TheMegaMan

TheMegaMan

Well-Known Member
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
 
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.
 
Rodeo Joe said:
It was mentioned and somebody posted a pic of it in another thread recently but I can't remember which one..
Click to expand...

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...
 
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
 
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
Click to expand...

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.
 
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