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Old 12-03-2006, 11:48 PM
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Join Date: Oct 2006
Posts: 46
Cold air instead of heat 1979 300D

MB mechanic did by-pass on heat, it was working, now blowing cold air. Any suggestions? Thanks...T.C.

1979 300D non-turbo silver
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Old 12-04-2006, 09:57 AM
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Gotta another one...
Join Date: Jun 2005
Location: NE Ohio
Posts: 1,685
Find out what he did to bypass it. I'm not sure, but you may have the servo unit ( ), someone else can chime in here. If not, the monovalve ( ) is easily "altered", or rebuilt. Also, if it stopped suddenly, you may want to check your coolant level. Low levels will reduce heat.
1989 300E 144K
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Old 12-04-2006, 10:03 AM
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Join Date: Jan 2002
Posts: 18,350
79 has the servo. You need to figure out how the bypass works and how the flow of hot coolant to the heater core is controlled. It could be that there is a valve and the valve is closed. If you get heat at high speed, it could mean that the auxilliary coolant pump has failed. Could also be a bad thermostat. What is the temperature guage showing?
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
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Old 12-04-2006, 10:32 AM
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"Just Passin' Thru"
Join Date: Sep 2005
Location: Springfield,MO
Posts: 277
Here is your servo info. Enjoy!

"Our Lady of Blessed Acceleration...
don't Fail Us Now"
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Old 12-04-2006, 12:08 PM
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Join Date: Oct 2006
Posts: 46
1979 300D does have the servo

I think that the servo was mechanically by-passed but not physically (second new re-maned servo from German Star in 2 1/2 years) . The auxillary pump is not hooked up. The heater hose that runs along the fire wall has a valve in the middle. Open for heat, closed for AC. This valve used for the by-pass is alright, still a little heat will come out sometimes when I first start her up for just a few seconds and then just cold air! Blower works great new MB part 2 years old. Heat only works on defrost which I understand means a vacuum problem within the system?

1979 300D non-turbo silver
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