Electrical System for All Caterpillar Products Caterpillar

Starting System


D3400 01T

How the Starting System Works

The starting system converts electrical energy from the battery into mechanical energy in order to start the engine.

Illustration 1g01073449

A basic starting system has four parts:

Battery - Supplies energy for the circuit

Starter switch - Activates the circuit

Solenoid (motor switch) - Engages the starting motor drive with the flywheel

Starting Motor - Drives the flywheel to crank the engine

When the starter switch is activated a small amount of current flows from the battery to the solenoid and back to the battery through the ground circuit.

The solenoid performs two functions. The solenoid engages the pinion with the flywheel and closes the switch inside the solenoid between the battery and starting motor, which completes the circuit and allows high current to flow into the starting motor.

The starting motor converts the electrical energy from the battery into rotary mechanical energy in order to crank the engine. The starting motor is similar to other electric motors. All electric motors produce a turning force through the interaction of magnetic fields inside the motor.

Starting Motor

Illustration 2g01079926

A review of some basic rules of magnetism is needed, to understand the basic operating principles of starting motors.

The following information is the basic rules of magnetism:

  • Like poles repel. Unlike poles attract.

  • Magnetic flux lines are continuous and exert force.

  • DuriCurrent-carrying conductors have a magnetic field that surrounds the conductor in a direction that is determined by the direction of the current flow.

If a conductor has a current that has passed through the conductor, there will be a magnetic field that is formed. A permanent magnet has a field between the two poles. When the current carrying conductor is placed in the permanent magnetic field, there will be a force that is exerted on the conductor because of the magnetic field. If the conductor is formed in a loop and placed in the magnetic field, the result is the same. Since current flow is in opposite directions in the coil, one side will be forced upward while the other side is forced downward. This will provide a rotational effect or a torque effect on the coil.

Starting Motor Principles

Illustration 3g01073452

The pole pieces in the field frame assembly can be compared to the ends of a magnet. The space between the poles is the magnetic field.

Illustration 4g01073454

If a wire (field winding) is wrapped around the pole pieces and current is passed through the wire, the strength of the magnetic field between the pole pieces increases.

Illustration 5g01073455

If you feed current from the battery into a loop of wire, a magnetic field is also formed around the wire.

Illustration 6g01073457

If the loop of wire is placed in the magnetic field between the two pole pieces and current is passed through the loop, a simple armature is created. The magnetic field around the loop and the field between the pole pieces repel each other, causing the loop to turn.

Illustration 7g01073459

A commutator and several brushes are used to keep the electric motor spinning by controlling the current that is passing through the wire loop. The commutator serves as a sliding electrical connection between the wire loop and the brushes. The commutator has many segments, which are insulated from each other.

The brushes ride on top of the commutator and slide on the commutator in order to carry battery current to the spinning wire loops. As the wire loops rotate away from the pole shoes, the commutator segments change the electrical connection between the brushes and the wire loops. This reverses the magnetic field around the wire loops. The wire loop is again pulled around and passes the other pole piece. The constantly changing electrical connection keeps the motor spinning. A push-pull action is set up as each loop moves around inside the pole pieces.

Several loops of wire and a commutator with many segments are used to increase motor power and smoothness. Each wire loop is connected to the wire loops own segment on the commutator in order to provide current flow through each wire loop as the brushes contact each segment. As the motor spins, many wire loops contribute to the motion in order to produce a constant and smooth turning force.

Illustration 8g01073460

A starting motor, unlike a simple electric motor, must produce very high torque and relatively high speed. Therefore, a system to support the wire loops and to increase the strength of each wire loop's magnetic field is needed.

A starter armature consists of the following components:

  • Armature core

  • Armature shaft

  • Commutator

  • Armature windings (wire loops)

The starting motor shaft supports the armature as the armature spins inside the starter housing. The commutator is mounted on one end of the armature shaft. The armatures core holds the windings in place. The core is made of iron in order to increase the strength of the magnetic field that is produced by the windings.

