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W124 heating issue

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by ronhugh, Dec 31, 2012.

  1. ronhugh

    ronhugh Member

    Likes Received:
    Aug 3, 2009
    The heating in my 1990 W124 230TE will only run hot (better than running cold!). Even in the winter car gets much too warm, with all air controls off and then with no decent airflow windows get all steamed up.

    So, driving any distance, need to have airflow control off and open one window.

    I have replaced the duovalve but not the solution.

    Any ideas/solutions?


  2. Bolide

    Bolide Senior Member

    Likes Received:
    Sep 17, 2002
    Your Mercedes:
    BMW 525 Diesel Touring
    There's a temperature sensor in the car which tells the heater what temp to regulate to. It then sends a PWM voltage to the duovalve to regulate the water flow. Assuming the new duovalve is good (never wise to assume) the the fault lies in the sensing / controlling setup

    If you have an old-fashioned multimeter with a needle you might be able to read the control voltage to the duovalve and see if it varies as you move the temperature setting

    Nick Froome
  3. television

    television Always remembered RIP

    Likes Received:
    Mar 14, 2005
    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    Yes you can measure the voltage on the duo valve, though a pulsed voltage.

    the center pin is the common and the pin for the valve each side.

    With the heater on max you should have 11 volt approx over each side and zero volts when set to minimum.

    When set to max you can feel the valves pulsing by touching each outlet pipe about 2 to4 pulses per second. When set to off or cold there should be no pulse.

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