Starter motor, starting system: how it works, problems, testing

Updated: January 25, 2023
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 (see the picture).
Starter motor Starter motor. Click for a larger photo.
In most cars, the starter motor is bolted to the engine or transmission, check these photos: photo 1, photo 2. See how the starter motor works inside below.

The starter motor is powered by the car's main 12-volt battery. To turn over the engine, the starter motor requires very 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 have 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.
Starter motor inside Starter motor inside. Read more below
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.

Starter solenoid

Starter solenoid Starter solenoid.
A typical starter solenoid has one small connector for the starter control wire (the white connector in the photo) and two large terminals: one for the positive battery cable and the other for the thick wire that powers the starter motor itself (see the diagram below).

The starter solenoid works as a powerful electric relay. When activated, through the control terminal, the solenoid closes the 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.

Battery cables

Starting system diagram Starting system simplified diagram.
As we mentioned, the starter motor requires very high electric current to turn over the engine. That's why it's connected to the battery with thick cables (see the diagram).

The negative (ground) cable connects the negative "-" battery terminal to the engine cylinder block, or transmission, close to the starter.

The positive cable connects the positive "+" battery terminal to the starter solenoid. Often, a poor connection at one of the battery cables can cause the starter motor not to work.

How the starting system works:

  1. When you turn the ignition key to ON position, the engine computer (PCM) checks if the ignition key security code matches (immobilizer). If yes, the engine is allowed to start.
  2. When you turn the key to the START position, or press the START button, the engine computer (PCM) checks if the transmission is in Park or Neutral, if the brake pedal (Automatic) or clutch pedal (Manual) is depressed and if the steering lock is unlocked (in some cars).
  3. If all checks pass, the engine computer activates the starter relay.
  4. The starter relay closes the starter control circuit and activates the starter solenoid.
  5. The starter solenoid closes the high-current circuit and sends power to the starter motor.
  6. At the same time, the starter solenoid throws the starter gear forward to mesh it with the engine flexplate gear (or flywheel gear in a manual transmission). The flexplate (flywheel) is attached to the engine crankshaft.
  7. The starter motor turns over the engine crankshaft fast enough to allow the engine to start. In cars with a push button start, the system disengages the starter motor as soon as the engine starts running.

Neutral safety switch

Automatic transmission range switch Automatic transmission range switch.
For safety reasons, the starter motor can only be operated when the automatic transmission is in Park or Neutral position. In a car with a manual transmission, the engine can only be started when the clutch pedal is depressed.

In cars with an automatic transmission, the transmission range switch works as a neutral safety switch. It sends the signal to the engine computer if the transmission is in Park or Neutral.
In vehicles with a manual transmission, the clutch pedal switch performs the same role. It tells the engine computer if the pedal is depressed.

The job of the transmission range switch is to tell the vehicle computer (PCM) which gear the transmission is in.
If your car has a gear indicator on the dash, you might be able to see when a transmission range indicator is not working.

The most common problem is when you shift the transmission into "Park" and the letter "P" doesn't show on the dash. This means that the vehicle computer (PCM) doesn't know that the transmission is in "Park" and will not allow the starter to operate.
Read more: 2 Reasons Why a Car Won't Start in Park but Starts in Neutral

Starting system problems

Starting system problems are common and not all of them are caused by a faulty starter motor. To find the cause of the problem, the starting system must be properly tested. If when you are trying to start the car, you hear that the starter cranks as usual, but the car doesn't start, then the problem is most likely not with the starting system. Read more: The engine cranks but won't start.

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 might 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 above).
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 other parts inside the starter motor wear out and the starter motor stops working.

For example, a failed starter motor was common in some Toyota Corolla and Matrix models. Even with a good battery, the starter would click, but would not turn over.

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.

The 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.

The transmission range switch can also fail or get out of adjustment. 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 checks the battery state of charge with the battery tester
If 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 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, but the starter motor doesn't work, it's the problem with the starter motor itself. The starter solenoid control terminal must also be checked for proper connection.

Starter motor repair options

Used starter motor Used starter motor.
If the starter motor is faulty, it will need to be replaced, which may cost from $250 to $850 depending on the repair difficulty and part price.

In some cars, it's easy to replace a starter motor, in others cars more parts will need to be removed to get access to the starter motor (e.g., intake manifold). There are several part options:
  1. New OEM part. It's usually the most expensive option but you know it will fit and the quality should be good.
  2. New Aftermarket part. Aftermarket starter motors are cheaper but the quality is not consistent. Some will work as good as OEM, but some may fail prematurely. Ask for the part brand with a good reputation and warranty coverage.
  3. Remanufactured part. They are usually cheaper, but could be hit and miss. Ask about the warranty.
  4. Used part. It's probably the cheapest option, but for how long it will work is a question. Inquire about the warranty. You can locate a used part from a local auto-recycling facility or your shop can locate it for you.
  5. Rebuilding the existing starter motor. If the replacement starter motor is not available, call your local starter/alternator shop or auto electric shop, they may be able to rebuild your starter motor. If you want to rebuild it yourself, repair kits are available online and you can probably find a Youtube video showing how to do it. Of course, it's not an easy job and needs to be done right to work.

How does the starter motor work inside?

Starter motor inside Starter motor inside.
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 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 bushings.