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I have a 3 connection brake light switch on my early S3, running a full DC system with a fused positive, but when the wires are connected to both outside terminals of the switch then the brake pedal is depressed it's causing a short and blowing the fuse I have on the positive side of the battery, but if you disconnect the wires from the switch and touch the 2 wires together manually the rear brake light works ok does this sound like a dodgy switch to you? (would of thought they were that simple inside there's nothing to go wrong really) spose it could be a duff one.

Try removing the wires. Then use a multimeter on the ohms scale and check that when you operate the switch it doesn't connect the incoming supply to the switch to ground.

dickie wrote:Try removing the wires. Then use a multimeter on the ohms scale and check that when you operate the switch it doesn't connect the incoming supply to the switch to ground.

On a setting of either 2k or 20k the meter goes to zero when the pedal is depressed on the outer connections, I'm not great with a meter so not sure if that's right or wrong hopefully you can explain

That means that when you press the pedal, the 2 outer connections are connected together; this is what you want.

So, connect one lead to earth or battery negative and the other to the switch terminal. It should say something like infinity (figure 8 on its side, or OL which means overload).

Try that and report back as detailed as you can please and we'll sort it.

dickie wrote:That means that when you press the pedal, the 2 outer connections are connected together; this is what you want.

So, connect one lead to earth or battery negative and the other to the switch terminal. It should say something like infinity (figure 8 on its side, or OL which means overload).

Try that and report back as detailed as you can please and we'll sort it.

Thanks for this, on ohms again 2k or 20k and black meter wire to battery negative none of the switch terminals are giving any reading what so ever when foot pedal depressed.

OK. Put the meter to DC voltage range. Black lead on battery negative and red to switch terminal.

On one terminal, it should show 12V all the time. On the other terminal it should only show 12V when the switch is operated.......

dickie wrote:OK. Put the meter to DC voltage range. Black lead on battery negative and red to switch terminal.

On one terminal, it should show 12V all the time. On the other terminal it should only show 12V when the switch is operated.......

With the yellow and pink connected it would just blow a fuse when pressing the pedal, but have just got the nipper to push the pedal while just the yellow was connected while i held the meter leads and she saw a spark from the brake switch, on closer inspection I did it and watched, I saw a spark jump from the metal switch pin to the metal pedal pin (both of which meet when pedal in the normal position) then at the same time i heard the fuse blow, guessing that is not correct

Turns out after testing every connection (all 3) (wires dissed) is to the switch pin when the pedal is depressed, hence the short on the brake pedal when the switch pin comes out....... surely the switch is shagged

Lord-Spanner wrote:Turns out after testing every connection (all 3) (wires dissed) is to the switch pin when the pedal is depressed, hence the short on the brake pedal when the switch pin comes out....... surely the switch is shagged

You're right, it's goosed.

One of the pins, the one on the load or brake light side of the switch has an internal connection to the activation pin. It obviously shouldn't. You can prove this by using your meter on ohms and measuring between the activation pin and the electrical connection pin. You may have to press the pin to see this.

dickie wrote:

Lord-Spanner wrote:Turns out after testing every connection (all 3) (wires dissed) is to the switch pin when the pedal is depressed, hence the short on the brake pedal when the switch pin comes out....... surely the switch is shagged

You're right, it's goosed.

One of the pins, the one on the load or brake light side of the switch has an internal connection to the activation pin. It obviously shouldn't. You can prove this by using your meter on ohms and measuring between the activation pin and the electrical connection pin. You may have to press the pin to see this.

Wouldn't mind but its only been fitted recently and never used on the road, only really used to fit the pedal and the rear brake cable and to set the switch at the correct height for the pedal knob, more bloody remade junk that's not fit for purpose, got an MB one on the way so that should work I will test the old one when I take it off within the next couple of days.

Thanks for your help though Dickie very much appreciated

woo hoo, we have a brake light and no blown fuses, MB brake switch tested before fitting and alls good and hunky doory and yes the old one (say old it was relatively new and unused) was shot, out of interest I opened it up and what a load of junk, no wonder it caused a short, cheap and nasty internals, Its right what they say buy cheap by twice

Glad it's sorted.

dickie wrote:Glad it's sorted.

Yeah me 2, caused some right grief that did, cheers for the help though Dick