C7 Industrial Engines Systems Operation – Electrical System

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

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

Uncontrolled electrical circuit paths can cause electrical noise which may degrade the engine and radio performance.

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

Illustration 1 g00864026
Grounding Stud To Battery Ground (“−”)

Illustration 2 g00864027
Alternate Grounding Stud To Battery Ground (“−”)

The engine must have a wire ground to the battery.

Ground wires or ground straps should be combined at ground studs that are only for ground use. You need to periodically check that the grounds are tight and grounds are free of corrosion.

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

When jump starting an engine, the instructions in 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 a 24 volt starting system. Only equal voltage for jump starting should be used. The use of a higher voltage will damage the electrical system.

The Electronic Control Module (ECM) must be disconnected at the J1/P1 and J2/P2 locations before welding on the vehicle.

Engine Electrical System

The electrical system has three separate circuits:


  • Charging circuit
  • Starting circuit
  • Low amperage circuit

Some of the electrical system components are used in more than one circuit. The following components are used in each of the three circuits:


  • Battery
  • Circuit breaker
  • Ammeter
  • Battery cables

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



The disconnect switch, if equipped, must be in the ON position in order to let the electrical system function. There will be damage to some of the charging circuit components if the engine is running with the disconnect switch in the OFF position.

If the vehicle has a disconnect switch, the starting circuit can operate only after the disconnect switch is put in the ON position.

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

Both the low amperage circuit and the charging circuit are connected to the same side of the ammeter. The starting circuit is connected to the opposite side of the ammeter.



Never operate the alternator without the battery in the circuit. Making or breaking an alternator connection with heavy load on the circuit can cause damage to the regulator.


Charging System Components


Illustration 3 g01363364
Alternator components

(1) Brush holder

(2) Rear frame

(3) Rotor

(4) Stator

(5) Drive end frame

(6) Fan assembly

(7) Slip rings

(8) Rectifier

The alternator has three-phase, full-wave, rectified output. The alternator uses brushes to generate electricity.

The alternator is an electrical component and a mechanical component that is driven by a belt from engine rotation. The alternator is used to charge the storage battery during engine operation. The alternator is cooled by a fan that is a part of the alternator. The fan pulls air through holes in the back of the alternator. The air exits the front of the alternator and the air cools the alternator in the process.

The alternator converts mechanical energy and magnetic energy into alternating current (AC) and voltage. This process is done by rotating an electromagnetic field (rotor) that is direct current (DC) inside a three-phase stator. The alternating current and the voltage that is generated by the stator are changed to direct current. This change is accomplished by a system that uses three-phase, full-wave, rectified outputs. The three-phase, full-wave, rectified outputs have been converted by six rectifier diodes that are made of silicon. The alternator also has a diode trio. A diode trio is an assembly that is made up of three exciter diodes. The diode trio rectifies field current that is needed to start the charging process. Direct current flows to the alternator output terminal.

A solid-state regulator is installed in the back of the alternator. Two brushes conduct the current through two slip rings to the field coil on the rotor.

Also, a capacitor is mounted in the back of the alternator. The capacitor protects the rectifier from high voltages. The capacitor also suppresses radio noise sources.

The voltage regulator is a solid-state electronic switch that controls the alternator output. The voltage regulator limits the alternator voltage to a preset value by controlling the field current. The voltage regulator feels the voltage in the system. The voltage regulator switches ON and OFF many times per second in order to control the field current for the alternator. The alternator uses the field current in order to generate the required voltage output.

Note: Refer to Service Manual, SENR3862 for detailed service information for the Delco Remy 27 SI Series Alternator.

Note: If the alternator is connected to an engine component, the ground strap must connect that engine component to the frame or to the battery ground.

Starting System Components

Starting Solenoid

A solenoid is a magnetic switch that does two basic operations:


  • The solenoid closes the high current starting motor circuit with a low current start switch circuit. 
  • The solenoid engages the electric starting motor pinion with the ring gear.

Illustration 4 g00285112

The solenoid has windings (one set or two sets) around a hollow cylinder. 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 cylinder. This moves the shift lever in order to engage the pinion drive gear with the ring gear. The front end of the plunger then makes contact across the battery and the motor terminals of the solenoid. After the contact is made, the starting motor begins to turn the flywheel of the engine.

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

When two sets of solenoid windings are used, the windings are called the hold-in winding and the pull-in winding. Both sets of windings have the same number of turns around the cylinder, but the pull-in winding uses a wire with a larger diameter. The wire with a larger diameter produces a greater magnetic field. When the start switch is closed, part of the current flows from the battery through the hold-in windings. The rest of the current flows through the pull-in windings to the motor terminal. The current then flows through the motor to ground. The solenoid is fully activated when the connection across the battery and the motor terminal is complete. When the solenoid is fully activated, the current is shut off through the pull-in windings. At this point, only the smaller hold-in windings are in operation. The hold-in windings operate for the duration of time that is required in order to start the engine. The solenoid will now draw less current from the battery, and the heat that is generated by the solenoid will be kept at an acceptable level.

Starting Motor

Illustration 5 g01363366
Electric starting motor components

(9) Brush assembly

(10) Field windings

(11) Solenoid

(12) Clutch

(13) Pinion

(14) Armature

The starting motor is used to turn the engine flywheel at a rate that will allow the engine to start running.

Note: Some starting motors have ground straps that connect the starting motor to the frame, but many of these starting motors are not grounded to the engine. These starting motors have electrical insulation systems. For this reason, the ground strap that connects the starting motor to the frame may not be an acceptable engine ground. Starting motors that were installed as original equipment are grounded to the engine. These starting motors have a ground wire from the starting motor to the negative terminal of the battery. When a starting motor must be changed, consult an authorized dealer for the proper grounding practices for that starting motor.

The starting motor has a solenoid. When the ignition switch is turned to the START position, the starting motor solenoid will be activated electrically. The solenoid plunger will now move a mechanical linkage. The mechanical linkage will push the pinion in order to engage with the flywheel ring gear. The pinion will engage with the ring gear before the electric contacts in the solenoid close the circuit between the battery and the starting motor. When the circuit between the battery and the starting motor is complete, the pinion will turn the engine flywheel. A clutch gives protection for the starting motor so that the engine cannot turn the starting motor too fast.

When the ignition switch is released from the START position, the starting motor solenoid is deactivated. The starting motor solenoid is deactivated when current no longer flows through the windings. The spring now pushes the plunger back to the original position of the plunger. At the same time, the spring moves the pinion gear away from the flywheel ring gear.

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