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Mercedes 300D
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Hi, my '81 300D is experiencing what I think is a strange problem. The heat, when set to MAX, will only get warm and not HOT. Also, when the dial is set to anything other than MIN, the car takes a few extra seconds to shut off. I've plugged the door locks because there is definitely a leak in that system. All of the other vacuum systems seem to operate normally. Even the vent louvers operate normally, if not quickly, when set to different settings. The A/C on the car doesn't work.

I'm assuming that a vacuum connection is activated when the heat is turned on. I've checked the connections/hoses behind the instrument cluster, the climate control buttons and the glove compartment. I did notice that there is a 'foam' hose that connects the ambient temp sensor hose to another hose that exits the firewall behind the glove compartment. It is spongy and falling apart. I didn't check the vacuum hoses under the battery tray.

Would anyone know why the heat doesn't get HOT, just warm? Also, where should I look for the connection between the heat being on and the engine taking just seconds more to shut off? Mono valve? Is there a vacuum hose line that is related to the heater? Any ideas/help would be great! Thanks!!
 

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1982 300D Turbo
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5 Posts
Maybe your mono valve?

Might be the Mono Valve, which is supposed to let hot water into the heating element and often goes bad. MZ makes a repair kit for it that gives new seals and element for a around $28. The job takes about 10 minutes with a screw driver and a 10mm socket. It is located behind your air filter housing against the firewall. If you remove it you can easily tell if the seals have gone bad.

Mercedessource.com also sell the kit with great instructions. I have bought several of their manuals and kits and have been very pleased.
 
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