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Diagnose Onan Generator Problems

Onan Manuals (.pdf files)

• Free Onan Troubleshooting Manual
• Onan Owners Handbook
• Onan BGM, NHM Series Owner's Manual

Onan Generator Troubleshooting

Contributed by: Mickey Mathias (with technical details provided by Alan Robinson, Certified Onan Tech)

(Note: The following is a compilation of technical correspondence regarding a specific generator problem. This information is intended for general reference. Verify your specific model number and specifications before performing repairs.)

Problem: Engine starts, but shuts off as soon as the start button is released (Onan Emerald 4kW).

Initial Checks: Oil Pressure Sensor & Field Flash

Check the oil level first. An oil pressure sensor automatically shuts down the generator if oil pressure drops. Pressing the start button manually bypasses this circuit, allowing the engine to run only while held down. Inspect the wire connected to the oil pressure sensor near the oil filter to ensure it hasn't disconnected.

If the oil system is fine, the issue is typically either:

1. The control board is not providing field flash voltage (initial startup power) to the voltage regulator while cranking.
2. The control board is supplying voltage, but an open circuit in the voltage regulator prevents power from reaching the brushes.

Testing the Control Board & Voltage Regulator

Locate the voltage regulator module inside the control box (bolted down at the bottom left, featuring a capacitor molded into the top and a harness connected to a 12-conductor socket J4/P4).

• Check Pin 7: Set your voltmeter to DC volts, ground the negative lead, and back-probe Pin 7 on connector J4/P4. You should see approximately 12V DC while cranking.
• If 12V is present at Pin 7: If you see 12V here, but do not read 10–11V at the brushes, the control board is functioning, but the voltage regulator is faulty.
• If 12V is NOT present at Pin 7: Check Pin 5 on connector P1 (the control board connector). If 12V is missing here as well, the control board is faulty. If 12V is present at P1 Pin 5 but missing at P4 Pin 7, inspect the wiring and connector contacts between them.
• Brush Continuity: Check continuity from P4 Pin 9 to one brush, and from P4 Pin 10 to the second brush. Resistance should be less than 1 ohm, with no continuity to ground on either line.

If the control board appears faulty, you can test the regulator and generator assembly by using a jumper lead to briefly apply +12V DC to P4 Pin 7 while cranking. If the regulator, brushes, and armature are functional, the engine should run and maintain power output independently.

Diode Static Test on Voltage Regulator

Unplug the 12-pin regulator connector. Set your multimeter to "diode check" and test between the following terminal pairs (positive lead on the first pin, negative on the second):

5-9, 7-9, 10-9, 11-9, 12-9, 5-10, 5-11, 5-12, 5-3

The voltage regulator is defective if any pair shows an "open" or "short," except for pair 5-10, which should indicate "open."

18V Manual Field Test

With the regulator disconnected, jump an external 18V DC power source (such as two 9V transistor batteries connected in series with a 5A inline fuse) to pins 9 and 10 in the harness plug leading to the brushes while starting the engine:

• This should generate roughly 120V AC output, allowing the control board to enter run mode.
• Measure AC voltage across pins 2 and 3 (should read ~120V AC).
• Measure exciter winding output across pins 11 and 12 (should read ~85% of main output voltage).

If the system enters run mode and output voltages match these readings, the generator windings and control board are intact, confirming a failed voltage regulator.

Governor & Mixture Adjustment Procedure

1. No-Load Idle Check: With the engine running under no load, verify that the throttle lever does not rest firmly against the idle stop screw. Manually pulling the lever back to the stop should drop engine frequency to 55 Hz.
2. Governor Linkage: With the engine stopped, unclip the linkage rod from the throttle lever. The rod end should align naturally with the mounting hole when the throttle is just short of wide-open. Adjust rod length as needed by loosening the locknut on the ball-and-socket joint.
3. Speed & Sensitivity Settings:
   • Adjust the speed nut on the governor spring bracket to set no-load frequency to 62–63 Hz.
   • Apply a full load; frequency should remain at or above 58 Hz (allowing a 2–4 Hz drop from no load to full load).
   • If frequency drops more than 4 Hz under load, turn the sensitivity screw counterclockwise 1 to 2 turns, reset the speed nut, and retest.
4. Carburetor Mixture Adjustment:
   • With the generator fully warmed up under full load, turn the main fuel adjustment screw inward until speed drops slightly, then outward until speed drops slightly. Set the screw to the midpoint where frequency peaks, then turn it out an additional 1/4 turn.
   • Remove the load and allow the engine to stabilize for 1 minute. Slowly turn the idle adjustment screw inward until speed drops or hunting begins, then back it out 1/8 to 1/4 turn until the engine runs smoothly without surging.