Generator
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Which generator is in the bay? Read the label on the unit.
- What is the complaint? Pick what the customer sees.
- Does it crank? Hold the start switch. Listen and watch the panel. Onan: hold START at the center position; prime is at the bottom. Expect: Engine cranks strong
- When does it die? Note how long it runs before it quits.
- Check the breaker on the generator Find the output breaker on the generator itself and reset it once.
- What kind of fuel? Read the label or look at the fuel source.
- Which generator? Pick the brand to see its codes.
- Is there fuel and spark? Check the fuel level or the LP valve. Then confirm spark at the plug. Onan QG 4000: pull the plug lead and check for a blue spark on a tester. Expect: Fuel present, strong spark
- Check the battery and the cables Meter the battery, then the cable ends at the starter. Onan gensets need a healthy house battery: cranking draws a lot. Expect: About 12.4 V or more at rest, clean tight ends
- No crank Check the fuse, the start switch, and the control board. Onan: check the 5 A control fuse on the board. Expect: 12V at the starter solenoid when the switch is held
- Dies right after start Most gensets shut down if the oil is low or the control sees no output. Onan checks oil pressure and AC output within seconds of start.
- No output Qualified person: check the transfer switch and the wiring, then test the generator output at the breaker. Expect: About 108 to 132 V AC at the generator output
- Lp fuel Check the tank level and the valve. Is LP reaching the coach? LP gensets draw a LOT of vapor: a small tank can freeze the regulator under load. Expect: Steady stove flame
- Gasoline fuel Check the tank and the fuel line. Old fuel is the #1 cause of a gasoline no-start. Onan QG gensets draw from the coach tank but only when the tank is above about 1/4. Expect: Fresh fuel, clean line
- Diesel fuel Check the fuel level and the shut-off. Diesel gensets also need good cranking speed and clean fuel. Expect: Fresh fuel, open shut-off
- Which code flashes after it dies? Oil is full. The shutdown code tells you which sensor stopped it.
- Code 36: engine stopped without a stop command Engine stopped without a stop command. Expect: Do this first: Check the fuel level, prime for about 30 seconds per the manual, and check the air filter. Stop if you find a fuel leak or damage.
- Code 12: output over voltage Output over voltage. Expect: Do this first: Generator output too high. Disconnect loads, check the voltage regulation and sensing, then service.
- Code 13: output under voltage Output under voltage. Expect: Do this first: Overloaded, engine running slow, or a weak alternator. Reduce the load, check engine speed, service.
- Code 14: output over frequency Output over frequency. Expect: Do this first: Engine speed too high. Check the governor and throttle actuator and linkage.
- Code 15: output under frequency Output under frequency. Expect: Do this first: Engine running slow or overloaded. Fuel, air, load and the governor.
- Code 19: throttle actuator circuit fault Throttle actuator circuit fault. Expect: Do this first: Check the actuator plug and wiring, and the linkage for binding. Replace the actuator if needed.
- Code 25: alternator over voltage Alternator over voltage. Expect: Do this first: Alternator control fault. Service.
- Code 26: alternator under voltage Alternator under voltage. Expect: Do this first: Alternator, wiring or load problem. Check load, then service.
- Code 27: pma sense lost PMA sense lost. Expect: Do this first: The permanent magnet alternator signal is lost. Check the plug and wiring, then the alternator.
- Code 29: battery over voltage Battery over voltage. Expect: Do this first: Check the start battery and the charge circuit. Measure the battery while running.
- Which code is flashing? Onan flashes a two-digit code on the start switch light: count the first group of flashes, pause, then the second group.
Causes and repairs
Fuel or vapor lock
It dies after it warms up. Suspect a vapor lock, a fuel pump, or a fuel line running too close to the exhaust. Onan QG gasoline gensets are known for fuel pump failures.
Tools: hand tools
Check the fuel line for heat and kinks, and check the fuel pump. On LP, check the regulator and the vaporizer.
