Question about Mercedes-Benz 420-Series

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1991 mercedes 420 sel..compressor comes on and the air will no longer blow out the vents in the dash..still gets air from defrost vents,but no air from vents in dash..

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  • Ronald Whetstone Jul 14, 2010

    Thank ..I need it to be very basic.. Very Very Helpful



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The vacuum motor nside the dash is not working properly either due to a vacuum hose being disconnected, or the door is somehow blocked from moving to the vent position.

Need to take dash apart to see mechanism to fix.

Good luck to you.

Posted on Jul 13, 2010

  • 1 more comment 
  • Ronald Whetstone Jul 14, 2010

    Thank you for your reply..I would like to add that when the compressor is off in the ec position there will be air flow through the dash vents. will the expansion valve have anything to do with this?

  • Duane Wong
    Duane Wong Jul 14, 2010

    I do not think that the expansion valve has anything to do with this problem. It's more of a logical flow problem of cool or hot air from the evaporator(cold) or the heater core (hot), and the mixture of those two that is causing your problem through the vents.
    Thermal expansion valve

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    Thermostatic expansion valve
    A thermal expansion valve (often abbreviated as TXV or TX valve) is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheat at the outlet of the evaporator. This is accomplished by use of a temperature sensing bulb filled with a similar gas as in the system that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As temperatures in the evaporator decrease, so does the pressure in the bulb and therefore on the spring causing the valve to close. An air conditioning system with a TX valve is often more efficient than other designs that do not use one.[1]

    A thermal expansion valve is a key element to a refrigeration cycle; the cycle that makes air conditioning, or air cooling, possible. A basic refrigeration cycle consists of four major elements, a compressor, a condenser, a metering device and an evaporator

  • Duane Wong
    Duane Wong Jul 14, 2010

    You see, there are two different parts to the a/c system and getting cold air through the vents.

    1. is the coldness of the evaporator which can be made to get cold, in some brands of a/c with an orifice tube or expansion valve.

    2. is the pathway to the vents as controlled by a simple flapper that may go through a convoluted series of pathways to reach the human being who feels the cold air, as a result.

    The a/c evaporator could be cold as ice, but if the flapper doesn't direct the air to the vents, the recipient of the cold air on a hot day will not feel it.

    Also, the fan forces air through the evaporator to cool the air and then it flows to the recipient making them, in turn, cooler and more comfortable on a hot day.

    So, it's not that hard to understand if you put it that way.

    I hope this helps you to understand the system better.



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