I hava 2006 F350 6.0 diesel and it won't crank when it's cold ?
It runs fine when it's warm
How cold is cold? If you are in extreme cold you may need a block heater. If your battery(s) are weak they may not have enough power for a cold start. Cold is hard on diesel engines.
If your in temperature zones that go down to 15 to 0 degrees, Definitely put block heater and a trickle charger in. Up here in Ak, these items are a must!