C27 and C32 Engines Electrical System Caterpillar

Electrical System
1.1. Reference
2.1. Grounding Practices
3.1. Engine Electrical System
4.1. Charging System Components
5.2. Alternator
6.1. Starting System Components
7.2. Solenoid
8.2. Starting Motor


Refer to the Schematic and the Troubleshooting, "Electronic Troubleshooting" for additional information on your engine.

Grounding Practices

Proper grounding for the electrical system is necessary for proper engine performance and reliability. Improper grounding will result in unreliable electrical circuit paths and in uncontrolled electrical circuit paths.

Uncontrolled engine electrical circuit paths can result in damage to the main bearings, to the crankshaft bearing journal surfaces, and to the aluminum components.

Uncontrolled electrical circuit paths can cause electrical noise which may degrade performance.

In order to ensure proper functioning of the electrical system, an engine-to-frame ground strap with a direct path to the battery must be used. This may be provided by a ground for the starting motor, by a frame to the ground for the starting motor, or by a direct frame to engine ground. An engine-to-frame ground strap must be run from the grounding stud of the engine to the frame and to the negative battery post.

Connect the battery negative post to the frame rail. From the frame rail, connect the ground wire to one of the following locations:

  • Cylinder head ground stud

  • Optional engine ground stud connection

The engine must be grounded to the frame rail. Connect the battery negative post to one of the following locations:

  • Cylinder head ground stud

  • Optional engine ground stud connection

The engine must have a ground wire to the battery.

Ground wires or ground straps should be combined at the studs that are only for ground use.

All of the ground paths must be capable of carrying any potential currents.

The engine alternator should be grounded to the battery with a wire size that is capable of managing the full charging current of the alternator.


When jump starting an engine, the instructions in the Operation and Maintenance Manual, "Starting with Jump Start Cables" should be followed in order to properly start the engine.

This engine may be equipped with a 12 volt starting system or with a 24 volt starting system. Only equal voltage for boost starting should be used. The use of a welder or of a higher voltage will damage the electrical system.

The engine has several input components which are electronic. These components require an operating voltage.

This engine is tolerant to common external sources of electrical noise. Electromechanical buzzers can cause disruptions in the power supply. If electromechanical buzzers are used near the system, the engine electronics should be powered directly from the battery system through a dedicated relay. The engine electronics should not be powered through a common power bus with other devices that are activated by the Engine Control Switch (ECS).

Engine Electrical System

The electrical system can have three separate circuits. The three circuits are the charging circuit, the starting circuit, and the low amperage circuit. Some of the electrical system components are used in more than one circuit.

The charging circuit is in operation when the engine is running. An alternator creates electricity for the charging circuit. A voltage regulator in the circuit controls the electrical output in order to maintain the battery at full charge.

The starting circuit is in operation when the start switch is activated.

The low amperage circuit and the charging circuit are connected through the ammeter. The starting circuit is not connected through the ammeter.

Charging System Components


The alternator is driven by the crankshaft pulley through a belt that is a Poly-vee type. This alternator is a three-phase self-rectifying charging unit. The regulator is part of the alternator.

The alternator design has no need for slip rings or for brushes. The only part of this alternator that moves is the rotor assembly. All of the conductors that carry current are stationary. The following components are the conductors: the field winding, the stator windings, six rectifying diodes and the regulator circuit.

The rotor assembly has many magnetic poles with air space between each of the opposite poles. The poles have residual magnetism that produces a small amount of magnet-like lines of force (magnetic field). This magnetic field is produced between the poles. As the rotor assembly begins to turn between the field winding and the stator windings, a small amount of Alternating Current (AC) is produced in the stator windings. The alternating current is produced from the small magnetic lines of force that are created by the residual magnetism of the poles. The AC is changed into Direct Current (DC) when the current passes through the diodes of the rectifier bridge. Most of this current provides the battery charge and the supply for the low amperage circuit. The remainder of current is sent to the field windings. The DC current flow through the field windings (wires around an iron core) increases the strength of the magnetic lines of force. These stronger magnetic lines of force increase the amount of AC that is produced in the stator windings. The increased speed of the rotor assembly also increases the current output of the alternator and the voltage output of the alternator.

The voltage regulator is a solid-state electronic switch. The voltage regulator senses the voltage of the system. The regulator then uses switches to control the current to the field windings. This controls the voltage output in order to meet the electrical demand of the system.


The alternator should never be operated without the battery in the circuit. The making or the breaking of an alternator connection with a heavy load on the circuit can cause damage to the regulator.

Illustration 1g01096944
Typical cross section of an alternator
(1) Regulator
(2) Roller bearing
(3) Stator winding
(4) Ball bearing
(5) Rectifier bridge
(6) Field winding
(7) Rotor assembly
(8) Fan

Starting System Components


Illustration 2g00292316
Typical cross section of a solenoid

A solenoid is an electromagnetic switch that performs two basic functions:

  • The solenoid closes the high current circuit for the starting motor with a low current start switch circuit.

  • The solenoid engages the pinion for the starting motor with the ring gear.

The solenoid has windings (one set or two sets) around a hollow cylinder or a hollow housing. A plunger that is spring loaded is located within the solenoid housing. The plunger can move forward and backward. When the start switch is closed and electricity is sent through the windings, a magnetic field is created. The magnetic field pulls the plunger forward in the solenoid housing. This moves the shift lever in order for the pinion drive gear to engage with the ring gear. The front end of the plunger then makes contact across the battery and across the motor terminals of the solenoid. The starting motor then begins to turn the flywheel of the engine.

When the start switch is opened, current no longer flows through the windings. The spring now returns the plunger to the original position. At the same time, the spring moves the pinion gear away from the flywheel.

When two sets of windings in the solenoid are used, the windings are called the hold-in winding and the pull-in winding. Both of the windings wind around the cylinder for an equal amount of times. The pull-in winding uses a wire with a larger diameter in order to produce a stronger magnetic field. When the start switch is closed, part of the current flows from the battery through the hold-in winding. The remainder of the current flows through the pull-in windings, to the motor terminal, and then to the ground. When the solenoid is activated, the current is shut off through the pull-in windings. Only the smaller hold-in windings are in operation for the extended period of time that is necessary for the engine to be started. The solenoid will now take a smaller amount of current from the battery. Heat that is created by the solenoid will be kept at an acceptable level.

Starting Motor

Illustration 3g01385598
Typical cross section of a starting motor
(9) Field
(10) Solenoid
(11) Clutch
(12) Pinion
(13) Commutator
(14) Brush assembly
(15) Armature

The starting motor rotates the engine flywheel at a rate that is fast enough to start the engine.

The starting motor has a solenoid. When the start switch is activated, the solenoid will move the starter pinion in order to engage the pinion and the ring gear on the engine flywheel. The starting motor pinion and the ring gear will engage before the circuit between the battery and the starting motor is closed by the electric contacts in the solenoid. When the circuit between the battery and the starting motor is complete, the pinion will rotate the engine flywheel. A clutch provides protection for the starting motor so that the engine cannot turn the starting motor too fast. When the switch is released, the starter pinion will move away from the ring gear.

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