The starter motor is an electric motor that turns over or "cranks" the engine to start. It consists of a powerful DC (Direct Current) electric motor and the starter solenoid that is attached to the motor. In most cars, the starter motor is bolted to the engine or transmission.
The starter motor is powered by the car's main 12-volt battery. To turn over the engine, the starter motor requires high electric current, which means the battery has to have sufficient power. If the battery is discharged, the lights in a car might work, but it won't be enough power (current) to turn over the starter motor.
What are the symptoms of a bad starter motor: When starting a car with a fully charged battery, there is a single click or nothing happens at all. The starter motor doesn't run, even though there is a 12-Volt power at the starter control terminal.
Another symptom is when the starter motor runs, but fails to turn over the engine. Often, this might cause a loud screeching noise when starting the car. Of course, this could also be caused by damaged teeth on the ring gear of the flex plate or flywheel
A typical starter solenoid has one small connector for the starter control wire and two large terminals: one for the positive battery cable and the other for the thick wire that powers the starter motor itself.
The starter solenoid works as a powerful electric relay. When activated, through the control terminal, the solenoid closes the hi-current electric circuit and sends the battery power to the starter motor. At the same time, the starter solenoid pushes the starter gear forward to mesh with the ring gear of the engine flexplate or flywheel.
The starter motor typically has four field windings (field coils) attached to the starter motor housing from the inside. The armature (the rotating part) is connected through the carbon brushes in series with the field coils. On the front end of the armature, there is a small gear that is attached to the armature through an overrunning clutch.
How does the starter motor work? When the driver turns the key or presses the Start button, the solenoid winding is energized. The solenoid plunger moves in the direction of the arrow and closes the solenoid contacts. This connects the battery power to the starter motor (field coils and armature). At the same time, the plunger pushes the starter gear forward through the lever. The gear then engages with the ring gear of the flexplate and turns it over. The flexplate is attached to the engine crankshaft.
Most of the starter problems are caused by worn out or burned solenoid contacts, worn brushes and a commutator and worn armature bushings. The symptom of worn out solenoid contacts is when the solenoid clicks but the starter motor doesn't run. When the starter brushes are worn out, the starter motor doesn't make any noises. When front and rear armature bushings wear out, the armature rubs against the field shoes causing the starter motor to run slow and noisy. Many modern starter motors have small ball bearings instead of the bushings. If you want to rebuild the starter motor, the starter motor rebuild kits that include common wear parts are sold online.