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| Illustration 1 | g00584419 |
Typical 24 V Starting Circuit (1) Test point. (2) Test point. (3) Test point. (4) Test point. (5) Test point. (X) Hold-in coil. (W) Pull-in coil. | |
General Information
All starting systems are made up of four elements. The elements are the key start switch, the start relay, the starting motor solenoid, and the starting motor. The start relay may not be required on smaller engines. In this case, the start switch powers the starting motor solenoid directly. Start switches are relatively low current devices. Start switches are designed to operate properly at 5 A to 20 A. The coil of a start relay between test point (1) and test point (2) draws about 1 A. The start switch easily turns on the start relay. The start relay can have a long life cycle. The contacts on a start relay are rated between 100 A and 300 A when the contacts are switched. Because a solenoid requires 5 A to 50 A, the start relay can easily switch this load.
The solenoid engages the pinion with the flywheel. The solenoid is a high current switch that is rated at about 1000 A. The solenoid activates the starting motor. The starting motor solenoid has two coils. Pull-in coil (W) draws about 40 A and the hold-in coil (X) requires about 5 A. When the start relay closes, both coils (W) and (X) receive power. Battery voltage is applied to the high end of both coils. Battery voltage is applied at test point (3) . Test point (3) and the start terminal (S) are located at the same points. The low end of hold-in coil (X) is permanently grounded to the ground post of the starting motor. Grounding for the low end at test point (4) of the pull-in coil (W) is momentary. Grounding takes place through the DC resistance of the starting motor.
When the magnetic force is built up in both coils, the pinion moves toward the flywheel ring gear. The pinion will stop short of engaging the flywheel ring gear. Power is applied to the starting motor when the solenoid contacts close. This will temporarily remove the ground from the pull-in coil (W) . Battery voltage is placed on both ends of the pull-in coil (W) while the motor cranks. During this period, the pull-in coil is out of the circuit. Cranking continues until power to the solenoid is broken by releasing the key start switch. The switches and relays allow a 5 A dash mounted switch to activate a 500 A to 1000 A motor.
System Voltage
The temperature of the batteries will affect the power that is available to crank the engine. The following table is a guide to typical system voltages from a normal system.
| TYPICAL SYSTEM VOLTAGE DURING CRANKING AT VARIOUS AMBIENT TEMPERATURES | ||
| Temperature | 24V System | 12V System |
| -23 °C (-9.4 °F) to -7 °C (19.4 °F) |
12 to 16 V | 6 to 8 V |
-23 °C (-9.4 °F) to -7 °C (19.4 °F) |
14 to 18 V | 7 to 9 V |
-23 °C (-9.4 °F) to -7 °C (19.4 °F) |
16 to 20 V | 8 to 10 V |
The maximum amount of voltage that is lost for a starting motor is shown in table 2. The values that are listed are the maximum for machines of approximately 2000+ service meter hours. The voltage loss for newer machines is less than the voltage loss shown in the table.
| MAXIMUM ACCEPTABLE SYSTEM VOLTAGE DROPS DURING CRANKING | ||
| Circuit | 24V System | 12V System |
| Battery negative post to starting motor negative terminal | 1.4 V | 0.7 V |
| Drop across disconnect switch | 1.0 V | 0.5 V |
| Battery positive post to solenoid positive terminal | 16 to 20 V | 8 to 10 V |
| Solenoid battery terminal to solenoid motor terminal | 0.8 V | 0.4 V |
System voltages that are greater are often caused by loose connections, corroded connections, or switch contacts that are faulty.
Procedure
| Tools Needed |
| 6v-7070 Digital Multimeter |
| 225-8266 Ammeter Tool Gp (AC/DC Clamp-On Ammeter) or equivalent |
| NOTICE |
|---|
Do not crank the engine continuously for more than 30 seconds. Allow the starting motor to cool for two minutes before cranking the engine again. |
If the starting motor cranks slowly or the starting motor does not crank at all, do the following procedure:
- Use a multimeter in order to measure the battery voltage at the battery posts. While the engine is cranking or attempting to crank the engine, battery voltage measurements should be taken.
Expected Result: The battery voltage is equal, or the battery voltage is greater than the voltage that is shown in Table 1.
Results:
- YES - The battery voltage is OK, proceed to 2.
- NO - The battery voltage is too low, test the batteries as shown in Special Instruction, SEHS7633.
- YES - The battery voltage is OK, proceed to 2.
- Measure the current draw on the positive battery cable between the battery and the starting motor solenoid with the clamp-on ammeter. The maximum amount of a current draw and the minimum amount of a voltage that is allowed is provided in the Specifications section.
The voltages and currents that is provided in the Specifications section are taken at a temperature of 27 °C (80.60 °F). Temperatures that are below 27 °C (80.60 °F) will require less voltage and the current draw will be higher. If the current draw is too large, the starting motor has a problem. The starting motor must be removed for repair or replacement.
Note: Low batteries can be caused by battery condition or a shorted starter.
- Measure the starting motor voltage from test point (4) to test point (5) with a multimeter. While the engine is cranking or attempting to crank the engine, measurements should be taken.
Expected Result: The battery voltage is equal, or the battery voltage is greater than the voltage that is shown in Table 1.
Results:
- Yes - The starting motor voltage is OK, the batteries and the starting motor cables down to the motor are within specifications, Proceed to 5.
- No - The starting motor voltage is low, the voltage drop between the batteries and the starting motor is too great. Proceed to 4.
- Yes - The starting motor voltage is OK, the batteries and the starting motor cables down to the motor are within specifications, Proceed to 5.
Note: If voltage at the battery post is within approximately 2 V of the lowest value in the applicable temperature range of Table 1 and if the large starting motor cables get hot, then the starting motor has a problem. The 225-8266 Ammeter Tool Gp is not needed.
- Measure the voltage drops in the cranking circuits with the multimeter. Compare the measured results with the maximum voltage drops that are allowed in Table 2.
Expected Result: The battery voltages are within specifications.
Results:
- Yes - The voltage drops are OK, proceed to 5 in order to check the engine.
- No - The voltage drops are too high, repair and/or replace the faulty electrical component.
- Yes - The voltage drops are OK, proceed to 5 in order to check the engine.
- Manually turn the engine in order to ensure that the engine has not locked up. Check oil viscosity and any external loads that would affect engine rotation.
Expected Result: The engine is locked up and/or the engine is hard to turn.
Results:
- Yes - The engine is locked up and/or hard to turn. The engine needs repaired.
- No - The engine is not hard to turn over. Proceed to 6.
- Yes - The engine is locked up and/or hard to turn. The engine needs repaired.
- Attempt to crank the starting motor.
Expected Result: The starting motor cranks.
Results:
- Yes - If the engine does crank, remove the starting motor for repair and/or replacement.
- No - If the engine does not crank, check for blocked engagement of the pinion and flywheel ring gear.
Note: Blocked engagement and open solenoid contacts will give the same electrical symptoms.
- Yes - If the engine does crank, remove the starting motor for repair and/or replacement.