Question about 1989 Mercedes-Benz 300-Class

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Car dies when heater turned on

Car runs perfectly fine when the heater is off but, as soon as I turn the heater/ac on the engine dies.

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  • bratt802 May 26, 2009

    It's when the heater is turned on.

  • Raymond Ramirez
    Raymond Ramirez May 11, 2010

    seems your alternator charging rate or battery condition should be check ,or your tensioning device for your belt,check your battery clamp for tightness . what happens if you turn your lights on while the car is on. go and look at the lights and see if they are dimm

  • Raymond Ramirez
    Raymond Ramirez May 11, 2010

    When you turn the heater on can you hear the compressor relay engage the compressor clutch before it dies

  • Raymond Ramirez
    Raymond Ramirez May 11, 2010

    is it the ac or heater. when the ac switches on the clutch for the compressor for the ac switches on. at this point the compressor may be froze. this causing the belt to stop causing the engine to die


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Assuming your charging system is working properly-You probably have an idle speed control problem. Behind the battery there is an over voltage relay with red top and a 10 amp fuse in it. This relay powers the a.b.s. and the idle speed control unit. Make sure fuse is o,k, and that the relay is not corroded. Next there is 2 micro switches on the throttle linkage(you will have to remove the air cleaner. This switches are tested with an ohm meter and show continuity in one position and open in the other(don't worry if the ohm reading is high).Next to the over voltage relay there is 2 black relays(one fuel pump the other is climate comtrol) The one that says klima is the climate control. Make sure it is not corroded and a lot of times the electrical connector gets bowed and you will lose contact. This relay is really more of a control unit and gives input to the idle and c.i.s. control units as far as the a/c and heater operation. Hopefully I have been helpful

Posted on May 30, 2009

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