The cause and diagnosis of idling starter are as follows:

The phenomenon of idling starter 

When the starter switch is on, the starter is idling and the pinion cannot engage with the flywheel gear ring to drive the engine.

Cause of idling starter

1. The shift fork of the starter is out of slot by mechanical force, and the drive pinion cannot be pushed, or its stroke is improperly adjusted and cannot engage with ring gear.

2. Travel of the solenoid switch iron core is too short.

3. The armature of starter auxiliary circuit has short circuit or open circuit, the armature can not get into its working position.

4. The overrunning clutch of starter slips.

5. Flywheel gear are badly worn or broken.

The starter idling fault diagnosis - there are two cases when a starter idles

Idling means that the drive pinion of the starter cannot engage with the ring gear of the flywheel. The failure is mainly in the operation and control part of the starter. The following examinations and diagnoses should be made.

1)  The mechanical starter(spring starter), first check whether the stroke of the shift fork is properly adjusted. If not properly adjusted, the main contact plate is connected to the contact and the starter idles, when the drive pinion does not engage with the flywheel gear ring. If properly adjusted, the shift fork may be removed from the slot.

2) The magnetic starter, check whether the travel of the contact plate of the main circuit is too short. If it is too short, the main circuit will conduct ahead of time, thus causing the armature to rotate at high speed.

3) Sliding armature starter, it is mainly the wear of the limit plate on the claw block, and cannot block the movement of the limit plate. Therefore, the lower contact of the movable contact is closed in advance, and the armature rotates at a high speed. If the clearance between the movable contact and the upper and lower contact of the fixed contact is not adjusted properly, it means the lower contact clearance is too small, then the armature will rotate in advance at a high speed.

The other is that the drive pinion of the starter has engaged with the gear ring of the flywheel. As a result of one-way clutch slips and idling. Most likely the starter one-way clutch has a faulty, the starter should be removed to check or replace it.