Seems your battery is in the not so good /excellent condition anymore in cold weather.

In cold weather, you battery gives less amps and on the contrary your diesel engine needs more amps because of more resistance/friction of the cold diesel engine(including longer glowing of the plugs).

To exclude/include ther is a battery problem,
try to JUMP-start your volvo 240 diesel (with cold engine) with another car / with the engine running.

/* carefull about polarity hey !!!!!!!!!!!!!!!*/

Compare how your diesel is now starting.
if its goes much better => the volvo 240 battery is suffering of age °)
if the starting encounters the same difficulties => problem is everything else
(to early to find out ...)


succes
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240 '88 B200K wite sedan 255K km Europe virgo+all turbo gauges, Phoenix Gold + Kenwood ampli, Focal door speakers + tweeters, Kenwood 6'x 9', 10 CD changer in enlarged globe cabinet, dubble rear coil springs , hitch, fog lights, 2 extra rectangular pencil lights, 2 X 3 rear red driving lights