Illustration 9g01073461

A field winding is a stationary insulated wire that is wrapped in a circular shape, which creates a strong magnetic field around the motor armature. When current flows through the field winding, the magnetic field that is between the pole pieces becomes vary large. The magnetic field can be 5 to 10 times that of a permanent magnet. As the magnetic field between the pole shoes acts against the field that is developed by the armature, the motor spins with extra power.

Starting Motor Characteristics

Starters are high capacity intermittent duty electric motors that tend to behave with the followign specific characteristics:

  • If starting motors are required to power a certain mechanical component (load), the starting motor will consume a specific amount of power in watts.

  • If the load is removed, speed increases and current draw will go down.

  • If the load is increased, speed decreases and current draw will go up.

The amount of torque that is developed by an electric motor increases as the current flowing through the motor increases. The starting motor is designed to operate for short periods of time under an extreme load. The starting motor produces a very high horsepower for the size.

Counter Electromotive Force (CEMF) is responsible for changes in current flow as the starter speed changes. CEMF increases the resistance to current flow from the battery, through the starter, as the starter speed increases. This occurs because, as the conductors in the armature are forced to spin, the conductors are cutting through the magnetic field that is created by the field windings. This induces a counter voltage in the armature that acts against battery voltage. This counter voltage increases as the armature speed increases. This acts as a speed control, and prevents high free running speeds.

Although, most electric motors have some form of current protection device in the circuit, most starter motors do not. Some starters have thermal protection. This is provided by a heat sensitive thermostatic switch. The thermostatic switch will open when the starter temperature rises due to excessive cranking. The switch will automatically reset when the starter temperature cools. The electric motor is classified as an intermittent operating motor. If the electric motor were a continuous operating motor, the electric motor would need to be almost as large as the engine. Because of the high torque demands on the starter motor, a great deal of heat is produced during operation. Extended operation of the starter motor will cause internal damage due to this high heat. All the parts of the starter motor's electrical circuit are very heavy. This enables the handling of the heavy current flow that is associated with the operation.

If higher loads require more power to operate, then each starter motor must have sufficient torque in order to provide turning speed that is necessary to crank the engine. This power is directly related to the strength of the magnetic field, since the strength of the field is what creates the power.

Illustration 10g01073464

As previously described, starting motors have a stationary member (field windings) and a rotating member (armature). The field windings and the armature are usually connected together so that all current that enters the motor passes both the field and the armature. This is the motor circuit.

The brushes are a means of carrying the current from the external circuit (field windings) to the internal circuit (armature windings).

The brushes are contained in brush holders. Normally, half the brushes are grounded to the end frame. The other half of the brushes are insulated and connected to the field windings.

Starter motor fields can be wired together in four different configurations in order to provide the necessary field strength:

  • Series

  • Compound (shunt)

  • Parallel

  • Series-parallel

Series wound starters (Illustration 9) are capable of producing a very high initial torque output when the starters are first engaged. This torque then decreases as the starters operate due to counter-electromotive force. This decreases the current flow since all the windings are in series.

Compound motors have three windings in series and one winding in parallel. This produces good initial torque for starting and the benefit of some load adjustment due to the parallel winding. This type of starter also has the added benefit of speed control due to the parallel field.

Parallel wound motors provide higher current flow and greater torque, by dividing the series windings into two parallel circuits.

Series-parallel motors combine the benefits of both the series and the parallel motors.

Many starters have four fields and four brushes. Starters that are required to produce very high torque, may have up to six fields and brushes. Some light duty starters may have only two fields.

Many heavy-duty starter motors are not grounded through the case of the starter. This type of starter motor is grounded through an insulated terminal that must be connected to the battery ground for the starter to work. A ground wire for the solenoid and other engine electrical devices must also be attached to the starter ground terminal for proper electrical operation.

