Cummins/Onan QSB5-G3 NR3 Engine

Shared across brands: Cummins/Onan, Magnum, Terex

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

• Fluctuating generator output voltage
  Cause: An “ON/OFF” type load may be the cause
  Fix: Redistribute load if possible
• Engine turns over (cranks), but won’t run
  Cause: Unit out of fuel
  Fix: Check fuel level in tank, fill as needed
• Engine shuts down automatically and TROUBLE LIGHT on CONTROL PANEL is illuminated
  Cause: Oil Pressure Switch Not opening
• Low generator output voltage
  Cause: Voltage adjustment set too low
  Fix: Adjust voltage potentiometer
• Engine won’t crank
  Cause: Loose battery cable or discharged battery
  Fix: Check cables and battery electrolyte level. Recharge as necessary
• No generator output voltage
  Cause: Circuit breaker tripped
  Fix: Reset circuit breaker
• High generator output voltage
  Cause: Voltage adjustment potentiometer
  Fix: Adjust potentiometer
• Engine runs, but loses power under load
  Cause: Clogged fuel system air in fuel lines
  Fix: Check fuel system
• Low Speed
  Cause: Defective injectors on engine
  Fix: Have injectors checked by a qualified technician
• Engine runs, but loses speed
  Cause: Unit is overloaded
  Fix: Reduce load
• Wet Stacking
  Cause: Diesel engine run at less than 35% of its full load capacity, causing unburned fuel to accumulate in the exhaust system; can also cause fouling of injectors and buildup on engine valves.
  Fix: Load the unit heavily for five hours or until the exhaust is clear; diesel engines operate properly when applied loads are between 35% and 100% capacity.
• Vgen<, > — The generator voltage is out of limits given by Gen <V and Gen <V setpoints. 27
• Underspeed — During starting of the engine, when the RPM reaches the value of Starting RPM setpoint, the starter is switched off and the speed of the engine can drop under Start RPM again. Then the underspeed protection becomes active. Protection evaluation starts 5 seconds after start up. 26
• Ubat — Battery voltage is out of limits given by Batt Overvolt and Batt Undervolt setpoints. 25
• Stop fail — Gen-set stop failed. 24

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan QSB5-G3 NR3.

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

Parts cataloged across all variants of this engine family — verify the exact part fits your specific variant and serial number before ordering.