664. The winch shown in the illustration will operate normally in all speeds in both directions, with the exception that it will not accelerate into 'fifth point' hoist or 'fifth point' lower. What would be a possible cause

The winch fails to reach the fifth speed in both hoist and lower directions. This indicates a failure affecting only the final acceleration stage, which is controlled by time-delay relay 3T; an open coil in this relay prevents the circuit from reaching the fifth point in either direction.