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Cummins/Onan 10C Troubleshooting

Cummins/Onan 10C troubleshooting: 140 symptom-cause-remedy entries from the manufacturer's own service documentation.

Cummins/Onan 10C — full specs, parts & maintenance.

Troubleshooting

140

Symptoms & remedies

Symptom	Cause	Remedy
Engine cranks too stiffly	Too heavy oil in crankcase	Drain. Refill with light oil.
Engine cranks too stiffly	Engine stuck	See PREPARATION. Disassemble and repair.
Engine cranks too slowly when cranked electrically	Discharged or defective battery	Recharge or replace.
Engine cranks too slowly when cranked electrically	Loose connections	Tighten loose connections.
Engine cranks too slowly when cranked electrically	Corroded battery terminals	Clean corroded terminals. Replace cable if necessary.
Engine cranks too slowly when cranked electrically	Brushes worn excessively or making poor contact	Replace brushes or clean commutator.
Engine cranks too slowly when cranked electrically	Short circuit in generator load circuit	Repair or replace parts necessary. Disconnect load.
Engine cranks too slowly when cranked electrically	Dirty or corroded points in start solenoid switch	Replace switch.
Engine will not start when cranked	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc., or retime ignition.
Engine will not start when cranked	Lack of fuel or faulty carburetion	Refill the tank. Check the fuel system. Clean, adjust, or replace parts necessary.
Engine will not start when cranked	Cylinders flooded	Ground spark plug cables. Crank engine with spark plugs removed.
Engine will not start when cranked	Poor fuel	Drain. Refill with good fuel.
Engine will not start when cranked	Poor compression	Tighten cylinder heads and spark plugs. If still not corrected, grind the valves. Replace piston rings if necessary.
Engine will not start when cranked	Wrong ignition timing	Reset breaker points or retime ignition. See IGNITION TIMING.
Engine runs but voltage does not build up	Poor brush contact	See that brushes seat well on commutator and collector rings, are free in holders, are not worn shorter than 1/2 inch, and have good spring tension.
Engine runs but voltage does not build up	Open circuit, short circuit, or ground in generator	Refer to the GENERATOR section of Maintenance.
Voltage unsteady but engine not misfiring	Speed too low	Adjust governor to correct speed.
Voltage unsteady but engine not misfiring	Poor commutation or brush contact	Refinish commutator or undercut mica if necessary. See that brushes seat well on commutator and collector rings, are free in holders, are not worn shorter than 1/2 inch, and have good spring tension.
Voltage unsteady but engine not misfiring	Loose connections	Tighten connections.
Generator overheating	Fluctuating load	Correct any abnormal load condition causing trouble.
Generator overheating	Short in load circuit	Correct short circuit.
Generator overheating	Generator overloaded	Reduce the load.
Generator overheating	Improper brush rig position	Refer to the GENERATOR section of MAINTENANCE - See Brush Rig.
Engine overheating	Improper lubrication	See Low Oil Pressure.
Engine overheating	Poor ventilation	Provide ample ventilation at all times.
Engine overheating	Dirty or oily cooling surfaces	Keep the engine clean.
Engine overheating	Retarded ignition timing	Retime ignition.
Engine overheating	Generator overloaded	Reduce load.
Voltage drops under heavy load	Engine lacks power	See remedies under "Engine Misfires at Heavy Load".
Voltage drops under heavy load	Poor compression	Tighten cylinder heads and spark plugs. If still not corrected, grind the valves. Replace piston rings if necessary.
Voltage drops under heavy load	Faulty carburetion	Check the fuel system. Clean, adjust or repair as needed.
Voltage drops under heavy load	Dirty carburetor air cleaner	Clean. Service with proper oil.
Voltage drops under heavy load	Choke partially closed	Choke plate must be wide open at operating temperature.
Voltage drops under heavy load	Carbon in cylinders or in carburetor venturi	Remove carbon.
Voltage drops under heavy load	Restricted exhaust line	Clean or increase the size.
Voltage drops under heavy load	Improper governor adjustment	Refer to ADJUSTMENTS.
Engine misfires at light load	Carburetor idle jet clogged or improperly adjusted	Clean or adjust.
Engine misfires at light load	Spark plug gaps too narrow	Adjust to correct gap - .025" (.018" for gas operation).
Engine misfires at light load	Intake air leak	Tighten manifold and carburetor mounting screws. Replace gaskets if necessary.
Engine misfires at light load	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc.
Engine misfires at heavy load	Defective spark plug	Replace.
