My 85 3000td has a temperature sensor that keeps the fan from blowing when its cold out. It takes about 6 short city blocks on a winters day for the engine to get warm enough for the fan to overcome that lockout. It only varies about 100 yards in the distance it takes to reach that temperature. The air is just barely warm when it begins blowing. This is in temperatures between about 5 degrees and 20 degrees. I've never experimented with it in below zero temperatures although it was about 10 below last week and it did seem to take a little longer before the fan came on.

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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