Understanding Engine Starter Motor

Do you know without a starter motor an engine and its component won’t work? Well, you should know. A starter motor is one of the major component use in an internal combustion engine which is electrically powered. Because an engine cannot rotate on its own, a part is required to start the first cycle. The starter motor help to initiate the engine’s which then continually operate on its own power.

starter motor

Today we’ll be looking at the definition, function, parts, diagram, types, working principle as well as bad symptoms and troubleshooting of starter motor.

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Starter Motor Definition

A starter or starter motor is an electrical device that used to rotate (crank) internal combustion engines so as to initiate the engine’s operation under its own power. As soon as the engine begins to run, it got disconnected from the engine, which now relies on the combustion process. The component is mounted on the engine’s gearbox housing, and the starter motor gear meets flywheel’s teeth.

Being an electrical component, a starter consists of a powerful DC (Direct Current) electric motor and a solenoid. The solenoid receives positive power direct from the battery and hearth current from the engine body. In order to turn the engine, a 12-volt battery must be used to run the starter. This means the battery has to be sufficient enough to power the device. In the most situation where the starter motor clicks but won’t start, the issue is either from the battery or starter itself.

The starter motor has only one function which is the first and second rotations the engine needed. Apart from that, the device is just an extra load to the vehicle. There should be another means of starting vehicles without starter motors right! I think the component is too big for one function.

Most drivers improvise if the starter is faulty or battery power is low. The improvising is done by pushing the vehicle back or fro just to begin the combustion process. Though this is not highly advisable, but in a situation where the battery needs to be charged by the vehicle’s alternator. We can discuss on the starter motor function and your experience improvising in the comment section.

Parts of Starter Motor

Below are starter motors parts and their functions:


An armature is an electromagnet component that is mounted on the driveshaft or bearings for a guide. It is made of a laminated soft iron core which is wrapped with numerous conductor loops or windings.


A commutator is a section of the shaft at the rear of the housing on which brushes run to conduct electricity. It is made of two plates mounted to the axle of the armature, the plates provide connections for the coil of the electromagnet.


Brushes are parts that run on a section of the commutator at the rear of the housing. it rubs the commutator and conducts electricity.


The solenoid features two coils of wire that are wrapped around the core. This solenoid serves as a switch that connects and close the electrical connection between the starter motor and the vehicle’s battery.


The function of a plunger in a starter motor is to push forward so the pinion can be engaged.

Lever Fork:

The lever fork is connected to the plunger which makes them push forward together to engage the pinion.


A pinion is a small mechanism containing gear and springs. It engages immediately the engine started, by extending the gear to the flywheel teeth. The flywheel is the source of engine rotation.

Field Coils:

The field coils are held in housing with screws as it consists of two or more coils connected in series. These coils receive power from the battery that converts them into an electromagnet that turns the armature. This creates a magnetic field around the armature.

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Below is the diagram of a starter motor:

major parts of a starter motor

Types of Starter Motor

Below are the five types of starter and their difference:

Direct Drive Starter Motor DD

Direct drive the most common and older types of starter motor available out there. it comes with different applications and construction design but it’s solenoid operated unit. Well, its working remains plain like other types.

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The solenoid is energized by the car battery when the ignition switch or key is pressed. This pushes the plunger to shift the lever that directs the pinion gear. The pinion gear which then meshes with the engine flywheel. So, as the starter motor spin, the flywheel spin along which then begin the engine combustion to run on its own.

Planetary Gear PLGR

The existence of these types of starter motor has vastly replaced the direct-drive types. It is a permanent magnet that transmits power between the pinion shaft and the armature. The armature spin with more speed and torque.

The essence of planetary gear is to reduce gear which further reduces the requirement of high current. There is a sun gear located at the end of the armature and three plenary carrier gears inside the ring gear held stationery.

Planetary gears are able to attain a great amount of gear reduction as the ring gear is held and inputting the sun gear while outputting the carrier.

