[wilkin250r]

Jim nailed it.

Oddly enough, a motor draws LESS power as it spins faster. Which means if it's spinning slowly, it's going to try to draw huge current (the end result being, if you stop the motor completely, it acts like a dead short and attempts to pull an unlimited amount of current)

Since heat is all current-based, that higher current that is caused by the low voltage welds the contacts of your solenoid. I've never had it happen with a car, but I've done it to a few motorcycles.