Engine misfires at heavy load	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condensers, etc. or retime ignition.
Engine misfires at heavy load	Clogged carburetor	Clean carburetor.
Engine misfires at heavy load	Clogged fuel screen	Clean.
Engine misfires at heavy load	Defective spark plug cable	Replace.
Engine misfires at all loads	Fouled spark plug	Clean and adjust.
Engine misfires at all loads	Defective or wrong spark plug	Replace. Use Champion Com. 8.
Engine misfires at all loads	Leaking valves	See VALVE SERVICE.
Engine misfires at all loads	Broken valve spring	Replace.
Engine misfires at all loads	Defective or improperly adjusted breaker points	Adjust or replace breaker points.
Low oil pressure	Oil too light	Drain, refill with proper oil.
Low oil pressure	Oil badly diluted	Drain, refill with proper oil.
Low oil pressure	Oil too low	Add oil.
Low oil pressure	Oil relief valve not seating	Remove and clean, or replace.
Low oil pressure	Badly worn bearings	Replace.
Low oil pressure	Sludge on oil screen	Remove and clean.
Low oil pressure	Badly worn oil pump	Replace.
Low oil pressure	Defective oil pressure gauge	Replace.
High oil pressure	Oil too heavy	Drain, refill with proper oil.
High oil pressure	Clogged oil passage	Clean all lines and passages.
High oil pressure	Oil relief valve stuck	Remove and clean.
High oil pressure	Defective oil pressure gauge	Replace.
Engine backfires	Lean fuel mixture	Clean carburetor. Adjust jets.
Engine backfires	Clogged fuel filter	Clean.
Engine backfires	Air leak at intake manifold or carburetor flange	Tighten mounting screws. Replace gaskets if necessary.
Engine backfires	Poor fuel	Refill with good, fresh fuel. See PREPARATION.
Engine backfires	Spark advanced too far	Reset breaker points or retime ignition.
Engine backfires	Intake valve leaking	Reseat or replace.
Excessive oil consumption, light blue exhaust	Poor compression. Usually due to worn pistons, rings, or cylinders	Refinish cylinders. Install oversize pistons and rings.
Excessive oil consumption, light blue exhaust	Oil too light or diluted	Drain. Refill with proper oil.
Excessive oil consumption, light blue exhaust	Too large bearing clearance	Replace bearings necessary.
Excessive oil consumption, light blue exhaust	Engine misfires	Refer to "Engine Misfires At All Loads".
Excessive oil consumption, light blue exhaust	Faulty ignition	Clean, adjust, or replace breaker points, spark plugs, condenser, etc., or retime the ignition.
Excessive oil consumption, light blue exhaust	Too much oil	Drain excess oil.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Fuel mixture too rich	See that choke opens properly. Adjust jets properly. Adjust the float level.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Choke not fully open	See that choke opens properly.
Black, smoky exhaust, excessive fuel consumption, fouling of spark plugs with black soot, possible lack of power under heavy load	Dirty air cleaner	Clean. Service with proper oil.
Light pounding knock	Loose connecting rod	Replace rod bearings.
Light pounding knock	Low oil supply	Add oil. Change if necessary.
Light pounding knock	Oil badly diluted	Drain. Refill with proper oil.
Light pounding knock	Low oil pressure	See Low Oil Pressure for remedies.
Engine stops unexpectedly	Empty fuel tank	Refill.
Engine stops unexpectedly	Defective ignition system	Check the ignition system. Repair or replace as needed. See that the STOP button lead is not grounded.
Engine stops unexpectedly	Fuel pump failure	Repair or replace.
Dull metallic thud, if not bad, may disappear after few minutes operation. If bad, increases with load and distance	Loose crankshaft bearing	Replace, unless one of the next two remedies permanently corrects the trouble.
Sharp metallic thud, especially when cold engine first started	Low oil supply	Add oil. Change if necessary.
Sharp metallic thud, especially when cold engine first started	Oil badly diluted	Drain. Refill with proper oil.
Pinging sound when engine is suddenly or heavily loaded	Carbon in cylinders	Remove the carbon.
Pinging sound when engine is suddenly or heavily loaded	Spark advanced too far	Reset breaker points or retime ignition.
Pinging sound when engine is suddenly or heavily loaded	Wrong spark plugs	Install correct spark plugs.
Pinging sound when engine is suddenly or heavily loaded	Spark plugs burned or carboned	Clean. Install new plugs if necessary.
Pinging sound when engine is suddenly or heavily loaded	Valves hot	Adjust tappet clearance. See VALVE SERVICE.
Pinging sound when engine is suddenly or heavily loaded	Fuel stale or low octane	Use good, fresh fuel. See PREPARATION.
Pinging sound when engine is suddenly or heavily loaded	Lean fuel mixture	Clean fuel system. Adjust carburetor jets properly.
