Generator Output Voltage Low or Absent

Common Symptoms

• Panel displays fault code 16 and the unit shuts down or refuses to energize lamps
• Metal halide or LED lamp heads do not strike or light after warm-up period
• AC voltmeter on the panel reads zero or significantly below rated voltage (120V or 240V)
• Unit cranks and runs normally but produces no usable light output
• Generator engine runs at correct RPM but load contactor does not close
• Fault LED flashes on the operator panel while engine continues to idle
• Repeated manual restarts result in same code 16 fault within seconds of lamp energization attempt

Probable Causes (Ranked by Likelihood)

• Failed or worn AVR (automatic voltage regulator) not supplying correct field excitation current Very Likely
• Brushes worn below minimum length or stuck in holder, interrupting field circuit to rotor slip rings Very Likely
• Slip rings on rotor shaft grooved, contaminated with carbon, or open-circuited Likely
• Open or shorted rotor field winding eliminating magnetic field needed for voltage buildup Likely
• Residual magnetism lost in generator head stator requiring flash excitation to restore Possible
• Stator winding failure (shorted or open winding) preventing voltage generation Possible
• Engine speed significantly below rated RPM causing under-frequency and low voltage output Less Likely

Step-by-Step Diagnostic Procedure

1. Check engine RPM first. Use the panel tachometer or a handheld tachometer. Kubota D902 and D1105 units should govern to 1800 RPM at 60 Hz. If RPM is low, diagnose the governor or throttle linkage before touching the generator head.

2. With the unit OFF and cool, open the generator end cover and visually inspect the brushes. Each brush should protrude at least 5mm from the holder. If either brush is worn flush, cracked, or stuck in the holder by carbon buildup, replace both brushes as a set. This is your most cost-effective first step.

3. Inspect the slip rings on the rotor shaft. They should be smooth, copper-colored, and free of heavy carbon tracking or pitting. Clean light contamination with electrical contact cleaner and a lint-free cloth. Deep grooves or burn marks mean the rotor needs professional attention.

4. Measure rotor field winding resistance. Disconnect the brush leads and measure resistance across the two slip rings with a multimeter set to Ohms. A healthy Kubota-paired generator rotor typically reads 15 to 30 ohms. A reading of OL (open) or under 5 ohms (shorted) indicates a failed rotor winding.

5. Check AVR output. With the unit running, carefully measure DC voltage across the AVR output terminals (brush holder supply terminals). You should see 10 to 30V DC depending on load. Zero volts DC from the AVR with correct AC sensing input voltage present means the AVR itself has failed. Replace the AVR module.

6. Flash the field if residual magnetism is suspected. With the engine running and generator producing low or no voltage, briefly apply 12V DC from a fully charged battery across the field brushes in the correct polarity (match the AVR wiring diagram label for F+ and F-). If voltage builds up immediately, the unit lost residual magnetism and a new AVR set to the correct voltage will fix it permanently.

7. Measure AC output at the generator output terminals with a multimeter on AC Volts. Compare to the nameplate rating. If you have RPM and field excitation confirmed correct but output is still low, the stator has likely failed. Stator testing and replacement requires specialty tools and a shop environment. Call a qualified generator technician at this point.

Frequently Asked Questions