Starting system, Starter motor

Updated: March 08, 2015

Starter motor

Starter motor
Starter motor. Click for larger photo

A starter is an electric motor that turns over or "cranks" the engine to start it. It consists of a powerful DC (Direct Current) electric motor and the starter solenoid that is attached to the motor (see the picture).

The starter motor is powered by the car battery. To turn over the engine the starter motor requires a very high electric current, which means the battery has to have sufficient power.

Starting system problems are common and not all problems are caused by a faulty starter motor. To find the cause of the problem the starting system must be properly tested. read more below.

Starter solenoid

Starter solenoid
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 it with the engine flywheel (flexplate) ring gear teeth.

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 (see the diagram below).

Battery cables

Starting system diagram
Starting system simplified diagram

The starter motor requires a very high current to turn over 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 (flexplate in an automatic transmission). The flywheel is attached to the engine crankshaft. The starter motor spins, turning over the engine crankshaft allowing the engine to start.

Neutral safety switch

Neutral Safety Switch
Neutral Safety Switch
(Transmission Range Sensor)

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.

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. Here are a few common starting system problems:

Battery terminals
Corroded batter terminal             Good connection

The 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, when attempting to start the engine you will probably hear a single click or repeated clicking, or the starter may turn over slowly and stop. 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).

Corroded starter terminal
Corroded starter solenoid control terminal

Sometimes the starter control terminal gets corroded (in the photo) or a starter control wire gets loose or disconnected from the terminal causing the starter not to work. For example, this corroded starter control terminal was the cause of a no-start, no-crank condition in the Mazda 3. We only noticed this after disconnecting the control wire connector. Cleaning the terminal and replacing the connector solved the problem.

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. If the starter motor is faulty, it will have to be replaced, which may cost from $250 to $650. Rebuilding the starter motor is usually cheaper, but takes more time.

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. In this case, the flywheel ring gear needs to be checked for damaged teeth.

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.

How the starting system is tested

A technician testing the battery
A technician checking the battery state of charge
with the battery tester

When the starter motor doesn't work, first the state of charge of the battery, battery terminals and battery cables must be checked. One of the symptoms of a weak battery is when the dash lights go dim when the key is turned to the START position.

The next step typically involves testing the starter control circuit. Your mechanic may start by measuring the battery voltage at the starter solenoid control terminal with the key in the START position. If there is no voltage, the problem is most likely in the starter control circuit (ignition switch, starter relay, neutral safety switch, control wire). If there is a battery voltage at the starter solenoid control terminal with the key in the START position, the starter motor itself could be bad. The starter solenoid control terminal must also be checked for proper connection.

What is inside the starter motor

Starter motor cutaway
Starter motor cutaway. Image courtesy of Robert Bosch GmbH

A starter motor has several (typically 4) electric 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 attached to the armature through an overrunning clutch. This part is commonly known as the Bendix.

Many starter motor problems are caused by worn carbon brushes or the armature bearings. The contact points inside the starter solenoid also can fail.

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Please note: the information provided in this article is for informational purposes only and cannot substitute for the advice of professional mechanic or authorized dealer. Consult your mechanic, and check your car's owner's manual for safety measures, precautions, warnings, tips and recommendations.