When the starter switch is turned on, the starter will make a knocking sound, and the starter can't drive the engine. 

Failure causes

1. Incorrect adjustment of starter switch or solenoid switch.

2. Improper adjustment of armature movable fixed contacts and movable contact clearance.

3. The pinion or flywheel ring gear wear or slip.

4. Fixed bolts of starter are loose or clutch housing is loose.

5. Internal failure of the starter.

Fault diagnosis 

This signs indicates that the starter drive pinion is difficult to mesh. First turn the crankshaft to an angle, then turn on the starter switch to test. If the sound of impact disappears and gets stuck by the starting engine, it indicates that the gear ring of the flywheel gear ring has been damaged and should be replaced.

1. If the crankshaft is rotated to any angle, the impact will not be eliminated. The drive pinion never meshes. This indicates the travel of shift fork and solenoid switch travel is too short, resulting in the drive pinion not engaged or rotating at high speed.

2. The starter housing shakes obviously when the starter switch is switched on, indicating the starter fixed bolt or clutch housing fixed screw is loose. Tighten it immediately, or it may damage the starter drive end cover.

3. In addition, the cause can be roughly determined according to the characteristics of the knocking sound. Generally, the impact of improper travel adjustment or idling is continuous, and the impact of loose starter bolts or clutch housing or damaged flywheel gear ring is intermittent, sometimes be engaged. The diagnosis of idling with a knocking sound is the same as that of a starter idling fault.