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?

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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
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13