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Starter Motor Guide of 2024

In order to start, most cars rely on an electric component called starter motor. The starter motor or starter, as it’s also called, is a complex device and more than a simple DC motor. It’s also one of most useful motors in your car, with the function to set the car moving. This guide explains the working of the starter, why your car needs it, the parts that make a typical starter motor, and more. At the end of the guide, we answer popular starter motor questions. In a nutshell, the guide encompasses every aspect of the automotive starter and its operation.

Everything about the Automotive Starter Motor

Starter Motor Definition: What is a Starter Motor?

The starter motor in car automotive starting systems is the part that provides the turning force to start the engine. An IC engine usually does not start on its own but requires an external source of rotational force. In most cars today, this force comes from an electrical component called starter motor or cranking motor.

The starter motor is installed in different places but usually close to the flywheel. From here, it thrusts a pinion gear to rotate the flywheel and start the engine. That happens whenever you turn the ignition key or press the starting button, depending on your type of vehicle.

Most starter motors are identical in construction. They only differ in the starting method, starting mechanism, and gear configuration. The image below shows a typical starter motor used in automobile applications. Notice the design that consists of the motor, a tubular solenoid on top, and the pinion drive on one side.

starter motor diagram showing the internal construction

The starter is one of the most powerful motors in a vehicle- and power hungry. A starter motor amp draw is usually so high that it goes up to 100A in normal situations. When under load, the current demand often increases to several hundred amps. Such currents require thick wires to convey. In fact, one of the ways to identify the starter location is by following the thick wire from the battery. This is usually the starter motor wire.

Starter motors also have to move huge loads when starting a vehicle engine. In addition to operating at extremely high currents, a gear reduction setup is usually used to increase the turning force. One of the most popular is planetary gears. The configuration used depends on the particular motor or application.

The starter motor function is simply to start the engine. In other words, it powers up the engine from its rest. How does it accomplish that? Find out below to understand how the starter motor on your car or truck operates.

the purpose of starter motor is to start the engine running

How a Starter Motor Works

Despite being a complex device, the starter motor working principle is a straightforward process; current flows, the motor spins and a gear moves to drive the flywheel. To understand the events that lead to the engine cranking, here is the starter motor working explained in more detail.

  • To start your car, you either turn the ignition key or press the START button. This causes current to flow through the ignition circuit. The ignition circuit then closes another circuit. This other circuit contains wires going to one of the starter motor components called starter solenoid.

  • Upon receiving voltage, the starter motor solenoid becomes energized and does two things. First, it moves a plunger and closes contacts to supply power to the starter motor. Second, it pushes an attached lever and moves the starter motor gear to interlock with the ring gear on the flywheel

  • With the motor turning and its drive gear in mesh with the flywheel, the engine starts to turn until reaches the power stroke. By now, the engine no longer needs to motor to turn it. That’s when you release the ignition key.

  • Releasing the ignition key cuts of power to the solenoid, causing it to lose magnetism. It now no longer pulls the plunger. A powerful spring causes the plunger to return to its original position.

  • At the same time, the retracting plunger pulls back the lever. This forces the pinion gear to return to its former position as well. Meanwhile, the conductive surfaces that the plunger had closed are no longer in contact. This causes the motor to stop spinning.

The starter motor operation steps explained here only take a few seconds to happen. They should not take any longer, or the motor will overheat. It’s even recommended to wait between trials to start the engine as the heat buildup could end up damaging the motor. The short duration also means that you do not have the starter motor draining the battery unnecessarily.

Besides overheating and power consumption, there are other reasons why the starter motor must not run when the engine has started. Doing so could end up damaging the motor from the back drive caused by the flywheel. To protect the motor further, a one way clutch called the starter motor overrunning clutch is also used. The clutch allows the motor to spin the flywheel, but prevents the flywheel from turning the motor.

starter motor exploded view to show the different parts

Starter motor Parts

Despite its name, the starter motor is more than an electric motor. It contains parts to serve different functions such as actuation, current switching, and gear reduction. In order to better understand the working of the starter and how it fails, it’s important that you’re aware of these components. The starter motor parts names and their functions are explained below. Note that the starter described here is the conventional type that consists of an electric motor and solenoid.

DC Motor

starter motor armature

This is one of the main parts of a starter motor. It’s the component that provides a turning force that helps to crank up the engine. The motor is designed like any other DC motor, and contains similar parts. The parts are:

  • Motor Housing/Yoke– the housing is the part that encloses the components inside. If helps to shield these parts from damage and mounts the assembly. The housing is usually a metal structure, tough and resistant to corrosion and impact.

  • Field Coils– the field coil in starter motor assemblies is the wire winding around a magnetic core and that attaches to the housing or yoke. The coils provide the magnetic field required by the motor to start spinning. The ends of the coils are connected to the commutator. When current flows into the motor, the coils energize and magnetize the core.