Fuel or governor
It dies under load. Suspect a fuel restriction or a governor issue. Onan mechanical governors can be adjusted; inverter gensets manage speed electronically.
Tools: hand tools
Check the fuel filter and the line, then the governor and its linkage.
Breaker was the problem
The breaker had tripped. It may have been an overload.
Tools: none
Watch the load. If it trips again, find the overload or a short before you reset it.
Ignition problem
No spark. Suspect the plug, the coil, or the igniter. Onan QG 4000: the plug is a common wear item; check the gap first.
Tools: spark tester, multimeter
Pull the plug. Check the gap against the service decal on the genset and look at the tip: wet with fuel, black with soot, or cracked. Use an inline spark tester: no blue spark with a new plug means the coil or ignition module. Check the kill wire isn't grounded.
Air, compression or control
Fuel and spark are present but it will not run. Suspect low compression or a control fault.
Tools: compression gauge
Screw a compression gauge into the plug hole and crank with the throttle open. Compare with the same model in a known good unit: very low or zero points to valves or rings. Good compression: check for a code on the start switch, and that the fuel pump clicks while cranking.
Battery low
A low battery will not crank the genset.
Tools: battery charger
Charge or replace the battery. Check the charging system after it starts.
Loose or corroded ends
Bad cable ends drop voltage at the starter.
Tools: hand tools
Clean the terminals and the cable ends, tighten them, and retest.
Starter
The battery and cables are good, so the starter is the fault.
Tools: multimeter
Confirm the starter gets 12V when the switch is held. Replace the starter if it does not spin.
Fuse, switch or wiring
No 12V at the solenoid. Suspect the fuse, the start switch, or the wiring.
Tools: multimeter
Meter the fuse, the switch, and the wire to the solenoid. Replace the failed part.
Starter failed
The starter gets 12V but does not spin.
Tools: multimeter
Replace the starter with the same part.
Oil low
Most gensets shut down on low oil to protect the engine.
Tools: none
With the coach level, pull the dipstick and fill to the FULL mark. Don't overfill: too much oil causes smoke and shutdowns too. Look for leaks under the genset.
Transfer switch or wiring
The generator makes power but the coach does not get it.
Tools: multimeter
Check the transfer switch and the wiring from the generator to the panel.
Generator fault
The generator runs but makes no 120V. Suspect the voltage regulator, the brushes, or the windings. Onan QG: the voltage regulator is a common failure.
Tools: multimeter
Meter output at the generator breaker under no load. 0 V: pull the brush cover and look for worn or stuck brushes and burned slip rings. Brushes and rings good: the voltage regulator is the common failure on Onan QG. Very low voltage (a few volts) means the field isn't exciting.
Lp tank empty or valve closed
The genset has no fuel.
Tools: none
Fill the tank or open the valve, then purge the line and retest.
Gasoline tank empty or low
The genset has no fuel, or the coach tank is below the genset draw tube.
Tools: none
Fill the tank past 1/4 and retest.
Old gasoline
Old fuel will not fire. It may also clog the carburetor.
Tools: hand tools
Drain the old fuel, refill with fresh, and clean the carburetor if needed.
Fuel pump or filter
The fuel is fresh but not reaching the engine. Suspect the pump or a clogged filter. Onan QG: the fuel pump is a known failure item.
Tools: hand tools
Check the fuel filter and the pump output. Replace the failed part.
Diesel tank empty
The genset has no fuel.
Tools: none
Fill the tank and retest.
Diesel filter or lift pump
Fresh fuel but the engine starves. Suspect the fuel filter or the lift pump.
Tools: hand tools
Check the filter and the lift pump. Replace the failed part.
Cleared
Cleared. Run it through a full cycle twice before you sign it off.
Write down what you did and the reading that proved it.
Control or output fault
It dies without a code.
Tools: Multimeter
Meter output at the generator breaker in the first seconds it runs: no AC output and the board shuts it down. Then check the low-oil switch wire for a chafe to ground, and the oil level again with the genset level. Onan models without a code readout: a failing low-oil switch is a common cause.