Tapping sound	Valve clearance too great	Adjust to proper clearance. See VALVE TAPPETS.
Tapping sound	Broken valve spring	Install new spring.
Hollow clicking sound with cool engine under load	Loose piston	If noise is only slight and disappears when engine warms up, no immediate attention needed. Otherwise replace parts necessary.
Sharp click when cranking engine	Magneto impulse coupling	Normal condition - should stop as soon as engine starts.
Voltage low at far end of line but normal near the plant	Too small line wire used for load and distance	Install larger or extra wires or reduce load.
Motors run too slowly and overheat at far end of line but OK near the plant	Too small line wire used for load and distance	Install larger or extra wires or reduce load.
Noisy brushes	High mica between bars of commutator or rings	Undercut mica.
Noisy brushes	Brushes not seating properly	Sand to a good seat or reduce load until worn in.
Excessive arcing of brushes	Rough commutator or rings	Turn down.
Excessive arcing of brushes	Open circuit in armature	Install a new armature.
Excessive arcing of brushes	Brush rig out of position	Line up properly.
Spark plugs foul up rapidly	Engine running cold	Restrict air flow. Install preheater hose.
Spark plugs foul up rapidly	Wrong plugs	Use Champion Com 8.
Spark plugs foul up rapidly	Carburetor too rich	Adjust.
Spark plugs foul up rapidly	One spark plug fouled	Clean plugs.
Oil dilution	Leaky carburetor	Replace.
Oil dilution	Worn oil seals	Clean or replace.
Oil dilution	Fouled breather valve	Clean.
Oil seal leak	Loose oil fill cap	Tighten - replace if gasket is damaged.
Voltage unsteady but engine not misfiring	Poor commutation or brushes	Refinish commutator or undercut mica if necessary. See that brushes seat well on commutator and collector rings, are free in holders, are not worn shorter than 1/2-inch, and have good spring tension.
Voltage unsteady but engine not misfiring	Fluctuating load	Correct any abnormal load condition causing trouble.
Engine overheating	Spar ventilation	Provide ample ventilation at all times.
Lean fuel mixture		Clean carburetor. Adjust jets.
Lean fuel mixture	Clogged fuel filter	Clean.
Lean fuel mixture	Air leak at intake manifold or carburetor flange	Tighten mounting screws. Replace gaskets if necessary.
Lean fuel mixture	Poor fuel	Refill with good, fresh fuel.
Lean fuel mixture	Spark advanced too far	Reset breaker points or retime ignition.
Lean fuel mixture	Intake valve leaking	Reseat or replace.
Loose connecting rod		Replace rod bearings.
Low oil supply		Add oil. Change if necessary.
Oil badly diluted		Drain. Refill with proper oil.
Low oil pressure		See Low Oil Pressure for remedies.
Dull metallic thud, if not sag, may disappear after few minutes operation. If bad, increases with load	Loose crankshaft bearing	Replace, unless one of the next two remedies permanently corrects the trouble.
Sharp click when cranking engine	Normal condition — should stop as soon as engine starts
Voltage low at far end of line but normal near plant	Too small line wire used for load and distance	Install larger or extra wires or reduce load.
Motors run too slowly and overheat at far end of line but OK near the plant	Too small wire used for load	Install larger or extra wires or reduce load.
Noisy brushes	High mica between bars of commutator	Undercut mica.
Excessive arcing of brushes	Dirty commutator or rings	Clean.
Excessive arcing of brushes	Brushes not seating properly	Sand to a good seat or reduce load until worn in.
Spark plugs foul rapidly	Engine running 'cold'	Restrict air flow. Install preheater hose.
Spark plugs foul rapidly	Wrong plugs	Replace with correct plugs.
Spark plugs foul rapidly	Carburetor too 'rich'	Adjust.
Oil dilution	One spark plug fouled	Clean plugs.
Oil dilution	Leaky carburetor valve	Clean.
Oil seal leak	Worn oil seals	Replace.
Oil seal leak	Fouled breather valve	Clean or replace.

Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.

Cross-brand guides for these symptoms

Engine overheating Low oil pressure

Common questions

Cummins/Onan 10C: Voltage drops under heavy load — what causes it?

Documented causes: Engine lacks power; Poor compression; Faulty carburetion; Dirty carburetor air cleaner. See the table for the matching fixes.

Cummins/Onan 10C: Low oil pressure — what causes it?

Documented causes: Oil too light; Oil badly diluted; Oil too low; Oil relief valve not seating. See the table for the matching fixes.

Cummins/Onan 10C: Pinging sound when engine is suddenly or heavily loaded — what causes it?

Documented causes: Carbon in cylinders; Spark advanced too far; Wrong spark plugs; Spark plugs burned or carboned. See the table for the matching fixes.