I'm pretty sure an 80 TD does not have a monovalve, but the ACC servo. It could be a bad servo, BUT I believe it does have an auxilliary heater pump, the failture of which would account for why you have heat at high rpm's with a lot of circulation from the main water pump, but no heat at low rpm's with the no additional circulation from the auxilliary pump. I'd check the pump first, and then the ACC servo. Pray it's the pump, the servo is $$$$$.


I just realized--why are we responding to a thread from 2000?

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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