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Old 02-01-2000, 11:33 AM
Nick Kotterman
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My '86 420SEL only blows warm air out of ALL the vents, even when the temp wheel is turned down. In that case, the center vents open up and hot air pours out of them. The system picks the right settings, it just won't blow cold air. It warms up the interior till it is uncomfortably hot, and then opens the center vents to (i assume) cool it back down, but only hot air blows out of them. I have resorted to pushing the min. fan setting, and then turning off the system when it gets too hot. I am just thankful it is only 35 degrees out, or else I don't know what I would do. Any suggestions?? Thanks for the help.

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Old 02-01-2000, 12:17 PM
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The clmate control unit in the dash applies a voltage to the servo valve which controls the flow of hot coolant through the heater core. The servo valve is a normally open valve. This means that if no voltage is applied fromt the control unit, it flows maximum hot coolant through the heater core causing hot air being blown under any conditions.

If you check for voltage at the connector to the servo valve under the hood, I'll bet you will find that no voltage is present under any condition. If this is indeed the case, the problem is most likely in the climate control unit. If there is voltage there, check the valve.

The control units are a few hundred dollars exchange and come with a chart which gives you the pinouts for checking the sensors and actuators which it controls using an ohmmeter. I'm sure that the mercedes shop, the people who host this site, can provide the control unit or the valve, whichever you need.

Good Luck,
Larry Bible
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Old 02-01-2000, 06:22 PM
Senior Member
Join Date: Apr 1999
Location: Walnut,CA,USA
Posts: 554
Replace the mono valve ins here is part number 000 835 06 44,just remove four screws at top of mono vlave,valve is locate right next to battery under hood. good luck.

MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner
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