  • Armature- the starter motor armature is the turning part of the motor and commonly called starter motor rotor. It contains wire coils that, upon receiving an electric current, become energized and generate a magnetic field. The armature is positioned to cause an opposing magnetic field to that of the field coil, thereby producing a turning effect. The armature also contains the motor’s shaft and commutator.

  • Brushes– the starter motor brushes are the blocks of carbon or graphite arranged around the commutator. Equipped with springs to tension then against the commutator, brushes ensure power is conducted to the coil windings while the armature rotates. They are one of the first starter motor components to wear down.
cut away image showing the starter solenoid on top

Starter Motor Solenoid

The starter solenoid is the cylindrical part that you see mounted to the top of the motor assembly. Its purpose is to act as a large switch or relay and close contacts to supply current to the motor field windings and armature. The solenoid also performs another important function; to push a lever fork and move the pinion gear to mesh with the flywheel’s ring gear. The parts of the solenoid are:

  • Housing- this is the solenoid’s cover that provides a protective structure to the internal components. Just like that of the motor, the solenoid housing is made from a tough metal that also resists corrosion.

  • Solenoid Coil- a starter solenoid contains two different coils. One coil pulls the plunger and is called the pull-in coil. The other, known as the hold-in coil, helps to keep the plunger in position until current stops flowing and the plunger retracts.

  • Plunger- the plunger is the sturdy metal part that’s positioned in front of the solenoid coil. It’s usually made from a magnetic material. When current flows and energizes the coil, the plunger gets pulled in. As it moves, it pulls at a lever that, in turn, moves the motors drive gear toward the flywheel.

  • End Cap- the solenoid cap is mounted to the end of the tubular solenoid housing. It function is to provide the mounting surface for the starter solenoid wires. It also holds the contacts that close to convey current to the motor.
planetary starter motor gear arrangement

Drive Mechanism

This is the part of the starter motor assembly that helps to transmit the motor rotation to the flywheel or engine. As soon as the motor starts to spin, several things happen that push a drive gear to the flywheel. The drive mechanism is commonly called the starter Bendix drive and consists of these main components.

  • Pinion Gear- this is the small but heavy drive gear on the starter motor drive mechanism. Its purpose is to mesh with the flywheel’s ring gear and rotate it. The starter motor gear must be made from a tough material or it will wear too quickly. Despite that, the constant engagement and disengagement when starting the engine often takes a toll on this gear, causing it to wear or even break.

  • Drive Shaft– the drive shaft holds the pinion gear. It’s also usually splined and equipped with a powerful spring to make the interlocking of the flywheel’s and pinion gears smooth. Just like the pinion, the shaft must be a heavy-duty material to prevent breakage.

  • Drive Lever/Fork- the starter motor fork is the part that moves the pinion drive gear to mesh with the flywheel. The fork or lever is attached to the shaft on one end and the solenoid’s plunger on the other. The fork moves when the plunger gets pulled by the solenoid coil and returns to its original position when the coil de-energizes and frees the plunger.

  • Gear Reduction Setup– starter motors produces a large turning force. However that’s often not enough to start an engine. In most starters, a great reduction setup is used to increase the turning force or motor torque. As mentioned earlier, the most used configuration is the planetary gears. This setup consists of a sun gear,

  • Over-Run Clutch– a starter motor is normally equipped with a one-way clutch. The clutch is positioned between the shaft and motor and helps to ensure a smooth engagement and disengagement of the drive pinion and flywheel’s ring gear.

a reconditioned starter motor armature

Starter Motor Problems, Symptoms

The starter motor is prone to damage over time. Components will wear down, burn out, or even corrode. There are also problems that result from bad starter motor electrical connections or other parts of the starting system but which appear as starter issues. To diagnose these issues, you need to understand their causes. The reasons for starter motor failure include the following.

  • Oil leaking from the engine and finding entry into the starter motor assembly
  • Dirty or corroded starter motor electrical connections
  • Loose starter motor wires or cables
  • Worn starter motor components such as gears, brushes, commutator, and bearings
  • Broken Bendix gear or any other starter motor component
  • Dead or weak battery

When your car’s starter motor fails, the engine does not start easily, sometimes not starting at all. You will also tell that you have a failing starter by listening in to the sounds when starting the car, among other ways. The symptoms of a bad starter motor are listed below.

