TAG 2066598 - Caterpillar

Pinion Damage
After removal of the starting motor, inspect the pinion for the following damage or wear:
Leading edge damage on one or more teeth (milling)
Wear on the side of the tooth opposite of rotationA pinion with milled teeth indicates that the starter motor received a faulty Start signal when the engine was still running. This could have been caused by a mechanical problem or by the operator. Check the start relay, key switch, and wiring harness. A loose connection at the solenoid or contamination in a connector can cause the starting motor to engage intermittently.Wear on the side of the tooth opposite of rotation can be caused when the pinion stays engaged and is being driven by the ring gear.If any of the above symptoms are noted but the exact cause cannot be identified, the recommendation is to replace the start switch. Locked Armature
A locked armature is an indication that the starting motor continued to receive a Start signal after the engine was started. This delayed Start signal caused the starting motor to be driven by the ring gear and to exceed the design limit for armature speed. Evidence of a locked armature would be blueing of the armature shaft near the nose bushing. If a locked armature is suspected, use the pinion and try to rotate the armature by hand.If any of the above symptoms are noted but the exact cause cannot be identified, the recommendation is to replace the start switch. Twisted Spline (50MT)
A failure mode common to the 50MT starting motor is a twisted spline. This failure mode can be identified by using a screwdriver to pry the pinion forward (engage position). The armature shaft has a twisted spline if the following symptoms are found:
The pinion does not slide forward easily.
The pinion does not return completely after the screwdriver is quickly removed.When the pinion does not return completely after the screwdriver is removed, the solenoid does not have enough force to move the pinion away from the binding splines. Milling of the pinion teeth will occasionally be observed in this failure mode.The twisted spline failure mode can occur from two conditions. The first condition occurs when the starting motor is engaged and the engine is in a "rock back" mode. The second condition occurs in the case of a dual starter application. In a dual starter application, one starting motor fails and the load is too high for the remaining starting motor.