Brake lamp lights up after replacing switches

Discussion in 'Mech Tech' started by Bugster, May 26, 2020.

  1. Hi

    My brake lamp was sometimes flashing when I pressed the brake pedal in my 74 bay. My local VW parts pusher told me to replace both the swiches located on the master brake cylinder underneath the bus. So I bought two and did what he told me to. I spilled a few drops of fluid when replacing them.

    Now the brake light is lit all the time, but goes dark when I press the brake pedal. What have gone wrong?

    I see no leaks, and the brake pedal feels normal and brakes works fine.

  2. Are they 3 pin switches?

    Or have you just crossed over the live and the return between the two switches?

    If you disconnect each of the switches in turn you should still get a brake light coming on if you want to test each switch separately, then make sure the connection between the two is not crossed over, in which case the light will stay on when not connected to the switches at all....

    Sent from my SM-T580 using Tapatalk
    Last edited: May 27, 2020
    snotty likes this.
  3. Sorry for my late reply. Seems I don't use the bus as much as i should. :(
    Yes, they are three pin.
    I doubt I connected the wrong plug to the wrong switch. The cables are ziptied in a way that makes it unnatural to swap them. I did discover a leak through my new switch, ans will replace it again and recheck.

    Thanks for your comment. :)
  4. There are three tags on each switch ? So why cant you just put the new switch in 180 degrees out.. its on a thread ?
    Then even though the wiring lines up with the tags its on the wrong tags.

    Look for a marking on the old and new switches and match that with the wires. Cut cable ties if needed.

    Or use the wiring diagram to get the right wires on the normally open amd normally closed contact.

    nobayinhell likes this.
  5. Unless you have the dash brake warning set up just ignore one switch and don't use it...,if you have a bunch of connectors test them or touch a black and a black/red one untill lights come on.. then youve found the feed and the line to the ligjts, pop the black on one terminal and the black and red on another..probably opposite sides of the switch if it's 3 pin.

    Sent from my SM-G930F using Tapatalk
    zedders likes this.
  6. It’s likely to be the switches causing the problem
    The non genuine ones available are terrible quality
    snotty and Purple like this.
  7. Have bled the system could be trapped air
  8. Swapped the contacts today. Bled all four corners. Didn't help. Disconnected the front switch. Light is now off! Reconnected the front, disconnected the rear switch. Same result. Light is off. Still not understanding what the cause is. Thanks for all your input so far. The search continues.
  9. This^ might be a clue – you could have air in one brake circuit.
    Mark Darby likes this.
  10. This is the most useful diagram I found for this

    Attached Files:

    mcswiggs and zedders like this.
  11. You can test your wiring by switching the ignition on and putting 2 of the 3 wires together in turn.

    One of the combinations will illuminate the brake lights.

    Ignoring the 3rd wire for a moment you can fit these to the brake light switch terminals until you find a combination that works.

    The 3rd wire goes to the spare terminal.

    In my experience they don’t need a great deal of pressure to operate and unless your brake pedal sinks to the floor with no resistance, I doubt that air in the system is your problem.

    Obviously it helps to have an assistant to press the brake pedal while you work under the bus.

    Good luck.

    zedders likes this.
  12. Did some measuring with the multimeter today. My conclusion so far is that one of my new switches are faulty or the wrong type. I am ordering one more and trying that first. Thanks again for all input. :)
  13. There are lots of cheap ones available and they really aren't very good. Coolair have the proper ATE ones but quality ain't cheap. They do work though.
  14. Does seem from Mark's experience that some cheapo switches simply don't work.
    Mark Darby likes this.
  15. And that is also the case here! Happy to let you guys know that replacing my new china-crap switch with a Meyle-switch seems to have solved the problem!
    paradox, Valveandy, zedders and 2 others like this.

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