Mob. Mech. was replacing alternator on D-10 tractor. Access is through a hole in floorboard of cab. He was reaching through the hole to install alternator when he felt pain in his L shoulder. He completed job & contacted Foreman re: incident. He was seen by co. dr. on 9/24 & referred for ortho consult. He saw ortho dr. on 9/30 & was prescribed PT & given a 20 lb. lifting rest.