Brake lights not working

[moggy]

This problem had me stumped. My brake lights have stopped working. The bulbs, wiring and fuses are fine. There are 2 wires coming down to the brake switch on the master cylinder. Using my multimeter, one seems to be an earth and the other seems to have 0.5V on it when the ignition is on. I assume the 0.5V goes back to some kind of relay, which then powers the 12V brake lights. Now to exclude the brake switch being a problem I touched the 2 wires coming to it together and would assume that the brake lights would come on. They do not. So does this mean a dodgy relay somewhere? one would assume so. However, what also has me puzzled is that when I put my multimeter across the brake switch set to Ohms and get someone to put their foot on the brakes, I see no difference in the ohms. So that would say to me that the brake switch is also faulty. So should I assume that both a relay and the brake switch have suddenly gone faulty?

Futhermore, I took the brake switch off the master cylinder. It's different from the usual brake switches I'm used to which usually work off the brake fluid pressure. This one seems to have a small plastic pushrod which seems like it should work off something making it move within the master cylinder. However, simulating this pushrod using a nail and getting someone to put their foot on the brake pedal. But I feel nothing happen to the nail. Should I then also assume that the master cylinder is also faulty (it's only a few months old).
I'm totally confused as to what is the cause of the brake lights not working - to me it seems everything related to the brake lights apart from the bulbs is not working. Help

[Mike911scrs]

HI moggy
is this on the 914, if so the switch is on the brake pedal for the light at the rear, the one on the cylinder is for the light on the dash (if it has one), this is to tell you one of the brake systems, front or rear has failed, as if you would not know already

switch is 911 613 411 01 and the price from Porsche is £49.89 + vat

regards mike

[moggy]

Thanks Mike. It is indeed on the 914. The master cylinder is a 19mm one from a 911 I believe. I got rid of that pedal switch many years ago and wired it instead to the brake witch on the master cylinder.

[914 KEV]

Thanks Mike. It is indeed on the 914. The master cylinder is a 19mm one from a 911 I believe. I got rid of that pedal switch many years ago and wired it instead to the brake witch on the master cylinder.

Original wiring on the 914 does not have a relay for the brake lights.

Did you fit a new brake switch in place of the pressure switch already on the master cylinder?
As this switch only switches when there is a loss of fluid/pressure, not when the pressure increases!

[moggy]

Thanks Kev. That explains it then. I recently replaced the old 19mm master cylinder. The old one had a fluid based brake switch on, like the one below:



But this new one came with a switch like this, which has this weird plastic plunger coming out of it:



It seems I can't install the old fluid based brake switch in this new master cylinder so I need to buy another master cylinder that is suitable for a fluid based brake switch (which should then make my brake lights work). Any links to a part number so I can get the right master cylinder this time - I don't want to end up with another one with that plunger type of brake switch on.

This is the one I bought previously. You can clearly see the brake switch with the plunger on. This is the one I don't want.

https://iapunitedkingdom.com/catalog/pr ... /id/170593

In case anyone is wondering - no I can't install the original brake pedal switch due to changes made around pedal cluster during rallying years.

Thanks for your help guys.

[moggy]

Ignore me guys. I'm an idiot

A wire had come off the brake switch behind the pedal. What a plum

[KS]

I am duly ignoring you.

You plum.

[911hillclimber]

Good to see there are others like me then!
Things like this happen to me, but I change the most expensive complicated things first.....