Proper grounding for the vehicle electrical system and 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 vehicle and radio performance.
To ensure proper functioning of the vehicle and engine electrical systems, an engine-to-frame ground strap with a direct path to the battery must be used. This connection 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 to connect the engine grounding stud to the frame of the vehicle and to the negative battery post.
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| Illustration 1 | g00864026 |
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Grounding Stud To Battery Ground ("−") | |
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| Illustration 2 | g00864027 |
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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. 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 can manage the full charging current of the alternator.
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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. |
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 to keep the battery at full charge.
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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.
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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. |
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| Illustration 3 | g01363364 |
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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 to control the field current for the alternator. The alternator uses the field current to generate the required voltage output.
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.
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.
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| Illustration 4 | g00285112 |
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Solenoid | |
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 action moves the shift lever 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. Now, only the smaller hold-in windings are in operation. The hold-in windings operate during that time that is required to start the engine. The solenoid will now draw less current from the battery, and the heat generated by the solenoid will be kept at an acceptable level.
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| Illustration 5 | g01363366 |
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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 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 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.
The machine ECM, machine starting circuit, and engine ECM must work properly for the engine to start. The role of the engine ECM is to provide the ground to the start relay and the engine ECM relay. To simplify the starting circuit, it can split into two operations:
- Starter operation
- Turning on the engine ECM
The electrical circuits shown below are for the D Series Skid Steer Loader (SSL), Multi-Terrain Loader (MTL), and Compact Track Loader (CTL). The machine ECM and circuits will be different for the Mini-Hydraulic Excavators (MHE) and Compact Wheel Loaders (CWL), but the operation of the engine ECM will be the same.
Note: For the MHE and CWL and to obtain the latest SSL/MTL/CTL schematic, refer to the Electrical Schematic in Service Information System (SIS) web.
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| Illustration 6 | g06085316 |
The engine ECM always provides the ground for the start relay using a sinking driver. When the engine ECM receives 12 V at J1-12, it will sink J1-5 to ground. This will provide the ground to the start relay.
The following is a detailed description for the operation of the SSL D Series starter circuit referring to the SSL D Series electrical schematic above. The machine ECM and circuits will be different for the MHE and CWL. For those two product series and for the latest SSL, MTL, CTL schematic, refer to SIS web.
The machine ECM provides the 12 V to the start relay through the 306 Wire that comes from the machine ECM J1-6 pin. This is powered after the machine ECM receives 12 V from the keyswitch to start through the 307 Wire to J1-79 pin.
Note: All machine safety interlocks must be met for this to occur. For example:
- SSL: arm bar must be down and the operator in the seat
- MHE: the hydraulic lock control lever must be down
- CWL: parking brake engaged
• The engine ECM J1-5 pin provides the ground to the start relay through a sinking driver. When the machine ECM receives power from the keyswitch to start (307 wire to machine ECM J1-79 pin), 12 V is provided from the machine ECM J1-6 pin to the engine ECM J1-12 pin. The engine ECM will then sink J1-5 to the ground to provide the ground the start relay.
Note: There are some engine diagnostic codes/events that will prevent the engine from starting. If troubleshooting a no start issue, resolve those codes first.
With power and ground to the start relay, the starter will operate.
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| Illustration 7 | g06085332 |
The engine ECM is not turned on until there is 12 V at the following pins: J1-1, J1-18, J1-21, J1-28, J1-38, J1-58, and J1-78.
Until the engine ECM is turned on, the engine will not start and Cat® Electronic Technician (ET) will not be able to connect to the engine ECM. When the engine ECM receives 12 V at J1-13 and J1-33, the ECM will sink C1-3 and J1-23 to the ground. This is the ground for the engine ECM power relay. The engine ECM power relay provides the 12 V to the engine ECM pin at the following locations: J1-1, J1-18, J1-21, J1-28, J1-38, J1-58, and J1-78.
The following is a detailed description for turning on the SSL D Series engine ECM. This refers to the SSL D Series electrical schematic above. The machine ECM and circuits will be different for the CWL and MHE. For those two product families and for the latest SSL schematic, refer to SIS web.
Turning on the Engine ECM Circuit
The keyswitch provides 12 V to the main power relay module 1 along with the top fuse panel that includes the engine ECM fuse. In the schematic, the main power relay is called "PWR_RLY_1". The main power relay module 1 is always grounded through a ground behind the cab.
The engine ECM power relay module receives 12 V from the battery. In the schematic the engine ECM power relay module is called "PWR_RLY_ECM".
Main power relay module 1 will provide start-up power to the engine ECM at J1-13 and J1-33. When the engine ECM receives 12 V, it will sink J1-3 and J1-23 to ground. This is the ground for the engine ECM power relay module.
With power and ground to the engine ECM power relay module, 12 V is provided to the engine ECM at the following pins: J1-1, J1-18, J1-21, J1-28, J1-38, J1-58, and J1-78.
The engine can now start and Cat ET will be able to connect to the engine ECM.