The starter motor system

A starter has two jobs: push a small gear into the engine's flywheel, and switch on a current big enough to melt a thin wire. To crank an engine it can draw well over 100 A. The ignition switch could never carry that, so a chain of parts shares the work.

• How the solenoid switches a huge current and pushes the pinion in
• Why it has two windings, not one
• How the pinion meshes, and the clutch that protects the motor
• How to diagnose "click, no crank" and "whirr, no crank"

The solenoid sits on top of the motor. Inside is a coil around an iron plunger. Turn the key, current flows through the coil and it pulls the plunger in. On the end of the plunger is a copper disc. At the end of its travel, the disc bridges two heavy terminals, one from the battery and one to the motor. That's job one: the big switch.

The plunger also works a lever, sometimes called a fork, which pushes the pinion along its shaft and into the ring gear. That's job two: the push. The pinion moves first and the contacts close right at the end, so the motor only gets full power once the gears are meshed.

Two windings

Pulling the plunger in takes a lot of magnetism; holding it takes much less. So most solenoids have a strong pull-in winding and a weaker hold-in winding. The pull-in winding earths through the motor. Once the main contacts close, both its ends sit at battery voltage, so it switches itself off and only hold-in keeps working. That stops the solenoid overheating.

The pinion might have 10 teeth and the ring gear 130. So the motor turns 13 times for every turn of the engine. That trades speed for turning force, and it's what lets a small motor crank a big engine.

Tooth to tooth

Sometimes the pinion lands tooth against tooth. So behind it there's a meshing spring. The pinion stops, the spring squashes and the plunger carries on, so the main contacts still close. As soon as the motor turns, the teeth slide into line and the spring snaps the pinion home. The pull-in winding's gentle turn often gets the teeth to mesh first time anyway.

The engine fires. With the key still held, the ring gear now tries to drive the pinion. At 3,000 rpm, 13 : 1 would spin the motor at 39,000 rpm, fast enough to throw the windings out of the armature.

The one-way clutch between the shaft and the pinion stops that. Inside are rollers in tapered slots, pushed towards the narrow end by small springs. When the motor drives, the rollers wedge in and lock the pinion to the shaft. When the pinion tries to go faster, the rollers roll back to the wide end and the pinion freewheels.

Let go of the key and a spring pulls the pinion back out of mesh, ready for next time.

First, put a full start in order. Then work through five starter symptoms. For each one, pick the most likely cause and the diagram shows where to look.

Ten questions covering the whole module. You need 8 out of 10 (75%) to pass. Each answer comes with a short explanation.