ALTERNATOR 3218902 - Caterpillar

Reference: Truck Engine News, May 1987, Page 3, "Alternator/Generator Output Test On The Engine. "Truck Engine News; December 17, 1984; Page 3; "Starters That Remain Engaged After Engine Is Running. "
Truck Engine News, April 1989, "Page 6, "Causes And Effects Of Low Battery Voltage On Electric Starters. "
Reference: Special Instruction, SEHS7768, "Use of 6V2150 Starting/Charging Analyzer Group. "This article replaces the article with the same title on Page 4 of the May 1987 Truck Engine News.
This is a general procedure to help aid the serviceman determine if a starter needs replacement. It is NOT intended to cover all possible problems and conditions, but to serve only as a guide. The most common 24V circuit is discussed and 12V circuits are indicated as applicable. On engines with a diagnostic connector, the 6V2150 Starting/Charging Analyzer Group helps you to do the following procedure in seconds.
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