Starting system, Starter motor

Automotive illustrated glossary

Starter motor. Click for larger photo

Starter solenoid

Starting system simplified diagram

Starter relay

Neutral Safety Switch (Transmission Range Sensor)

Corroded batter terminal             Good connection

Starter motor

A starter is an electric motor that turns over or "cranks" the engine to start it.
A starter consists of the very powerful DC electric motor and the starter solenoid that is usually attached to the motor (see the picture). Inside, a typical starter motor has the electric windings (coils) attached to the starter motor housing and the armature (the rotating part) that is connected through the carbon brushes in series with the windings. On the front end of the armature, there is a small gear that attached to the armature through an overrunning clutch. This part is commonly known as the Bendix.

Starter solenoid

The starter solenoid works as a powerful electric relay - when activated, it closes the electric circuit and sends the battery power to the starter motor. At the same, the starter solenoid pushes the starter gear forward to mesh with the engine's flywheel. A typical starter solenoid has one small connector for the control wire (the white connector in the photo) and two large terminals: one for the positive battery cable and the other for the starter motor.

Battery cables

A starter motor requires a very high current to crank the engine, that's why it's connected to the battery with thick (large gauge) cables (see the diagram). The negative (ground) cable connects the "-" battery terminal to the engine cylinder block close to the starter. The positive cable connects the "+" battery terminal to the starter solenoid.

How a starting system works:

When you turn the ignition key to the Start position, the battery voltage goes through the starter control circuit and activates the starter solenoid, which in turn energizes the starter motor. At the same time, the starter solenoid pushes the starter gear forward to mesh it with the engine flywheel. The flywheel is attached to the engine crankshaft. The starter motor spins, turning the engine crankshaft allowing the engine to start.

Neutral safety switch

For safety reasons, the starter motor can only be operated when the automatic transmission is in Park or Neutral position; or if the car has a manual transmission, when the clutch pedal is depressed.
To accomplish this, there is a Neutral Safety Switch installed at the automatic transmission shifter mechanism or at the clutch pedal in case of a manual transmission. Often a transmission range sensor - the part that tells the powertrain computer which position (P R N D) the transmission is in, is used as a neutral safety switch (in the photo).
When the automatic transmission is not in Park or Neutral (or when the clutch pedal is not depressed), the neutral safety switch is open and the starter control circuit is disconnected.
Read more How a car starts, step by step.

Starting system problems

If when you are trying to start the car, you hear the starter cranks as usually, but the car doesn't start, then the problem is most likely not with the starting system - read our car no-start troubleshooting guide for tips how to find a problem if a car won't start.
Here are few common starting system problems:

A battery is very common to fail. Sometimes one of the electrical components that was left on or has a defect causing parasitic current draw drains the battery. Sometimes, an old battery can just die one day, with no warning. In either case, if the battery is low on charge, it won't have enough power for the starter motor to turn over the engine. If the battery is low on charge, you will probably hear a clicking noise or the starter may turn very slowly when attempting to start the engine.

Poor connection at the cable terminals can cause the starter not to work or run very slow too. Often the battery terminals or the ground cable connection get corroded causing starter problems (see the photo).

Another part that often fails is the starter motor itself. Sometimes the carbon brushes or some other parts inside the starter motor wear out and the starter motor stops working. Sometimes the starter gear for some reason won't mesh properly with the engine flywheel. This may cause a very loud metal grinding or screeching sound when attempting to start the car. If the starter motor is defective, it will have to be rebuilt or replaced, which is typically a few hundred dollar job.

An ignition switch also fails often. The contact points inside the ignition switch wear out, so when you turn the ignition switch to the "Start" position, no electric current is going through the starter control circuit to activate the starter solenoid. If jiggling the key in the ignition helps start the car, it's possible that the ignition switch is defective.

A neutral safety switch also can fail or get misadjusted. For example, if a car starts in "Neutral" but doesn't start in "Park", the neutral safety switch should be checked first.

by Vlad Samarin