RV Generator Maintenance: What Onan Owners Actually Need to Do
The design quirks people mistake for faults, and the short list of habits that decide whether a generator starts after a Florida summer.
Two things account for most generator calls, and neither is a broken generator. The first is fuel: a generator set's pickup tube is deliberately terminated higher in the tank than the chassis engine's, so it starves at roughly a quarter tank on purpose. The second is stale fuel and a varnished carburetor after a coach sat through a Florida summer unexercised.
Apex RV Repair is mobile across Florida - we come to your RV. Call 888-759-1438.
An onboard generator is the least used and most complained-about system in most coaches. It sits idle for months, then gets asked to start instantly and carry an air conditioner in August.
Two of its most alarming behaviors are not faults at all - they are design decisions that nobody explains at handover. Knowing which is which saves a service call, and knowing the maintenance that actually matters saves a carburetor.
Why does my generator shut off when the fuel tank gets low?
Because it is supposed to, and this single fact resolves a large share of "my generator keeps quitting" calls.
On a motorhome that shares one fuel tank between the chassis engine and the generator set, the generator's fuel pickup is deliberately terminated higher in the tank than the engine's. The reason is straightforward: without that arrangement, a generator left running overnight could empty the tank and leave you unable to drive anywhere. Cummins states this requirement in its RV generator installation documentation - the genset pickup must sit above the vehicle engine pickup precisely to prevent the generator running the vehicle out of fuel.
It is a design feature rather than a fault, and it exists so that running the generator overnight cannot strand the coach with an empty tank and no way to drive it anywhere.
How do you tell a fuel pickup cutoff from a real generator fault?
The practical result is that the generator loses its fuel supply and shuts down while the dash gauge still shows something in the region of a quarter tank. It will start again and run fine once you add fuel. Nothing is broken.
So the diagnostic question is simple. If your generator quits and the tank is low, fill it and try again before spending anything. If it quits with a comfortably full tank, that is a genuine fault and the causes below apply.
That single test separates the most common false alarm on this page from an actual repair, and it costs a tank of fuel rather than a diagnostic visit. Only if the set quits with a genuinely full tank are you looking at something worth paying a technician to chase down properly.
How often should I run my generator, and does load matter?
Monthly, and yes, the load is the part that matters most.
Exercising a generator is not about proving it starts. It is about getting the whole set hot enough for long enough to drive moisture out of the windings and the oil, and to move fresh fuel through the entire fuel path so it does not sit and varnish.
That is why running it for five minutes at no load is close to useless and can be worse than nothing. The set never reaches proper operating temperature, moisture condenses rather than being driven off, and on some engines light-load running encourages carbon buildup.
Monthly is the interval, but the load is the part that decides whether the run achieved anything at all. A set that idles for five minutes has been started, not exercised.
What does a proper generator exercise run look like?
- Run it long enough to reach full operating temperature rather than to a stopwatch number - the set should be properly hot, not warm.
- Load it while it runs. An air conditioner is the honest test because it is what you will actually ask of it.
- Do it monthly, including through the months you are not traveling. Storage is when generators die, not use.
- Watch and listen while it runs. Surging, hunting, a change in exhaust color or a new noise is information you only get by being present.
In Florida this matters more than in a dry climate, because humidity is relentless and a set that never gets hot never dries out.
Put it in the calendar on the same day each month rather than trusting yourself to remember it. The generators we are called out to are almost always the ones nobody ran.
Why won't my generator start after sitting all summer?
Almost always fuel, and specifically what fuel turns into while sitting in Florida heat.
Gasoline degrades. Light fractions evaporate, the remainder oxidizes, and what is left behind in the small passages of a carburetor is a varnish that no amount of cranking clears. The jets and passages in a generator carburetor are small by design, so it takes very little deposit to stop the engine getting a combustible mixture.
Ethanol-blended fuel adds a second mechanism. It is hygroscopic, meaning it pulls moisture out of humid air, and Florida supplies humid air year-round. Given enough time and enough water, the blend can separate, leaving a water-rich layer at the bottom of the fuel system - exactly where the pickup is.
Heat accelerates every part of that. A set left full through a Florida summer spends months in exactly the conditions that turn usable fuel into varnish.
What does ethanol fuel do to a generator over a Florida summer?
The pattern that follows is familiar: the engine cranks strongly, may fire briefly on whatever is left in the bowl, then dies. That is a fuel delivery failure, not a starting or ignition failure, and cranking it repeatedly only flattens the battery.
- Prevention beats repair by a wide margin here. Monthly exercise moves fresh fuel through before the old fuel can turn.
