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

Updated: February 12, 2019

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).

Starter motor
Starter motor. Click for a larger photo.

In most cars, a starter motor is bolted to the engine or transmission, check these photos: photo 1, photo 2. See how the starter motor works inside on the next page.

The starter motor is powered by the car 12-volt battery. To turn over the engine, the starter motor requires a very high electric current, which means the battery has to have sufficient power. If the battery is discharged, the lights in a car might come on, 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.

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 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.

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Battery cables

Starting system diagram
Starting system simplified diagram.

As we mentioned, the starter motor requires a very high electric 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 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 run.

How the starting system works:

When you turn the ignition key to the START position, or press the START button, if the transmission is in Park or Neutral, the battery voltage goes through the starter control circuit and activates the starter solenoid. The starter solenoid powers 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. In cars with a push button start, the system disengages the starter 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; or if the car has a manual transmission, when the clutch pedal is depressed. In vehicles with a manual transmission, the clutch pedal switch completes the starter motor circuit when pressed. In cars with an automatic transmission, the transmission range switch allows the starter operation only when a transmission is in Park or Neutral.

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. The symptom of this problem is when the vehicle starts in Neutral, but doesn't start in "Park".

This problem is often caused by a corroded or seized cable, or cable lever (see the photo). The rust at restricts the cable movement and prevents the switch from working properly. The solution is to lubricate the cable connection point and, if needed, replace rusted parts. The transmission range switch position might need to be readjusted too.

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 the starter cranks as usual, but the car doesn't start, then the problem is most likely

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