Cummins/Onan LRG-4251 Engine

How to read your Cummins/Onan engine serial & model number →

• The Time Delay Start/Stop Module (A15) is malfunctioning.
  Fix: Check for constant B+ at A15 terminal TB1-4. Check for run signal at ECM TB1-6. Voltage at A15 TB1-6 should be at B+ at the end of the start delay period. Check wiring and connections from A15 TB1-6 to ECM TB1-6.
• The starter motor or solenoid is malfunctioning.
  Fix: Push the Run-Stop-Remote switch to Run and check for battery voltage (B+) at starter solenoid terminal SW. Replace the starter motor if there is voltage but the motor does not function.
• The Run-Stop-Remote switch (S12) or wiring is faulty.
  Fix: A. Disconnect pin connector J4 from the ECM and check for electrical continuity (zero ohms) between switch terminals 2 and 3 when the switch is in the Run position and between terminals 1 and 2 when it is in the Remote position. Replace the switch if either set of contacts is faulty. b. If the switch works, check for electrical continuity (zero ohms) between J4-6 and J4-7 on the wire harness when the switch is in the Run position and between J4-5 and J4-7 when the switch is in the Remote position. Repair the wire harness if there is no electrical continuity in either position of the switch.
• The OVERSPEED lamp comes on and the engine shuts down. 1
  Fix: A. Reset the ECM by pushing the Run-Stop-Remote switch to Stop and the Reset switch to Reset and restart the set, monitoring engine speed. Spec A & B: Readjust the cutout speed if it is lower than specified. Replace the overspeed switch if it cannot be readjusted (see Section 3). Begin Spec C: If engine speed is correct and the set still shuts down, replace the ignition control module. b. Adjust the governor (see Section 8). c. If the governor cannot be adjusted to prevent shutdown due to overspeed, check for binding in the linkage. Repair and adjust the linkage as necessary. d. If the set still shuts down due to overspeed, re-install the magnetic speed pick-up unit to make sure the clearance with the flywheel gear teeth is correct. Replace the speed-pickup unit if output voltage at cranking speed is less than 1.5 VAC. (Intermittent open on magnetic speed pick-up lead can also cause an overspeed condition.) e. Replace the governor controller if the set still shuts down due to overspeed.
• The grounding strap between the control box and the battery negative (-) terminal or the ground wire between the control and engine ground is loose, damaged or missing.
  Fix: Check for continuity (zero ohms) between the grounding stud on the bottom of the control box and the battery negative (-) terminal. If there is no continuity or the grounding strap is loose or damaged, repair as necessary.
• The engine shuts down due to over-speed.
  Cause: The Fault Shutdown is being indicated by the fault circuit breaker reset button (extended out).
  Fix: 5. Indicates engine has exceeded normal operating speed. Contact an authorized service center for service.
• The engine shuts down due to low coolant level.
  Cause: The Fault Shutdown is being indicated by the fault circuit breaker reset button (extended out).
  Fix: 6. Indicates low coolant level. Replenish coolant and look for possible coolant leakage points and repair as required.
• The engine shuts down due to high engine temperature or low coolant level.
  Cause: The Fault Shutdown is being indicated by the fault circuit breaker reset button (extended out).
  Fix: 3. Indicates engine has overheated (engine temperature has risen above 230° F/110° C). Allow engine to cool down completely before proceeding with the following checks: a. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset the tripped common fault circuit breaker. Contact an authorized service center for service if none of the above.
• The engine does not crank in run mode
  Cause: The Emergency Stop switch has been used. (The switch button is lit.)
  Fix: Pull the Emergency Switch button. To reset the engine control, push the Run-Stop-Remote switch to Stop and the Reset switch to Reset. Then push the Run-Stop-Remote switch to Run to start the genset.
• The engine does not crank in Remote mode.
  Cause: The Run-Stop-Remote switch is at Stop. (The Switch-Off light will be flashing.); The Emergency Stop switch has been used. (The switch button is lit.); A Fault Shutdown is being indicated by one of the red lights on the control panel.; There is no remote circuit signal (12 VDC at auxiliary relay board A28-TB1-6) because fuse F3 on the ECM has blown.; There is no remote circuit signal (12 VDC at auxiliary relay board A28-TB1-6) because the remote circuit is not functioning properly.; Auxiliary relay board A28 is not functioning properly.
  Fix: Push the Run-Stop-Remote switch to Remote.; Pull the Emergency Switch button. To reset the engine control, push the Run-Stop-Remote switch to Stop and the Reset switch to Reset. Then push the Run-Stop-Remote switch to Remote.; Service the set as necessary. To reset the engine control, push the Run-Stop-Remote switch to Stop and the Reset switch to Reset. Then push the Run-Stop-Remote switch to Remote.; a. Replace the fuse with one of the same type and amp rating (15 A). b. If fuse F3 blows again, find and repair the fault in the remote control circuit, such as a loose wire that may be shorting to ground or a shorted relay coil or other component. See Section 9 for remote connections.; Apply 12 VDC to A28-TB1-6. If the engine cranks, find and repair the fault in the remote control circuit. See Section 9 for remote connections.; Check for misconnections (see Section 9) or loose connections and replace auxiliary relay board A28 if there is 12 VDC at terminal A28-TB1-6 but not at A28-J2-6.
• The engine cranks but does not start.
  Cause: The engine is not getting fuel.; The air cleaner is blocked.; Fuse F2 on the ECM has blown.; Fuel solenoid K1 does not energize.
  Fix: A. Open any closed fuel shutoff valve. b. Fill the propane supply tank. c. For natural gas fueled sets, check with the gas utility. d. Check fuel pressure at regulator (refer to Fuel Pressure in Section 8.) (High fuel pressure will prevent solenoid from opening.) e. Check fuel solenoid (VDC present at K1+ lead during cranking). Duel Fuel Sets Only: f. Check relay K99. Refer to wiring diagrams in Section 9. g. Check pressure switch wiring. Propane - normally closed connected to common. Natural gas - normally open connected to common. h. Check pressure switch function (@ 3.5 inch H2O or less, should switch from natural gas to LPG). i. Check S14 vacuum switch (optional) for proper operation.; Service as necessary.; Replace fuse with one of the same type and amp rating. If fuse F2 blows again, the wire between ECM TB1-10 and engine block terminal T26, or a wire between terminal T26 and an accessory may be loose or shorting to ground.; a. Fuel solenoid not energized by the ECM. Check for B+ at ECM TB1-10 when cranking. If no voltage present and fuse F2 is good replace ECM. b. Connect B+ to fuel solenoid (K1) terminal BAT. Replace the fuel solenoid if it does not “click” when energized. If fuel solenoid is working, check for blocked fuel line or fuel filter.
• The ECM is faulty.
  Fix: Push the Run-Stop-Remote switch to Run and check for battery voltage (12 VDC) at ECM TB1-8. Replace ECM if there is no voltage at ECM TB1-8 but 12 VDC at ECM TB1-9.
• Temperature is causing too low a cranking speed for starting.
  Cause: A. Increase room temperature. b. Plug in, repair or install engine coolant heater. c. Replace the engine oil if it is not of the recommended viscosity for the ambient temperature.
• PRE LO OIL PRES lamp lights. Engine continues to operate. (Engine can be programmed to shut down.)
  Fix: Indicates engine oil pressure has dropped to 20 psi (138 kPa). If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure. If engine can be stopped, follow procedures in step 5.
• PRE HI ENGINE TEMP lamp lights.
  Fix: Indicates engine has begun to overheat and engine temperature has risen to approximately 220°F (104° C) or coolant level is low on optionally wired sets. If generator is powering non-critical and critical loads and cannot be shut down, use the following: a. Reduce load if possible by turning off non-critical loads. b. Check air inlets and outlets and remove any obstructions to airflow. If engine can be stopped, follow procedure in step 3.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan LRG-4251.

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.