Cummins/Onan LRG-4251 Engine

Tools: Tachometer or frequency meter

1. Check the dip switch (SW1-SW4) settings for the application frequency and spec.
2. Start the set, warm up under a partial load (at least 1/4 rated load), then disconnect all loads.
3. If the governor was replaced, adjust the Gain to approximately 5 and Stability to 3.5.
4. Adjust the Gain pot until the engine is stable and responsive; turn counterclockwise to eliminate hunting.
5. Bump the throttle lever a couple of times to check for hunting.
6. Apply full load and adjust the Stability pot to minimize overshoot.
7. Attach a tachometer or frequency meter and adjust the Speed Trim pot to the desired speed.
8. Shut down and restart the genset to check for overspeed shutdown on startup.

⚠ Gaseous fuels are flammable and explosive and can cause severe personal injury or death. Keep a type ABC fire extinguisher handy.

Tools: Tachometer

1. Start the engine and let the set warm up under a partial load (at least 1/4 rated load).
2. Disconnect all loads, shut down, connect a tachometer and disconnect the governor linkage at the carburetor.
3. Start the engine and hold the throttle closed by hand so the engine does not overspeed.
4. Adjust the throttle idle position screw to obtain 800 rpm; reconnect the governor linkage.
5. Connect full rated load and turn the main fuel valve to full-rich.
6. Slowly turn the main fuel valve toward lean until the engine begins to lose speed.
7. Slowly back toward rich until the engine carries the full load smoothly.
8. For dual fuel, switch to LPG by closing the manual shutoff valve in the natural gas supply line.
9. Turn the LPG flow adjustment valve clockwise until the engine begins to lose speed.
10. Slowly turn it back counterclockwise until the engine carries full load smoothly.

1. Modification of connections is required only when converting the generator set fuel system.
2. Natural gas only (Spec A & B): connect EH lead P10 to IM pin #6.
3. Natural gas only (Spec A & B): connect EH lead P11 to IM pin #7.
4. LPG only (Spec A & B): connect EH lead P10 to IM pin #6; leave EH lead P11 and IM pin #7 disconnected.
5. Dual fuel (Spec A & B): connect EH lead P10 to IM pin #6; connect EH lead P11 to DFH lead P11; leave DFH lead P7 and IM pin #7 disconnected.
6. Begin Spec C: refer to the DC Wiring Diagram in Section 9.

1. With the genset stopped, remove the governor rod and spring from the throttle lever.
2. With the actuator shaft full counterclockwise, check the governor arm angle is 64° ±2°.
3. Loosen the governor arm screw to adjust the governor arm.
4. Verify the ball joint screw is in the third hole from the outside end of the governor arm.
5. Attach the governor rod and spring to the throttle lever.
6. With the throttle lever closed, adjust the governor rod so the arm angle is 64° ±2°.

Tools: Ohmmeter

1. Measure MPU resistance; replace if not between 1,000 and 1,050 ohms.
2. With the MPU removed, rotate the ring gear until a tooth lines up in the center of the mounting hole.
3. Thread the pickup in gently by hand until it just touches the ring gear tooth.
4. Back it out 1/4 turn and set the locknut.
5. Check output voltage; replace the MPU if output voltage at cranking speed is less than 1.5 VAC.

1. Run the generator set at rated frequency and no load.
2. Slowly turn the STABILITY pot clockwise until voltage becomes stable.
3. Turn counterclockwise until voltage again becomes unstable.
4. Turn the pot slightly clockwise from this position for maximum stability.
5. Readjust voltage after a stability adjustment.

Tools: Frequency meter

1. Check that the frequency selection jumper is connected for the application (50 Hz or 60 Hz).
2. Adjust the UFRO pot so the LED lights as frequency drops to 47 Hz (50 Hz) or 57 Hz (60 Hz).
3. Turn the pot clockwise to reduce the knee point frequency.

⚠ HAZARDOUS VOLTAGE! Touching uninsulated parts inside the control and power output boxes can result in severe personal injury or death.

Tools: AC-DC multimeter

1. Turn the voltage trimmer (Detector control) to its mid position.
2. Turn the VOLTS pot on the voltage regulator fully counterclockwise.
3. Turn the STABILITY pot on the voltage regulator to its mid position.
4. Start and operate the generator set at rated frequency and no load.
5. If the LED lights, see UFRO Adjustment.
6. Slowly turn the VOLTS pot clockwise until rated voltage is obtained.
7. See Stability Adjustment if voltage is unstable.

⚠ Disconnect starting battery cables (negative first) before performing tests.

Tools: 500 VAC megger

1. Apply the test potential between the winding and ground (winding laminations) for 10 minutes.
2. Record resistance at 1 minute and again at 10 minutes.
3. For a new generator with dry windings, obtain at least 5 megohms.
4. Polarization index should be at least 2.
5. For a set in service, resistance should not be less than 1 megohm.

⚠ The voltage regulator could be damaged if the flashing circuit is connected for more than 5 seconds.

⚠ HAZARDOUS VOLTAGE! Stand on a dry wooden platform or rubber insulating mat, make sure clothing and shoes are dry, remove jewelry and use tools with insulated handles.

Tools: 12 volt battery, 10 ohm resistor, 18 volt voltage suppressor, 12 A, 300 V diode

1. Assemble a 12 volt battery, 10 ohm resistor, 18 volt voltage suppressor and 12 amp 300 volt diode.
2. While the set runs at nominal frequency, momentarily connect the positive (+) side to voltage regulator terminal VR21-F1 (X).
3. Connect the negative (-) side to voltage regulator terminal VR21-F2 (XX).
4. Check output voltage, shut down the set and restart it.

⚠ The generator is heavy; you will need an assistant and hoist of sufficient capacity.

⚠ Disconnect battery charger from AC source before disconnecting battery cables.

⚠ Disconnect the negative (-) cable first and reconnect last.

Tools: Hoist and straps, Gear puller

1. Disconnect all power output and remote control connections and conduit; mark each lead.
2. Remove the end bell cover; disconnect the overspeed/start disconnect module plug and F1/F2 wires.
3. Remove the overspeed/start disconnect rotor element or overspeed switch assembly from the shaft end.
4. Disconnect generator leads and engine wiring harness from all terminals inside the control box.
5. Remove the four bolts securing the two saddle brackets of the control box to the generator.
6. Remove the two nuts and washers of the top generator through-studs.
7. Slide the control housing assembly off of the through-studs.
8. Cinch a hoisting strap around the middle of the generator stator and take up slack.
9. Scribe lines to register the parts before separating stator, flywheel housing and end bell.
10. Remove the remaining two through-stud nuts and tap the end bell free of the stator assembly.
11. Tap the generator stator free of the adaptor housing and draw it straight back.
12. Remove the generator through-studs.
13. Remove the air baffle, then the drive disc mounting bolts if rotor removal is needed.
14. Remove the two mounting screws if the exciter stator must be removed from the end bell.
15. Use a gear puller if the rotor bearing must be removed.

⚠ Do not start the generator set until the DC start disconnect module has been remounted and reconnected.

Tools: Torque wrench

1. Press a new rotor bearing up to the shoulder of the shaft (do not lubricate); apply force to the inner race.
2. Torque the eight drive disc bolts to 37-42 ft-lb (50-57 N-m).
3. Torque the six drive disc-to-flywheel bolts to 27-29 ft-lb (37-39 N-m).
4. Install the air baffle using four bolts.
5. Torque the flywheel housing bolts to 60 ft-lb (81 N-m).
6. Thread generator through-studs into the flywheel housing with the short-thread ends in the housing.
7. Mate the generator stator and flywheel housing making the scribed index lines register.
8. Torque the exciter stator two screws to 8 ft-lb (11 N-m); connect F1 and F2 leads.
9. Wipe the bearing bore in the end bell with molybdenum disulfide grease; make sure the O-ring is in place.
10. Mount the end bell; torque the through-stud nuts to 19-21 ft-lb (26-28 N-m).
11. Mount the overspeed switch assembly on the rotor and torque the bolt to 18 ft-lb (25 N-m).
12. Remount and reconnect all other components disconnected or removed under Disassembly.
13. Route and reconnect the control box wires and F1/F2 leads to the stator connection board.
14. Secure the end bell cover plate and torque the four screws to 8 in-lb (3.8 N-m).