Illustration 11g01073469

After electrical power is transmitted to the starting motor, some type of connection is needed to put this energy to work. The starting motor drive makes it possible to use the mechanical energy that is produced by the starting motor.

Although torque that is produced by the starter motor is high, the torque does not have the ability to crank the engine directly. Other means must be used to provide both adequate cranking speed and the necessary torque.

To provide adequate torque for cranking the engine, the speed of the starter is altered by the ratio between the pinion gear on the starter and the engine flywheel. This ratio varies from 15:1 to 20:1. For example, if the starter drive gear had 10 teeth, the ring gear might have 200 to provide a ratio of 200:10 or 20:1.

Starter drive mechanisms

If the starter were left engaged to the flywheel after the engine started, damage would occur to the armature due to very high speeds that are created as the engine rpm increased. At high speed, the armature would throw the windings due to centrifugal force.

The gear that engages and drives the flywheel is called a pinion gear. The gear on the flywheel is called a ring gear. How the starter pinion gear engages with the flywheel ring gear depends on the type of drive that is used.

Starter pinion gears and starter drive mechanisms can be of two different types:

  • Inertia drive

  • Overrunning clutch

Inertia drives are actuated by rotational force when the armature turns. This type engages after the motor begins to move. The drive sleeve has a very coarse screw thread that is cut into the drive sleeve, which corresponds to threads that are on the inside of the pinion.

As the motor begins to turn, the inertia that is created at the drive causes the pinion to move up the threads until the pinion engages with the ring gear on the flywheel. You can recreate this action by spinning a heavy nut on a bolt and watch the rotary motion change to linear motion as the nut moves up or down.

One disadvantage of inertia starters is that the pinion is not positively engaged before the starter begins to turn. If the drive does not engage with the flywheel, the starter will spin at a high speed without cranking the engine. If the pinion lags the pinion will strike the gear with heavy force. This will damage the teeth.

Illustration 12g01073470

The overrunning clutch drive is the most common type of clutch drive. The overrunning clutch drive requires a lever to move the pinion into mesh with the flywheel ring gear. The pinion is engaged with the flywheel ring gear before the armature starts to rotate.

With this type of drive system, a different method must be used in order to prevent armature overspeeding. A lever pulls the drive out of engagement while an overrunning clutch prevents overspeeding.

The overrunning clutch locks the pinion in one direction and releases the pinion in the other direction. This allows the pinion gear to turn the flywheel ring gear for starting. The overrunning clutch also allows the pinion gear to freewheel when the engine begins to run.

The overrunning clutch consists of rollers that are held in position by springs against a roller clutch. This roller clutch has tapered ramps that allow the roller to lock the pinion to the shaft during cranking.

The torque travels through the clutch housing. The torque is transferred by the rollers to the pinion gear. When the engine starts and the speed of the drive pinion exceeds the speed of the armature shaft, the rollers are pushed down the ramps. This permits the pinion to rotate independently from the armature shaft. Once the starter drive pinion is disengaged from the flywheel and is not operating, spring tension will force the rollers into contact with the ramps in preparation for the next starting sequence. There are various heavy duty designs of this drive.

Starting Circuit Controls

Illustration 13g01073471

The starting circuit contains control and protection devices. The control and protection devices are necessary to allow the intermittent operation of the starter motor and to prevent operation during some machine operation modes for safety reasons.

The starter electrical circuit may consist of the following devices:

  • Battery

  • Cables and wires

  • Key start switch

  • Neutral safety switch and clutch safety switch (if equipped)

  • Starter relay

  • Starter solenoid


The battery supplies all of the electrical energy to the starter which enables the battery to crank the engine. It is important that the battery be fully charged and in good condition if the starting system is to operate at full potential.

Cables and Wires

The high current flow through the starter motor requires cables that must be large enough to have low resistance. In a series circuit, any added resistance in the circuit will affect the operation of the load due to a reduction in the total current flow in the circuit.

