Do a search on heater or ACC servo and you'll find threads describing a bypass. People take a piece of pipe (copper) and join the heater hoses under the servo. I was going to do that on mine when I was having problems with the servo but decided by the time I did that, it would be better to spend my time replacing the servo.
If you end up replacing the servo, there is an metal bodied version that is superior to the original plastic body. You also might find a used one on an old Chrysler Imperial since they used the same servo.

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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