Dodge 2500, 2001, Diesel, automatic, 4x4 - on a road trip right now, starting to experience some heating/cooling problems. Temp gage reads normal while on highway, but when slows down (through towns) the temp gage goes to zero. When put in neutral reads normal. Only reads zero when in gear and driving slow.

Optional Information:
2001 Dodge Ram 2500 diesel

Already Tried:
nothing, don't know where to start. Son is driving from CA to MT and is in ID now and just called - please help. He is very tight on cash. Moving to his first base (in Air Force)