C3.3 Industrial Engine Caterpillar

General Information

All electrical starting systems have four elements:

• Ignition switch

• Start relay

• Starting motor solenoid

• Starting motor

Start switches have a capacity of 5 to 20 amperes. The coil of a start relay draws about 1 ampere between test points. The switch contacts of the start relay for the starting motor are rated between 100 and 300 amperes. The start relay can easily switch the load of 5 to 50 amperes for the starting motor solenoid.

The starting motor solenoid is a switch with a capacity of about 1000 amperes. The starting motor solenoid supplies power to the starter drive. The starting motor solenoid also engages the pinion to the flywheel.

The starting motor solenoid has two coils. The pull-in coil draws about 40 amperes. The hold-in coil requires about 5 amperes.

When the magnetic force increases in both coils, the pinion gear moves toward the ring gear of the flywheel. Then, the solenoid contacts close in order to provide power to the starting motor. When the solenoid contacts close, the ground is temporarily removed from the pull-in coil. Battery voltage is supplied on both ends of the pull-in coil while the starting motor cranks. During this period, the pull-in coil is out of the circuit.

Cranking of the engine continues until current to the solenoid is stopped by releasing the ignition switch.

Power which is available during cranking varies according to the temperature and condition of the batteries. The following chart shows the voltages which are expected from a battery at the various temperature ranges.

The following table shows the maximum acceptable loss of voltage in the battery circuit. The battery circuit supplies high current to the starting motor. The values in the table are for engines which have service of 2000 hours or more.

Voltage drops that are greater than the amounts in Table 2 are caused most often by the following conditions:

• Loose connections

• Corroded connections

• Faulty switch contacts

Diagnosis Procedure

The procedures for diagnosing the starting motor are intended to help the technician determine if a starting motor needs to be replaced or repaired. The procedures are not intended to cover all possible faults and conditions. The procedures serve only as a guide.

If the starting motor does not crank or cranks slow, perform the following procedure:

1. Use Tooling (A) in order to measure the voltage of the battery.

   Measure the voltage across the battery posts with Tooling (A) when you are cranking the engine or attempting to crank the engine. Do not measure the voltage across the cable post clamps.

   1. If the voltage is equal or greater than the voltage in Table 1, then go to Step 2.

   2. The battery voltage is less than the voltage in Table 1.

      A low charge in a battery can be caused by several conditions.

      • Deterioration of the battery

      • A shorted starting motor

      • A faulty alternator

      • Loose drive belts

      • Current leakage in another part of the electrical system

      Refer to Special Instruction, SEHS7633 in order to test the battery.

2. Use Tooling (B) in order to measure the current that is sent to the starting motor solenoid from the positive post of the battery.

   Note: If the following conditions exist, do not perform the test in Step 2 because the starting motor has a fault.

   • The voltage at the battery post is within 2 volts of the lowest value in the applicable temperature range of Table 1.

   • The large starting motor cables get hot.

   Use the Tooling (B) in order to measure the current. Place the jaws of the ammeter around the cable that is connected to the "bat" terminal. Refer to Specifications, "Starting Motor" for the maximum current that is allowed for no load conditions.

   The current and the voltages that are specified in Specifications are measured at a temperature of 27°C (80°F). When the temperature is below 27°C (80°F), the voltage will be lower through the starting motor. When the temperature is below 27°C (80°F), the current through the starting motor will be higher. If the current is too great, a fault exists in the starting motor. Repair the fault or replace the starting motor.

   If the current is within the specification, proceed to Step 3.

3. Use the Tooling (A) in order to measure the voltage of the starting motor, when you are cranking or attempting to start the engine.

   1. If the voltage is equal or greater than the voltage that is given in Table 1, then the battery and the starting motor cable that goes to the starting motor are within specifications. Go to Step 5.

   2. The starting motor voltage is less than the voltage specified in Table 1. The voltage drop between the battery and the starting motor is too great. Go to Step 4.

4. Use Tooling (A) in order to measure the voltage.

   1. Measure the voltage drops in the cranking circuits with the multimeter. Compare the results with the voltage drops which are allowed in Table 2.

   2. Voltage drops are equal to the voltage drops that are given in Table 2 or the voltage drops are less than the voltage drops that are given in Table 2. Go to Step 5 in order to check the engine.

   3. The voltage drops are greater than the voltage drops that are given in Table 2. The faulty component should be repaired or replaced.

5. Rotate the crankshaft by hand in order to ensure that the crankshaft is not stuck. Check the oil viscosity and any external loads that could affect the engine rotation.

   1. If the crankshaft is stuck or difficult to turn, repair the engine.

   2. If the engine is not difficult to turn, go to Step 6.

6. Attempt to crank the starting motor.

   1. The starting motor cranks slowly.

      Remove the starting motor for repair or replacement.

   2. The starting motor does not crank.

      Check for the blocked engagement of the pinion gear and flywheel ring gear.

      Note: Blocked engagement and open solenoid contacts will give the same electrical symptoms.