In some systems, the cables will connect the battery to the relay and the relay to the starter motor. In other systems the cable will go directly from the battery to the starter.

Ground cables must also be large enough to handle the current flow. All connectors and connections in the starting system must have as little resistance as possible.

Key Start Switch

The key start switch activates the starter motor by providing power to the starter relay from the battery. The key start switch can be operated directly by a key, a button, or remotely by linkage from a key activated control. The key start switch can be mounted in the dashboard assembly or on the steering column.

Illustration 14g01073473

Neutral Safety Switch or Clutch Safety Switch

All vehicles that are equipped with a power shift or an automatic transmission require a neutral safety switch that will only permit starter operation in park or in neutral. This switch can be mounted on the transmission, at the shifter or in the linkage. The switch contacts are closed when the transmission selector is in park or in neutral. The switch contacts are open when the transmission selector is in any gear.

Some vehicles may use a clutch safety switch that is open when the clutch is in the engaged position and closed when the operator depresses the clutch pedal. This prevents starter operation as long as the clutch is engaged. Some transmissions also use a neutral gear switch that will prevent starter operation, unless the transmission is placed in the neutral position.

All safety switches of this type should be maintained in good operating condition. All safety switches should never be bypassed or removed.

Starter relay

Illustration 15g01073474

The starter relay (magnetic switch) may be used in some starting systems. The starter relay is located between the key start switch and starter solenoid. The starter relay is a magnetic switch that is activated by power from the battery that is supplied through the key start switch. Relays are usually placed so that the cables between the starter and the battery are as short as possible.

The starter relay uses a small current from the key start switch to control the larger current to the starter solenoid, which reduces the load on the key start switch. Energizing the relay windings will cause the plunger to be pulled up due to the magnetism that is caused by the current flow through the windings. The contact disk will also be pulled up and will contact the battery and starter terminal ends. Current will flow from the battery to the starter solenoid.

Illustration 16g01073475

Solenoids combine the operation of a magnetic switch (relay) with the ability to perform a mechanical task (engage the drive). The starter solenoid produces a magnetic field that pulls the solenoid plunger and disc into the coil windings, which completes the starting system circuit. The solenoid is mounted on the starter motor so that linkage may be attached to the overrunning clutch drive in order to engage the drive.

Solenoids contain two different windings for effective operation. When the ignition switch is turned to the start position, current from the battery flows through a pull-in winding and a hold-in winding. These windings contain many coils of wire and produce a strong magnetic field to pull the heavy plunger forward and engage the starter drive.

When a plunger reaches the end of the travel through the solenoid, the plunger engages a contact disk that will operate like a relay and allow current to flow to the starter motor from the battery. This also serves to disconnect the series pull-in winding from the circuit and allows current to flow through a shunt hold-in winding only. Only the lighter magnetic field that is created by the hold-in winding is required to hold the plunger in position. This reduces the amount of control current that is required to eliminate heat buildup and provides more current for the starter motor.

Illustration 17g01073476

When the ignition switch is closed, battery current flows in two directions. Current flows from the battery to the start switch and then through the pull-in winding, field winding, armature, brushes and to ground.

The activation of the pull-in winding and the hold-in winding produces a magnetic force. The magnetic force pulls the plunger to the left, which moves the overrunning clutch and the pinion toward the flywheel ring gear.

Illustration 18g01073477

When the plunger is pulled to the left, the solenoid contacts close. At this point the pinion begins to mesh with the flywheel ring gear and the pull-in winding is shorted. This causes current flow through the solenoid contacts to the field winding, armature, brushes and to ground. Current still flows through the hold-in winding to ground. The starting motor is energized. The pinion engages the flywheel ring gear. The engine begins to crank. At this time the plunger is kept in the pull-in position only by the magnetic force of the hold-in winding.

