it was from some kind of Italian car.

The switch would be open when not showing a fault, ie. there is a vacuum. As I don't have a vacuum, that explains why my switch is closed and the light is on. It goes out when the switch is disconnected, so it's definitely the servo switch that's showing the fault condition.

Rodeo Joe · Mar 1, 2020

Is it a 109 or 88, lwb had servos a lot earlier that 88's.

TheMegaMan · Mar 1, 2020

Rodeo Joe said:
Is it a 109 or 88, lwb had servos a lot earlier that 88's.

88.

Brake Servo failure switch

TheMegaMan

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