The mechanic and electrician were moving the generator exciter outside of the crane cab. They had one come-along attached to the rear of the exciter and a chain fall attached to the front of the exciter. While pulling the exciter outside the door, the rear chain fall was above in the trolley guide. As the exciter was being pulled outside of the crane cab with the chain fall the lifting eye that the come-along was attached to, slipped out of the trolley guide rail. The come-along fell striking the employee on the left thumb causing a fracture and the employee to lose time from work.