GROMMET 2725121 - Caterpillar

General Information
Starting systems usually have four major components: ...Start switch...Start relay...Starter solenoid...Starter motor
Illustration 1. Typical cranking circuit.The only exception to this typical cranking circuit is that some small engines do not require the start relay. In this case, the start switch is connected directly to the starter solenoid.
Start switches are relatively low current devices. They are rated to switch approximately 5 to 20 amps. Because the coil of a start relay between TP2 (Test Point 2) and TP1 in Illustration 1 draws about 1 amp, the start switch can easily turn on the start relay and have long life.
The switch contacts of a typical start relay are rated to switch between 100 and 300 amps. Because a starter solenoid requires only 5 to 50 amps, the start relay can easily switch this load.
The starter solenoid has two functions: it engages the pinion with the flywheel, and it is a high current switch rated about 1000 amps that actually turns on the starting motor.
The starter solenoid has two coils: the pull-in coil (W) draws about 40 amps and hold-in coil (X) requires about 5 amps. The instant the start relay closes, both coils (W and X) receive power. Battery voltage is applied to the high end of both coils, at Test Point (3) which is the "start" terminal (S). The low end of hold-in coil (X) is permanently connected to the ground post of the starter motor. Grounding for the low end, Test Point (4), of pull-in coil (W) is momentary, and takes place through the DC resistance of the starter motor. When magnetic force builds in both coils, the starter pinion is moved to engage the ring gear. Only then will the solenoid contacts close to power the starter motor. This temporarily removes the ground from pull-in coil (W), and puts battery voltage on both ends of it while the motor cranks. During this period, the pull-in coil is out of the circuit and draws no current. Cranking continues until power to the motor solenoid is turned off by the start switch.
The result of these switches and relays is to permit a 5 amp dash-mounted switch to turn on a 500 to 1000 amp motor for cranking an engine.
Battery voltage (power) available during cranking depends on the temperature of the batteries. See Chart A. This chart is only a GUIDE as to what to expect from a NORMAL system.
Chart B shows maximum allowable voltage drops in the high current battery circuit to the starter. These values are maximums for engines with more than about 2000 SMH. Newer engines have less voltage drops.
Voltage drops greater than those listed are usually caused by loose and/or corroded connections or bad switch contacts.
The entire diagnostic procedure can be reduced to: ...confirming that batteries are within specifications...switches and cables/wiring from the batteries to the starter are not causing too much voltage loss.
The flow chart in Illustration 2 shows the entire procedure.
If the engine is equipped with the diagnostic connector, the 6V2150