Fan-related brake servo knocking (!)

Discussion in 'Electronics' started by arai1200, Oct 2, 2007.

  1. arai1200

    arai1200 New Member

    Messages:
    11
    Joined:
    Oct 2, 2007
    Car:
    '99 CLK 320
    A bit of an odd one this. I'm not even sure if it belongs under this category, so Administrators feel free to move it! I checked a load of the existing posts for something relevant in this category and wheels and tyres, but I couldn't find anything. Hopefully one of you clever chaps will be able to help me out a bit. I'm usually pretty good but this has got me stumped!!

    I was driving my '99 CLK320 to work this morning and got stuck in traffic. I noticed that there seemed to be a knocking sensation coming through the pedal every second or so. I pumped the pedal, stuck it in neutral, then park, but nothing made any difference. I thought I might be able to hear it so I turned off the radio, but still couldn't make it out over the noise of the heater fan. I turned the fan off and, to my considerable confusion, the noise/sensation stopped!

    On returning from work (still happening on the way home when the heater's on) I had a chance to take a closer look. It appears that the knocking occurs roughly once a second when the engine is running (in any gear) and the fan is on (any speed). On opening the bonnet, the knocking can be felt when touching the brake servo even when no pressure is being applied to the pedal.

    Any thoughts/help would be gratefully received. I cannot think of any reasons why the braking system would be linked to the fan.
     
  2. Dieselman

    Dieselman Banned

    Messages:
    34,206
    Joined:
    Jul 13, 2003
    Car:
    Peugeot 403 Convertible
    Hi.
    The kncocking is the heater duo valves pulsing, perfectly normal.

    As a test put your hand on the heater pipes.
     
  3. OP
    OP
    arai1200

    arai1200 New Member

    Messages:
    11
    Joined:
    Oct 2, 2007
    Car:
    '99 CLK 320
    Thanks for the reply. It sounds like nothing to worry about then. Out of interest, why does the sensation get transmitted to the brake pedal?
     
  4. Dieselman

    Dieselman Banned

    Messages:
    34,206
    Joined:
    Jul 13, 2003
    Car:
    Peugeot 403 Convertible
    Because the valve is right next to the servo and master cylinder.
     

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