EK9: Electrical gremlins part 2 – Brake light switch

Well OK. last post i told you about the three problems i have with my EK9. While the Battery was solved, i later found out i’m an idiot and problem Nr. 2 was not even a problem. Just me being stupid. The parking light would not light even after replacement of the bulb. A four hours later it turned out that i replaced the wrong (H4) bulb istead of the correct 12V 5W bulb. Well two dollars later i had that one solved too 🙂
The first, wrong (headlight) bulbs which i exchanged instead of the small one:

Now to problem Nr. 3 (a real problem): The brake lights mostly don’t work. Curiously they sometimes DID work after a long drive…
Since i’m an electronic engineer i decided i’ll have a look at the schematics before i bring it to the workshop…
As you can see the orange (brake lights) and red (signal  horn) circuits are connected to the same fuse. So when i hit the Horn and it worked, i knew the fuse for the 24V DC supply to the Brakelight swich must be OK as well…

What worried me a bit that not only the brake lights won’t work if the “stop signal” is missing, but also the ABS unit is connected to it and won’t work either since the schematics revealed it’s connected to the brake light switch directly as well.
Double safety issue here! I really wanted to fix this ASAP!
Since i knew the lights were OK and the 24V supply is OK too, the most possible reason (aside from wires, which rarely fail on a car this age) would be the brake light switch (“brems schalter” in the german schematics above). How can you check that?
First, remove the cover below the steering wheel (three screws and a few clips):

Have a look at the wiring mess underneath 🙂

Search for the Brake light switch, witch is mechanically connected to the top of the brake pedal (when pedal is pressed, the brake light switch is actuated):

Remove the two-pole connecting-plug from the switch and make a short circuit at the two poles using two measuring heads / pins and a piece of wire connecting both of them.

If they’re short-circuited you directly feed the 24V DC to the brake lights and ABS unit and the brake lights should turn on now. Which they did in my case. So it’s clear: The brake light switch does not make contact when the pedal is pressed anymore. Solution? Get a new one. It seems to be an easy install so i decided to order it myself:

Partnr: 35350-S04-003

Well let’s complete this job once it arrives here. I guess i saved some money by investigating myself. For additional fun here’s a picture of the complete wiring harness in the car.  


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