Why Your RV Generator Will Not Start, and What to Check First

Why does the generator quit while the gauge still reads a quarter tank?

The generator draws from the same tank as the engine on a gas or diesel motorhome, but its pickup tube stops higher. On most coaches that leaves roughly a quarter of the tank the generator simply cannot reach. So a set that ran fine at half a tank will crank, catch briefly on whatever is left in the line, and quit. That is not a fault. That is the design doing its job. Add fifteen or twenty gallons, then hold the start switch a little longer than feels comfortable, because the fuel pump has to refill an empty line before the engine will stay running.

What does storage do to the fuel and the carburetor?

Fuel that sits is the next suspect. Gasoline blended with ethanol pulls moisture out of the air, and once enough water accumulates, the ethanol and water drop out as a separate layer at the bottom of the tank. That layer is exactly what the pickup finds first. Engines will not run on it. In our climate this can happen over a single storage season, and it is worse in a coach stored with a half empty tank, because there is more air volume inside to give up moisture.

The lighter fractions in gasoline also evaporate and leave varnish behind. Generator carburetors use tiny fixed jets, and a main jet with a varnish film has effectively changed size. The symptoms are consistent: it starts on choke, dies as the choke opens, or it runs fine until you ask it for load. Cleaning usually means pulling the carburetor rather than spraying something down the intake. Stabilizer added before storage, plus a run long enough to pull treated fuel through the bowl, prevents nearly all of it.

Which battery does the generator actually start from?

Ask which battery the set pulls from, because it varies by coach. On most gas motorhomes the generator starts from the chassis battery, so a rig with healthy house batteries and a dead chassis battery gives you a generator that clicks or cranks slowly while the interior lights burn bright. Many diesel pushers do the opposite and start the generator from the house bank, and some have a dedicated start battery in the generator bay. Find yours in the manual before you conclude the starter is bad. Then measure at the generator start solenoid while someone holds the switch. Under 10 volts there means a battery or cable problem, not a generator problem.

What should you check, in order?

Work through these in order and stop at the first one that gives you a bad answer. Most no-start calls end inside the first three steps, and the rest usually land on the fourth or fifth. Have a helper hold the start switch at the interior panel while you watch or measure at the generator itself, because the remote switch, the harness, and the set's own control board all sit in that path, and any of them can be the thing that is broken.

The ordered checks we run before anything comes apart:

1. Add fuel until the tank is at least half full, then crank in ten second bursts with a full minute of rest between them.
2. Identify the correct start battery, then measure its voltage while the switch is held. Anything under 10 volts is a battery or cable fault.
3. Check the generator's own breaker and its inline fuses. Many sets carry a small fuse at the control board that disables starting entirely.
4. Reset the set: hold the stop side of the switch for several seconds, then try start. That clears a surprising number of control board lockouts.
5. Pull the spark plug on a gas set. A plug wet with fuel points at spark or compression. A dry plug points at fuel delivery.
6. Inspect the fuel filter and line for cloudiness or a visible separated layer, which points at water and phase separation.
7. If it starts and runs but produces no voltage, check the brushes and the output before assuming the generator head is finished.

Why does a worn brush crank normally but produce no power?

A generator that cranks, starts, and runs smoothly while producing nothing is a completely different problem. On the older Onan sets, field current reaches the rotor through a pair of carbon brushes riding on slip rings. Brushes wear. As they shorten, spring pressure drops, contact goes intermittent, and eventually the field never energizes at all. The engine has no idea any of this is happening. It runs perfectly and makes no power. Brushes are a wear item, they cost a small fraction of a generator head, and on many sets they are reachable from the end cover. Always worth checking before anyone talks about replacing the whole unit.

How often does the set really need to run, and under what load?

Run the generator once a month for at least two hours, with a real load on it. A no-load run is close to useless. It does not get the exhaust hot enough to drive out condensation, it does not exercise the field under demand, and on a gas set it barely moves fresh fuel through the carburetor. Turn on both roof air units, or one roof air plus an electric water heater element, and let the set work. That is what keeps the carburetor wet, the brushes seated, and the windings dry between trips.

What exhaust problems should stop you immediately?

Two rules, and neither one has an exception. Never run the set with the tailpipe extension damaged, missing, or ending short of the sidewall, because exhaust that hugs the coach finds its way into a window or a bay vent. And never run it in a closed bay or with any leak upstream of the tip. Carbon monoxide from a generator has no smell, and it builds fastest when the coach is closed up at night. Test the detector with its button every trip, replace it at the date printed on the housing, and if anyone gets a headache while the set runs, shut it down and open everything.

Frequently asked questions