DISC-FRICTION 3714319 - Caterpillar

General Information
All electrical starting systems have four elements:
Ignition switch
Start relay
Starting motor solenoid
Starting motorStart switches have a capacity of 5 to 20 A. The coil of a start relay draws about 1 A between test points. The switch contacts of the start relay for the starting motor are rated between 100 and 300 A. The start relay can easily switch the load of 5 to 50 A for the starting motor solenoid.The starting motor solenoid is a switch with a capacity of about 1000 A. 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 A. The hold-in coil requires about 5 A.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 negative 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.
Table 1
Typical Voltage Of Electrical System During Cranking At Various Ambient Temperatures    
Temperature     12 V System     24 V System    
−23 to −7°C (−10 to 20°F)     6 to 8 V     12 to 16 V    
−7 to 10°C (20 to 50°F)     7 to 9 V     14 to 18 V    
10 to 27°C (50 to 80°F)     8 to 10 V     16 to 24 V    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.
Table 2
Maximum Acceptable Voltage Drop In The Starting Motor Circuit During Cranking    
Circuit     12 V System     24 V System    
Battery post "-" to the starting motor terminal "-"     0.7 V     1.4 V    
Drop across the disconnect switch     0.5 V     1.0 V    
Battery post "+" to the terminal of the starting motor solenoid "+"     0.5 V     1.0 V    
Solenoid terminal "Bat" to the solenoid terminal "Mtr"     0.4 V     0.8 V     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 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