Fan spins freely (or should). Yours is fried at bushings or commutator if it doesn't. This may be a symptom rather than a cause of overheating (overuse to try to cool rad. leads to premature failure) or it could have seized first and now rad. with viscous fan coupling can't deal with summertime stop and go, A/C, slow speed driving, etc.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)