TL;DR

Low power from a generator is either a governor fault limiting engine speed below 60 Hz or an AVR fault reducing field excitation and voltage. Check output frequency first: if it is below 59.5 Hz, the governor needs adjustment. If frequency is correct but voltage is low, check the AVR.

What you might see

output voltage below nameplate (less than 460V on a 480V unit)
output frequency below 59.5 Hz
generator trips on low voltage under load
EMCP shows governor or AVR fault

Likely causes

Tap to mark as ruled out

Governor speed setpoint drifted, allowing frequency to drop below 60 Hz under load
AVR (automatic voltage regulator) fault reducing field current and output voltage
Restricted fuel supply reducing engine torque capacity
Mechanical governor linkage worn or binding, limiting fuel rack travel

Required tools

True-RMS multimeter (voltage) and frequency meter
Insulated screwdrivers for AVR and governor adjustment
CAT EMCP programming tool (if electronic governor)
Arc-flash PPE for 480V output measurements

Safety first

Generator output terminals carry 480V or higher and are lethal. All measurements at the output terminals require arc-flash PPE and category III or IV rated meter probes.
Do not adjust the AVR or governor while the generator is supplying a critical load without a trained operator monitoring the output and a backup source available.

Procedure

6 steps · ~45 min

1
Connect a true-RMS multimeter and a frequency meter to the generator output terminals. Measure voltage and frequency under no-load and then at rated load.
Warning: Generator output terminals carry 480V or higher. Use category III rated equipment and appropriate PPE.
2
If frequency is below 59.5 Hz under load, the governor speed setpoint needs adjustment. On a mechanical governor, turn the speed adjusting screw clockwise in small increments. On an electronic governor, access the speed trim parameter on the EMCP.[1]
3
If frequency is correct at 60 Hz but voltage is below 460V (for a 480V unit), the AVR output is insufficient. Most AVRs have a voltage trim potentiometer: adjust it to bring voltage into spec.[1]
4
If frequency drops sharply under rated load (droop), inspect the fuel filters and fuel delivery pressure at the injection pump.
5
Check the air filter restriction indicator. A restricted air filter reduces combustion efficiency and limits power output.
6
If frequency and voltage are both normal but load capacity is low, consult a CAT service technician for injection timing and compression testing.

Sources

Caterpillar C-Series Generator Set Operation and Maintenance Manual, Caterpillar Inc.
Caterpillar C-Series generator set operation and maintenance manual, governor and AVR adjustment procedures (general)
View source

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