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

You might also be interested: