AXLE AR-NON-ENGINE END 8V2822 - Caterpillar

Illustration 1. Location of external electrical terminals.This alternator is not self-energizing as is the more familiar brushless alternator (the 5S9088 Alternator is an example of a brushless alternator). The 7T2876 Alternator requires battery voltage in order to start up. Illustration 2 shows the connections to a 7T2876 Alternator in a typical charging circuit.
Illustration 2. Typical charging circuit with 7T2876 Alternator.When the key start switch is in either the ON or Start position, current is sent through the 7T1183 Limiting Resistor and the 9R895 Diode Wire Assembly, to the D+ terminal. From the D+ terminal, the current flows through the field coil of the alternator. This provides the necessary field current for the alternator to start up. After the alternator has started up, it begins to supply charging current to the battery and the rest of the vehicle's electrical circuits. The limiting resistor protects the field coil in the alternator from excessive current. In addition, the limiting resistor and diode allow the fuel shutoff solenoid to de-energize (normally energized to run), and the main power relay to shut off when the key switch is put into the OFF position. Without the diode, the alternator would keep those components powered up, and the engine would not shut down.
Troubleshooting alternator problems and locating the source of the failure can be tricky. For example, consider the case when the engine will not crank because the battery is low. And after jump starting, the alternator does not have any output voltage. Do not jump to the conclusion that the alternator has failed. Remember that this is not a self-energizing alternator. If the battery is too low, it will not be able to provide enough current to start the alternator charging sequence. So when troubleshooting a charging system problem, first check and charge the battery as required. Then check the alternator output. Also, always check the circuit from the key start switch (R terminal) to the alternator D+ terminal for an open circuit. An open circuit here will falsely indicate an alternator problem.