- If the coach genuinely will not be run, a fuel stabiliser added before storage and then circulated by running the set is the next best thing. Adding stabiliser to the tank without running the generator does nothing for the carburetor, because the treated fuel never reaches it.
- Keep the tank reasonably full during storage. Less air space means less moisture drawn in as the tank breathes through daily temperature swings.
- Do not keep cranking a generator that will not catch. You will flatten the starting battery and, on some sets, wash the cylinder with unburnt fuel.
What is the generator telling me through its sound and its output?
More than most owners realize, and you can read it without opening anything.
These sets produce alternating current directly from an alternator turned by the engine, and the output frequency is fixed by engine speed. That makes frequency a direct readout of engine rpm. A cheap plug-in meter that displays frequency at an outlet is effectively a tachometer for a generator you cannot easily put a tachometer on.
That turns vague symptoms into measurable ones:
| What you observe | What it usually indicates |
|---|---|
| Frequency sits low and steady under load | The engine is not holding its governed speed - fuel delivery, air, or a governor adjustment. Low frequency also means low voltage, which is hard on whatever you are running. |
| Frequency hunts up and down | Governor instability, often from a partially restricted fuel supply or air in the system. This is the classic surging that owners describe as the generator 'breathing'. |
| Frequency fine, voltage low | Points at the generating end rather than the engine - which is a different repair from everything else on this list. |
| Runs fine unloaded, stumbles when the AC starts | The set cannot meet the surge demand. Could be the generator's condition, could simply be that the load is at the edge of its rating. |
| Black exhaust smoke under load | Running rich - air restriction or fueling fault. A dirty air filter is the cheap end of this and the first thing to check. |
None of that requires taking anything apart, and all of it gives a technician a far better starting point than "it runs rough."
Can my generator run the air conditioner?
One rooftop air conditioner, usually yes if the set is sized for it and healthy. Two, generally not without help.
The difficulty is not the running load, which is modest - it is the instant of start-up, when a compressor draws several times its running current for a fraction of a second. A generator has to absorb that surge without its engine speed collapsing.
The number on the data plate is a running figure, and the moment that decides whether this works is not a running moment at all. It is the half second when the compressor kicks in and draws several times what it settles down to, and that spike is what a generator either absorbs or trips on. Two otherwise identical sets can behave differently on exactly that point, which is why the answer is rarely a simple yes.
Does a soft start let a smaller generator run an RV air conditioner?
This is where a soft start device earns its place. By reducing that inrush, it can bring a start-up within what a smaller generator can absorb. It does not make the air conditioner colder and it does not add generator capacity - it changes the shape of the demand at the moment of starting. Our air conditioning article covers what these devices do and do not do in more detail.
Two habits help regardless of hardware. Start big loads one at a time rather than together, and let the set warm up before asking it for the air conditioner. A cold engine has less margin to absorb a surge than a hot one.
Neither habit costs anything, and between them they settle a good share of the calls we get about a generator that supposedly cannot run an air conditioner it is sized for.
What maintenance is actually on the schedule?
Generator maintenance is specified in engine running hours, which is why coach mileage tells you nothing about it. The hour meter on the set is the number that matters, and the intervals for your exact model are in the Cummins operator manual for that unit rather than in general advice.
- Oil and filter on the manufacturer's hour interval, and by calendar as well if the set does very few hours - oil degrades sitting in a hot Florida bay whether or not it is being used.
- Air filter checked more often than the book suggests if you camp on unpaved sites, because dust load is what drives that interval and the book assumes an average.
- Spark plug on schedule. Its condition also reads back the mixture the engine has been running.
- Fuel filter on schedule, and sooner if the coach has been stored with fuel in it for a long stretch.
- The enclosure and cooling air path. Rodents nest in generator compartments, and a blocked cooling inlet in Florida heat is a fast route to a shutdown or worse.
- The exhaust system, checked for security and leaks every time you are under there. This one is a safety item, not a performance one.
Look up your own model
Cummins publishes operator, installation and service manuals for its RV generator sets by model and spec. The intervals, capacities and specifications differ between series, so the right document is the one matching the model number on your set - not one for a similar unit.
Is an old generator worth repairing?
Usually yes, and more often than for most RV systems, because the failures are typically fuel-system and consumable failures rather than the engine or the generating end wearing out.
A set that has sat unexercised for years and now will not start is generally suffering from stale fuel, a varnished carburetor, degraded rubber and a flat battery - all of which are serviceable. The rotating machinery inside has usually done very few hours, because that is the whole problem with how RV generators are used.
