SFWR GP-A4032 5094233 - Caterpillar

Electric Starting System - Test
General Information
All electrical starting systems have four elements:
Keyswitch
Start relay
Starting motor solenoid
Starting motorKeyswitches 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 keyswitch.Power which is available during cranking varies according to the temperature and condition of the batteries. Table 2 shows the voltages which are expected from a battery at the various temperature ranges.
Table 2
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 3 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 3
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 3 are caused most often by the following conditions:
Loose connections
Corroded connections
Faulty switch contactsDiagnosis 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