  • Starter motor not turning- there are many reasons for the starter motor not spinning but burnt out electrical parts are often the cause of loose and corroded connections and contacts

  • Starter motor grinding noise- the teeth of the pinion drive gear are worn and not engaging or disengaging smoothly. It could also be dirt that’s causing the gear to produce grinding noise or worn parts of the starter assembly

  • Starter freewheeling- the starter motor runs but does not engage the flywheel. In such cases, your car will not start even when the battery is providing sufficient power

  • Starter motor keeps spinning- the starter will not stop even long after the engine has started. If that happens, the starter motor circuit is faulty or the solenoid contacts welded together

  • Smoke or burning smell when you try to start your car- this happens as a result of the starter motor getting too hot and burning up lubricant. There’s likely an increase electrical resistance in the circuit
typical electric starter motor

Starter Motor FAQs

Q1. Why is the starter motor amp draw higher than that of other motors?

A. Starter motors need to be powerful enough to crank the engine. However, they run off the car battery which only provides about 12V of electricity. To produce the large amount of power required to turn the engine, the motor has to draw a lot of current. It’s not unlikely to find a starter motor that’s rated 400A. However, the amount of current drawn depends on the load, which usually varies.

Q2. What is a Bendix in a starter motor?

A. The starter Bendix is a type of mechanism that most starter motors use to engage with the flywheel. The mechanism got its name from the person who first made it, Vincent Bendix. It essentially consists of a pinion gear on a shaft. When starting the engine, the pinion thrusts to engage the flywheel gear, thereby transmit the turning force of the starter motor to the engine. The starter Bendix drive can be the traditional inertial type or the modern and actuated by a solenoid mechanism.

Q3. How long does a starter motor last?

A. The starter motor lifespan depends on many factors such as quality, how frequently it’s used and how well you maintain it. Under normal conditions and average use, the motor should last a car’s lifetime. That said, most there are starters that need replacing after around 100 miles. If most of your driving involves a lot of stopping and switching off the engine, the starter may not last the life of the car.

Q4. Can you fix a bad starter motor?

A. You can. Starter motor repair involves taking the assembly apart, both the starter motor and solenoid, and cleaning it. Damaged parts are also replaced. These include the brushes and other parts that may have worn off or burnt out. It’s not always easy or a good idea to fix a failed motor. If the wear is excessive or major parts are blown and corroded, you need to replace the entire unit.

Q5. How do you test a starter motor?

A. There are various ways to test a starter motor that has stopped working (or one that’s showing signs of failure). One of the most common is using a digital multimeter or test lamp to check for voltage. A starter motor test can also be done with the motor removed. Doing so should provide better results. However, only attempt that if you know how to or have a professional do it instead. In all these starter motor testing methods, the idea is to determine if the device is getting power or not.

Q6. How much is the starter motor repair cost?

A. Different repair shops will charge different amounts to fix your car starter motor. The cost varies from $100 to $200 or even more depending on the car make and model. Starter motor troubleshooting and repair often involves changing faulty parts. In some instances, the motor itself is not the problem but the circuit and its components. Before considering repair, the wires and connectors should be checked for fraying and corrosion.

Q7. How much does a starter motor cost?

A. The starter motor price largely depends whether the motor is an OEM or Aftermarket type. An OEM starter motor will cost you above $200 in most cases, while an aftermarket starter goes for between $100 and $200. The price also varies by brand and type of engine. The starter motor for the average vehicle costs from $100 to about $200. If buying the starter for a large engine such as V8, for example, the price jumps to around $200-$400.

Q8. Is it advisable to install a reconditioned starter motor?

A. The cost to rebuild starter motor assemblies ranges from $50 to around $150. That can mean money saved. However, you should only consider the option if working on a budget. A rebuilt unit will serve you but only for a short time and if rebuilt correctly. You could even consider doing it yourself and saving on costs even more. This video shows how to rebuild a starter motor.

Q9. Can you replace a starter motor yourself?

A. You can. Starter motor replacement does not require much skill, only repair. However, ensure that you have the tools for the project. Most importantly, have your car manual with you. You will need it to locate the motor and more. Make sure that you also understand how to wire a starter motor to a battery. You do not want to have the wrong starter motor circuit and causing costly damage. Also, the steps to remove the motor and install a new one.

Q10. How much does it cost to replace a starter motor?

A. The cost varies but mostly ranges from about $100 to $200 or more depending on your type of car. The amount does not include the cost of the starter itself which, as we have seen, varies widely. You can take your car to a repair shop or change the starter yourself. In addition to having the right type of starter motor for your car, it’s advisable to understand how to remove and install it. Again, the process varies across different vehicle types and models, although only slightly. Do not just buy any starter motor for sale. Always use your car’s information to shop for one.


Starter motor is a critical vehicle component. Without it, you cannot bring your car to start moving. It’s also one of the most complicated motors in an automobile, with different parts other than the motor itself. To understand a vehicle’s starting system, you need to know how the starter motor works as well as how it fails. We hope this guide helped to familiarize you with the starter motor used in a car, among other things.

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