Heater doesn't work properly at cruising speed!
Thread Starter
Newbie
Joined: Oct 2010
Posts: 4
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1981 300sd
Heater doesn't work properly at cruising speed!
High guys! I'm a newbie with this being my first post. I just bought a very clean 1981 300SD, (0-60 16sec, Slight upphill 0-60 - 21.18sec). Ok, the question - What could be the problem with my heater! After it is warmed up I get in and it is hot! However, when I get up to cruising speed cold air is getting in somehow or the heat just doesn't come out. The defrost is on for sure but not Hot - just warm. I checked the anti-freeze level and it is on par (I added maybe 12oz's max). Also there is a little vent just below the arm rest that blows outside temp air while cruising and wont shut off. Can anybody help me? Oklahoma is about to get cold. jmag
MBWorld Fanatic!
Joined: Apr 2007
Posts: 6,171
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300E a couple 1994 w124wagon E320 Wagon/,1971MGB Track/Rally, MG Midget Autocross ,2000 E320 wagon.
Watch your
coolant temp if it is in the normal range at rest or slow speed and drops cooler at speed you could have a thermostat stuck in the open position.
easy to replace. Part at www.autohausaz.com cheap.
easy to replace. Part at www.autohausaz.com cheap.
Junior Member
Joined: Nov 2008
Posts: 49
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From: Vancouver Canada and Darwin Australia
126,123
High guys! I'm a newbie with this being my first post. I just bought a very clean 1981 300SD, (0-60 16sec, Slight upphill 0-60 - 21.18sec). Ok, the question - What could be the problem with my heater! After it is warmed up I get in and it is hot! However, when I get up to cruising speed cold air is getting in somehow or the heat just doesn't come out. The defrost is on for sure but not Hot - just warm. I checked the anti-freeze level and it is on par (I added maybe 12oz's max). Also there is a little vent just below the arm rest that blows outside temp air while cruising and wont shut off. Can anybody help me? Oklahoma is about to get cold. jmag
In regards to the rear vent,its normal to have air come out unfortunately is does not shut off.They are all like this.
Keep us posted.
Thread Starter
Newbie
Joined: Oct 2010
Posts: 4
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1981 300sd
Wow! Thanks for the responses. I'll be doing both the Thermostat and Mono valve ASAP! Great to have a forum like this with knowledeable people that will share. A first time diesel user needs this type of connection. Thanks again, jmag
Senior Member
Joined: Jun 2006
Posts: 303
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From: Northern California
It changes all the time
A torn diaphragm will cause that symptom. Also you may experience the heater gowing full hot sometimes regardless of temp wheel. If this happens, remove the electrical connectors from their holder at the mono valve and squeaze them down. All MB heater systems since 1972 have been designed to fail in the full heat position in the interest of passenger safety. To shut the mono valve off it needs 12V. These contacts get tired over time and lose connection.
