Generator Operation and Troubleshooting

Over the past few model years, GM has improved generator reliability by installing larger generator bearings, higher temperature greases, and high temperature press-fit diodes across all generator lines. However, while these design changes have improved generator warranty over-all, the percentage of good generator replacements has increased. A recently completed analysis of generators that were replaced shows that roughly half were functioning properly. This raises concerns with proper on-vehicle generator diagnosis.
Generator Operation
The generator's purpose is to provide electrical power and maintain the battery voltage at the proper level. When electrical loads are switched on, the generator senses this change and increases its output current (amperage) to maintain the proper voltage, until maximum generator output is reached. Electrical loads in excess of generator capability will cause the system voltage to drop below normal. Example: a generator is heavily loaded (summer city traffic) and is attempting to recharge a discharged battery. Generator maximum output increases with engine speed, and typically at low idle speed the generator output capability can be exceeded.
Troubleshooting
When measuring generator output amperage, make sure you are measuring DC amps in the cable attached to the generator B+ stud and that the current probe is fully closed. Do not measure generator current at the battery. Also make sure your voltage measurement is correct and that the meter is operating properly.

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