Cummins/Onan AJ series troubleshooting: 84 symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan AJ series — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Generator will not crank engine (electric cranking models only) | Battery discharged | Recharge |
| Generator will not crank engine (electric cranking models only) | Loose connections | Tighten connections |
| Generator will not crank engine (electric cranking models only) | Defective starting circuit | Repair or replace as necessary |
| Generator will not crank engine (electric cranking models only) | Defective switch | Replace |
| Engine cranks too stiffly | Too heavy oil in crankcase | Drain, refill with lighter oil |
| Engine cranks too stiffly | Engine stuck | Disassemble and repair |
| Engine will not start when cranked | Faulty ignition | Clean, adjust, or replace breaker points, plug, condenser, etc., or retime magneto |
| 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 | Clogged fuel screen | Clean |
| Engine will not start when cranked | Cylinder flooded | Crank few times with spark plug removed |
| Engine will not start when cranked | Poor fuel | Drain, refill with good fuel |
| Engine will not start when cranked | Poor compression | Tighten cylinder head and spark plug. If still not corrected, grind the valves. Replace piston rings, if necessary |
| Engine will not start when cranked | Wrong timing | Reset breaker points or retime magneto |
| Engine runs but voltage does not build up | Poor commutation | See that brushes seat well on commutator, are free in their holders, are not worn too short, and have good spring tension |
| Engine runs but voltage does not build up | Open circuit, short circuit, or ground in generator | See GENERATOR. Replace part necessary |
| Engine runs but voltage does not build up | Residual magnetism lost | Consult your dealer |
| Excessive oil consumption, light blue smoky exhaust | Poor compression, usually due to worn piston, rings, or cylinder | Refinish cylinder. Replace piston and rings |
| Excessive oil consumption, light blue smoky exhaust | Oil leaks from oil base or connections. This does not cause smoky exhaust | Replace gaskets. Tighten screws and connections. Check breather valve |
| Excessive oil consumption, light blue smoky exhaust | Oil too light or diluted | Drain, refill with correct oil |
| Excessive oil consumption, light blue smoky exhaust | Worn engine | Repair as necessary |
| Excessive oil consumption, light blue smoky exhaust | Worn intake valve guide or valve stem | Replace |
| Excessive oil consumption, light blue smoky exhaust | Engine misfiring | Refer to symptom of engine misfiring |
| Excessive oil consumption, light blue smoky exhaust | Faulty ignition | Clean, adjust, or replace breaker points, plug, condenser, etc., or retime magneto |
| Excessive oil consumption, light blue smoky exhaust | Too much oil | Drain excess oil |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plug with black soot, possible lack of power under heavy load | Fuel mixture too rich | Adjust carburetor or choke. Install needed carburetor parts |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plug with black soot, possible lack of power under heavy load | Choke not open | See that choke opens properly |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plug with black soot, possible lack of power under heavy load | Dirty air cleaner | Clean |
| Black, smoky exhaust, excessive fuel consumption, fouling of spark plug with black soot, possible lack of power under heavy load | Excessive crankcase pressure, causing excessive fuel pump pressure | Clean breather valve |
| Light pounding knock | Loose connecting rod bearing | Adjust or replace |
| Light pounding knock | Low oil supply | Add oil |
| Light pounding knock | Oil badly diluted | Change oil |
| Engine stops unexpectedly | Fuel tank empty | Refill |
| Engine stops unexpectedly | Defective ignition | Check the ignition system. Repair or replace parts necessary |
| Dull metallic thud. If not bad, 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 metallic thud, especially when cold engine first started | Low oil supply | Add oil |
| Sharp metallic thud, especially when cold engine first started | Oil badly diluted | Change oil |
| Pinging sound when engine is suddenly or heavily loaded | Carbon in cylinder | Remove carbon |
| Pinging sound when engine is suddenly or heavily loaded | Spark too early | Adjust breaker points or retime magneto |
| Pinging sound when engine is suddenly or heavily loaded | Wrong spark plug | Install correct spark plug |
| Pinging sound when engine is suddenly or heavily loaded | Spark plug burned or carboned | Install new plug |
| Pinging sound when engine is suddenly or heavily loaded | Valves hot | Adjust tappet clearance |
| Pinging sound when engine is suddenly or heavily loaded | Fuel stale or low octane | Use good fresh fuel |