Illustration 19g01073478

As soon as the engine starts, the flywheel ring gear turns the pinion faster than the starting motor is rotating. The overrunning clutch breaks the mechanical connection between the clutch and the starting motor. When the ignition switch is released, current flow through the hold-in winding and the pull-in winding is in the same direction, which causes the hold-in winding magnetic force to be reduced. The solenoid contacts are opened. The plunger and overrunning clutch are pulled back to the original position by the return spring force. The armature stops and the motor is OFF.

Series-Parallel Systems

Machines with larger diesel engines require high power starters to provide adequate cranking speed for the engine. To achieve this some machines use 24V starters. Using 24V allows the starter to produce the same power with less current flow.

In a series-parallel system the starter operates on 24V but the rest of the machine electrical system operates on 12V. A special series-parallel switch is used that connects two or more batteries in parallel for normal accessory and charging operation and then connects the batteries in series with the starter when cranking. 12V accessories are preferred because they are much less expensive than 24V lights and accessories.

12/24V Electrical Systems

In another system of this type, the starter is connected in series with two 12V batteries. The alternator charges the batteries with 24V.

Starter System Testing

Accurate testing of a starting system begins with an understanding of how the system functions. If your knowledge of the operation is complete, you can logically determine the fault through visual inspection and electrical testing.

Inspecting and Troubleshooting

An organized procedure for testing and inspection is necessary to prevent the replacement of good parts or the unnecessary repair of operational components.

Verify the Complaint

Operate the system yourself to see how the system functions. Starter system problems will normally fall into the following categories:

  • The starter spins but does not crank the engine.

  • The engine cranks very slowly.

  • The engine does not crank at all.

  • The starter motor is too noisy.

Do not crank the engine for more than 30 seconds at a time. Allow the starter motor to cool between cranking periods in order to prevent damage.

Define the Problem

Determine whether the problem is mechanical or electrical. For instance, if the starter rotates but does not crank the engine, the problem is most likely mechanical, since the drive does not seem to be engaging.

Mechanical problems can be corrected by repairing the component or by replacing parts.

Electrical problems require additional testing to determine the cause of the fault and the repair that will be required.

Isolate the Problem

Regardless of whether the problem is mechanical or electrical, you will have to determine where the problem is occurring, so that you can quickly and accurately make your repairs.

The steps involved in testing and isolating a starting circuit are:

  1. Test the battery in order to determine if the battery is fully charged and capable of delivering sufficient current.

  1. Test the wiring and switches to determine if both are in good operating condition.

  1. If the engine, battery and wiring are all OK, but the starter is not operating correctly, the fault must be with the starter.

Visual Inspection

Begin all starting system testing with a thorough visual inspection. Check for the following problems:

  • Loose or corroded battery terminals

  • Worn or frayed insulation on the battery cables

  • Corroded solenoid or relay connections

  • A damaged starter solenoid or relay

  • Cracked or broken insulators on the starter relay

  • Loose engine or chassis grounds

  • Damaged neutral safety switches

  • Damaged ignition switch or actuating mechanisms

  • Loose starter

Battery test

Continue the inspection with a complete test and service of the battery.

Preform all required tests in order to verify that the battery is in good operating condition. Correct battery output is vital for good starting system operation and for correct diagnosis of the starting system.

Starting System Tests

On machine starting motor tests should be performed first in order to determine whether the starter must be removed and tested further.

The following test should be preformed while the starting motor is still on the machine:

  • Starting system voltage during cranking

  • Current draw during cranking

  • Voltage drops during cranking

  • Engine rotation

  • Starter motor pinion and flywheel ring gear inspection

Bench tests will determine if the starter must be repaired or replaced. Bench tests include a no load test and starting motor component tests.

Back to top
The names Caterpillar, John Deere, JD, JCB, Hyundai or any other original equipment manufacturers are registered trademarks of the respective original equipment manufacturers. All names, descriptions, numbers and symbols are used for reference purposes only.
CH-Part.com is in no way associated with any of the manufacturers we have listed. All manufacturer's names and descriptions are for reference only.