For those who don't know what and where is Mono valve, I am attaching a photo of its location in 1985 300D Turbo(123)

Monovalve is electrically operated selenoid valve which opens and shuts based on the signals received from ACC( Automatic climate control), This valve control the amount of hot coolant that enters the heating core and thus regulated the heating of the interior.
If this valve fails, its usually a rubber diaphragm, that tears. Its very easy to repair and repair kits are available at many places including FastLane and the dealers.

Thanks

MVK

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1985 300D Turbo 179,000miles