Permanent Magnet Gear Reduction PMGR

The permanent magnet gear reduction is designed to offer less weight, easy construction and less heat generation. It features four to six magnet field assemblies rather than field coil starters. It has three terminals on the 12V solenoid, and also heavy-duty making to require less current. Because there are no field coils, the commutator and brushes transfer current directly to the armature.

Permanent Magnet Direct Drive PMDD

The PMDD types of starter motors are similar to the direct drive in several ways. Their difference is that the field coil is replaced with permanent magnets in the permanent magnet direct drive type.

Off-Set Gear Reduction OSGR

These starter motor types work under high speed at low current. They are lighter and more compact which makes them easy to assemble. Off-set gear reduction starters are common among the four-wheel-drive vehicle as they increase cranking torque.

Inertial Starter

Inertial starters are the electric types that achieve the feature of all types of starter motors. It works perfectly fine during the cranking and even ensures the motor part safety. It starts the engine strong and very fast, making it the best option for cracking speed. The amount of weight associated with the torque capacity of the starter is extremely minimized.

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Working Principle

The working of a starter is quite easy and interesting, but most drivers really don’t know the secret behind motor cranking. When the ignition key or bottom is press, the transmission should be in park or neutral state. The battery voltage goes to through the starter control circuit to activates the solenoid.

The starter motor is powered by the solenoid, which helps to push the starter gear forward to mesh with the engine flywheel. This flywheel is mounted on the engine crankshaft, so as the starter motor spins it turns the flywheel so as the crankshaft. As soon as the engine start, the system disengages from the flywheel.

Note in an automatic transmission, starter motors can only be operated when the vehicle is in park or neutral position. And in manual transmission, the clutch pedal must be depressed.

The working inside the starter motor, there are four field windings attached to the housing from inside. The armature (rotating parts) is connected through the carbon brushes in series with the field coils. But remember in some starters field coils are replaced with magnet fields. There is a small gear at the front of the armature attached with an overrunning clutch.

Watch the video to have more understanding of how a starter motor work:

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Symptoms of Bad or Failing Starter Motor

Below are the symptoms of bad starter motors:

Engine Won’t Crank Or Start:

This is one of the most common symptoms that occurs if a starter motor is bad. Well, failing battery, bad ignition switch, or engine mechanical problem can also because. Which is why the problem must be diagnosed by a professional in the field.


If you notice your engine crank slowly, then you should know the starter motor has an internal problem. Although a weak battery and internal engine problem can also cause this issue. So, seeing a good mechanic will help you determine and fix the problem.

Grinding Noise:

You’ll notice a grinding noise while starting the engine if the teeth on the starter pinion are damaged. If the starter fails to quickly retract soon enough once the engine starts running. the flywheel teeth can also because of this problem.

Whirring Noise:

A whirring noise occurs when the starter pinion gear freewheels during the engine cranking. This is because the pinion gear is not engaging the flywheel properly, which will also cause the engine to fail to start.

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Troubleshooting Starter Motor Problems

Below are the ways to troubleshoot starter motor problems as a DIYer:

Looking Under the Hood:

Inspecting the vehicles battery and battery cables to see if they are in order. A weak or dead battery or faulty cables could be the cause.

Tap the Starter:

Using a light spinner to tap the body of a starter motor could also help. It should be lightly done to avoid breaking the component. Tapping might get the electrical part back in contact with each other or clear dirt blocking the parts.

Adjust the Transmission:

If a starter refuses to start in an automatic transmission, you should try changing from Park to Neutral. If it starts in neutral, then there may a technical glitch that’s preventing the car from starting in Park.

Check the Fuel Gauge:

This sound silly right! In modern combustion engines, an empty tank can cause a vehicle not to start due to the sensor installed on them.

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In conclusion, we’ve examined thoroughly on the engine starter motor, its function, and parts. We also saw the different types of a starter motor, how they work and their bad symptoms and troubleshooting.

I hope you enjoyed the reading, if so, kindly comment, share and recommend this site, other technical students. Thanks!

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