| Pinging sound when engine is suddenly or heavily loaded | Lean fuel mixture | Clean and adjust carburetor |
| Pinging sound when engine is suddenly or heavily loaded | Engine hot | Check air circulation |
| Pinging sound when engine is suddenly or heavily loaded | Speed too low | Adjust governor to correct speed |
| Pinging sound when engine is suddenly or heavily loaded | Poor commutation or brush contact | See that brushes seat well on commutator, are free in their holders, are not worn too short, and have good spring tension |
| Tapping sound | Tappet clearance too great | Adjust or replace tappets |
| Tapping sound | Broken valve spring | Install new spring |
| Hollow clicking sound with cool engine under load | Loose piston | If noise only slight and disappears when engine warms up, no immediate attention needed, otherwise replace worn parts |
| Voltage low at far end of line but normal near power plant | Too small line wire for load and distance | Install larger or extra wires or reduce load |
| Electric motor runs too slowly and overheats at far end of line but OK if used near power unit | Too small line wire for load and distance | Install larger or extra wires or reduce load |
| Voltage unsteady but engine not misfiring | Loose connections | Tighten connections |
| Voltage unsteady but engine not misfiring | Fluctuating load | Correct any abnormal load condition causing trouble |
| Engine backfires at carburetor | Lean fuel mixture | Clean or adjust carburetor |
| Engine backfires at carburetor | Clogged fuel screen | Clean screen |
| Engine backfires at carburetor | Poor fuel | Refill with good, fresh fuel |
| Engine backfires at carburetor | Spark too late | Adjust breaker points or retime magneto |
| Engine backfires at carburetor | Intake valve leaking | Grind or replace |
| Noisy brushes | High mica between bars of commutator | Undercut mica |
| Excessive arcing of brushes | Rough commutator | Turn down |
| Excessive arcing of brushes | Dirty commutator | Clean |
| Excessive arcing of brushes | Brushes not seating properly | Sand to a good seat |
| Excessive arcing of brushes | Open circuit in armature | Replace |
| Excessive arcing of brushes | Brush rig out of position | Line up properly |
| Generator overheating | Brush rig out of position | Adjust |
| Generator overheating | Overloaded | Reduce load |
| Voltage drops under heavy load | Engine lacks power | See remedies for engine misfires under heavy load |
| Voltage drops under heavy load | Poor compression | Tighten cylinder head and spark plug. 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 replace parts necessary |
| Voltage drops under heavy load | Dirty air cleaner | Clean |
| Voltage drops under heavy load | Choke partially closed | See that it opens wide |
| Voltage drops under heavy load | Carbon in cylinders | Remove carbon |
| Voltage drops under heavy load | Restricted exhaust line | Clean or increase the size |
| Engine misfires at light load | Spark plug gap too narrow | Adjust to correct gap |
| Engine misfires at light load | Intake air leak | Tighten or replace gaskets |
| Engine misfires at light load | Faulty ignition | Clean, adjust or replace breaker points, plug, condenser, etc., or retime ignition |
| Engine misfires at light load | Low compression | Tighten cylinder head and spark plug. If still not corrected, grind valves. Replace piston rings, if necessary |
| Engine misfires at heavy load | Spark plug gap too wide | Adjust gap |
| Engine misfires at heavy load | Faulty ignition | Clean, adjust or replace breaker points, plug, condenser, etc., or retime magneto |
| Engine misfires at heavy load | Clogged carburetor | Clean jet |
| Engine misfires at heavy load | Clogged fuel screen | Clean |
| Recoil starter friction shoe fails to function and engage with the cup | Failure of brake spring #8; lubrication getting onto brake washers #10; improper position of friction shoe sharp edge and friction shoe lever | Check for failure of brake spring #8; check for lubrication getting onto brake washers #10; check for proper position of friction shoe sharp edge and friction shoe lever |
| Broken re-wind spring | Replace with a new one | |
| Fatigued re-wind spring | Add turns of the rotor to increase pre-tension and then reinstall the middle flange; or try forming new loops and coiling spring inside-out |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Carbon in cylinder; Spark too early; Wrong spark plug; Spark plug burned or carboned. See the table for the matching fixes.
Documented causes: Poor compression, usually due to worn piston, rings, or cylinder; Oil leaks from oil base or connections. This does not cause smoky exhaust; Oil too light or diluted; Worn engine. See the table for the matching fixes.
Documented causes: Faulty ignition; Lack of fuel or faulty carburetion; Clogged fuel screen; Cylinder flooded. See the table for the matching fixes.