a friend of mine picked up a Massey Ferguson model 300 that was not running. he added a couple batteries and turned the key (found out it was in gear) the motor turned over, smoked the negative cable. he released the key and the starter stayed engaged and continued to crank the engine over. that is when the negative cable smoked and melted. he has since replaced the cable, solenoid and the key switch and it continues to crank when the key is released. any idea on why it would do this. thank you.