I'm pretty sure it's not compression or voltage. The glow plugs are new in both vehicles and seem to be glowing appropriately in all conditions. I'm almost certain it is just the speed at which the engine is turning over. I don't have a tach on the 77 but I would estimate that it is turning over at only half the speed of the 85 or possibly less.
Last year when I had this problem with the 77 I put a charger on the battery and warmed it up. When I did the engine turned over much faster and it started right away. (I've since replaced the battery so I think I can discount battery condition) I'm just puzzled about why two vehicles sitting next to each other in the same conditions have such a big difference in the speed at which the starter turns them. I know I could put a battery heater in the 77 and probably take care of the problem, but I'd like to know why it is happening. Since the engine was acutally warmer in the 77 than in the 85 since the 77 was plugged in, the answer must lie somewhere in the electrical--battery--starter component of the system. I think??

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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