2008 super duty
 

Cruise control only turns on when cold weather WARM WEATHER WILL NOT ACTIVATE ANYONE SEEN THIS BEFORE HELP:confused::confused::confused:

I have never come across that situation, however, take a look at the stoplite switch by the brake pedal , if the pedal is holding the switch from releasing completely it could be preventing the cruise from setting.

Or if the parking brake switch malfunctions the CC will kick off. I noticed on mine that if the parking brake pedal was not fully retracted (the up position) or if it wiggled down due to a weak return spring the CC would kick off. If you have a manual check the clutch pedal switch also.
Regards,
Eric