C7.1 Engines for Caterpillar Built Machines Electric Starting System - Test Caterpillar

Electric Starting System - Test
C7.1 Engines for Caterpillar Built Machines [UENR3387]
C7.1 Engines for Caterpillar Built Machines Electric Starting System - Test
1.1. General Information
2.1. Diagnosis Procedure

General Information

All electrical starting systems have four elements:

  • Keyswitch

  • Start relay

  • Starting motor solenoid

  • Starting motor

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 keyswitch.

Power which is available during cranking varies according to the temperature and condition of the batteries. Table 1 shows the voltages which are expected from a battery at the various temperature ranges.

Table 1
Typical Voltage Of Electrical System During Cranking At Various Ambient Temperatures 
Temperature  12 Volt System  24 Volt System 
−23 to −7°C (−10 to 20°F)  6 to 8 volts  12 to 16 volts 
−7 to 10°C (20 to 50°F)  7 to 9 volts  14 to 18 volts 
10 to 27°C (50 to 80°F)  8 to 10 volts  16 to 24 volts 

Table 2 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.

Table 2
Maximum Acceptable Voltage Drop In The Starting Motor Circuit During Cranking 
Circuit  12 Volt System  24 Volt System 
Battery post "-" to the starting motor terminal "-"  0.7 volts  1.4 volts 
Drop across the disconnect switch  0.5 volts  1.0 volts 
Battery post "+" to the terminal of the starting motor solenoid "+"  0.5 volts  1.0 volts 
Solenoid terminal "Bat" to the solenoid terminal "Mtr"   0.4 volts  0.8 volts 

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 problems and conditions. The procedures serve only as a guide.


If equipped with electric start, do not crank the engine for more than 30 seconds. Allow the starter to cool for two minutes before cranking again.

Never turn the disconnect switch off while the engine is running. Serious damage to the electrical system can result.

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

  1. Measure the voltage of the battery.

    Measure the voltage across the battery posts with the multimeter 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 to 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

  2. 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 problem.

    • 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 a suitable ammeter 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, "Starter 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 problem exists in the starting motor. Repair the problem or replace the starting motor.

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

  3. Measure the voltage of the starting motor.

    1. Use the multimeter in order to measure the voltage of the starting motor, when you are cranking or attempting to crank the engine.

    2. If the voltage is equal to 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.

    3. 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. 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.