What's this sensor?

[Ads1987]

Hi guys,

Took my instrument cluster off today to try and fix one problem and in the usual fashion came across another. This sensor on the brake servo has no plug connected to it and it looks like it should have two pins but one is broken.

I've found the connector but can't fit it on properly, can anyone tell me what it is so I can replace it please?

Thanks!

Adam.

https://www.dropbox.com/s/l6477ehyoo5it ... 6%2020.jpg

[ScienceBoy]

Problem with your photo?

[California Dreamin]

Hi Adam.....it's your brake light switch.
Hydraulic pressure operated and screws into the brake master cylinder. Some vehicles have two, has yours? do your brake lights operate?

Martin

[Ads1987]

I thought it might be that but then I saw that there is another sensor fitted and connected closer to the actual servo unit, my brake lights work so that's why I wasn't sure what this switch should do considering it isn't plugged in? Unless it's been bodged and bypassed somehow?

Thanks for the help

[California Dreamin]

I believe only one is 'used' so if your brake lights are functioning correctly then leave it as be......perhaps someone will be along to explain the function of the second switch? i can't remember, however I have this nagging feeling that this has been talked about before.
Perhaps it is to retain brake light function if one of the split circuits has a 'total loss' ?

Martin

[Ads1987]

Well that seems to make sense, hopefully someone will confirm thanks Martin, really helpful!

[AdrianC]

perhaps someone will be along to explain the function of the second switch?

For vans with cruise control? Would seem odd that they'd fit it to all vans, though.

[nevill3]

I recently replaced my brake master cylinder and the new one that came had two holes for switches like yours but my original cylinder only had one. I went to buy a blank from the motorfactors but they had none in stock so I just purchased a second switch and fitted that. Maybe yours has had the same "Bodge".

[Woodyyellow]

I'm sure mine has two fitted, wired in series I think. I have the dash out at the moment so will take a photo later.

Here you go, just taken with flash, two brake light sensors wired in series



E D I T. Just looked at this again, they are wired in parallel not series, Doh!

[Ads1987]

Why would they be wired in series?

Maybe vw modified the cylinder to fit more models and you just fit the extra sensor to plug the hole then?

I guess as my brake lights work I don't need to worry then?

Thanks for the pics and info guys!

[California Dreamin]

Why would they be wired in series?

Like I said...'tandem' split braking system with two sealed chambers, the reasoning being, one line fails the other keeps on working, it's a fail safe device.....as far as the two brake switches are concerned, this would ensure the continued function of the brake lights despite no pressure on one of the lines.

The wiring certainly suggests that this is the reason....not cruise control....at least, not wired this way.

Martin

[icosahedron]

The dual brake light switches with three terminals were only fitted to early T3's. They are connected in parallel (see Bentley) for switching the brake lights. The third terminal connects to the brake warning light module. If one of the switches malfunctions, the brake lights will still operate through the good switch. The faulty switch will however trigger a silicon controlled rectifier in the brake warning light module and turn it on permanently. One can reset it by disconnecting the module but it will come on again the next time the switch fails.

Bentley shows that the two terminal switches in later models are simply connected in parallel.

[AdrianC]

The wiring certainly suggests that this is the reason....not cruise control....at least, not wired this way.

Agreed. Definitely looks like it's a fail-safe. As well as keeping the brake lights working if one circuit fails, it also keeps them working if one switch fails. Except, of course, you won't know that until the other switch also fails...

I like the sound of that earlier system, but it also introduces a stack of scope for new failures...