Yes, I should have added that. Length of actual glow is independent of length glow light stays lit. Relays can vary. My 85 is around 25 seconds. My 79SD is around 40 seconds.
Early models sensed ambient temperature in the head. (of the engine, not the driver)

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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