The economics are better than for most RV systems because the parts that fail are usually consumables and fuel-system components rather than the engine or the generating end. That is the opposite of most appliances on a coach, where the expensive part is usually the one that fails first.
When is an RV generator genuinely past repairing?
The picture changes if the engine has genuine wear - loss of compression, heavy oil consumption, a knock under load - or if the generating end has failed, since that is often the more expensive half of the machine. That is the point at which a replacement conversation is honest rather than an upsell.
Worth saying plainly: if a set has sat for years, the useful first step is a diagnosis, not a rebuild quote. It is fairly common for the answer to be fuel system service and a battery, and it would be a poor outcome to replace a generator over that.
So buy the diagnosis before the rebuild quote. It is common for the answer to be a service rather than the overhaul the owner was braced for, and it is the cheapest way to find out which of the two you are actually facing.
When should you stop and call a technician?
Checking the fuel level, changing an air filter, adding stabiliser, running a monthly exercise and reading frequency at an outlet are all owner tasks and they narrow the diagnosis considerably.
Where it stops: fuel system work, exhaust work and anything involving the generating end. Those combine fuel, high current and carbon monoxide risk in a confined bay, which is not a good place to learn.
If you do call, having three things ready shortens the visit - the model and spec number from the set's data plate, the hours on the meter, and a plain description of the behavior including whether it changes with load. Our generator service page covers what a mobile visit includes.
Note whether the behavior changes hot versus cold as well. That one detail narrows a fuel fault from an ignition fault faster than anything else you can tell us.
What a visit costs
A service call is $299 and covers the trip out to your RV and the first hour on site. Work past that first hour is quoted to you once our technician has found the fault, before we carry on. In Orlando, Lakeland, Kissimmee, Apopka and nearby the service call is $399 and time past the first hour runs $200 per hour. Written estimates are free and you approve one before any work starts. Call 888-759-1438.
Sources and further reading
Every figure above that came from a manufacturer or a standards body is linked here so you can check it yourself.
- Cummins RV generator installation manual (PDF) - Contains the fuel system requirement that the genset pickup must terminate above the vehicle engine pickup - the reason a generator starves at a low tank.
- Cummins RV generator operator manual (PDF) - Operating and maintenance detail for one of the common gasoline RV generator families.
- Cummins RV generator operator manual, KY series (PDF) - Operation and maintenance schedule for another common gasoline RV generator family - check the model and spec on your own data plate.
- Onan generator quick start guide (PDF) - Manufacturer summary of starting, exercising and basic operation.
Related pages on this site
- RV generator service - diagnosis, fuel system work and scheduled maintenance
- RV electrical repair - transfer switches, breakers and the coach side of the power path
- Why your RV AC is not cooling - what a soft start does and does not do
- Book a service call - mobile across Florida
RV generator questions Florida owners ask
My generator runs but nothing has power inside the RV. What is wrong?
Check the generator's own circuit breaker first - most sets have one on the unit itself, separate from the coach panel. After that, look at the transfer switch, which is what decides whether the coach draws from shore power or the generator. A generator running happily while nothing inside works is far more often a transfer switch or breaker issue than a generator fault.
How many hours does an RV generator last?
Far more than most ever accumulate, which is why storage rather than wear is what usually kills them. A set that has done a few hundred hours has barely started its life. The failures that end RV generators are overwhelmingly fuel system deterioration and neglect during long idle periods, not worn-out engines.
Should I use ethanol-free fuel in my RV generator?
It removes the moisture absorption problem that causes trouble during long Florida storage, which is a real advantage for a machine that sits. It is not a substitute for exercising the set, because stale fuel of any kind still varnishes a carburetor. If you can get it and you store the coach for months at a time, it is worth the extra cost.
Why does my generator surge or hunt up and down?
Usually a fuel delivery restriction or governor instability. A partially blocked jet or filter starves the engine, the governor opens up to compensate, the engine overshoots, and the cycle repeats. It is one of the more reliable indicators that the fuel system needs service rather than an ignition or electrical fault.
Can I run my generator overnight while sleeping?
Only with working carbon monoxide detection in the coach, an exhaust system you have verified is sound, and awareness of where the exhaust is going relative to windows, vents and neighboring rigs. This is the one generator topic where being casual is genuinely dangerous rather than merely expensive. Test your CO alarm rather than assuming it works.
How do I know what model generator I have?
There is a data plate on the generator itself carrying the model and spec number. That combination is what identifies the correct manual, service intervals and parts. Model families look similar and specifications differ between them, so the spec suffix matters as much as the model.
Do you service generators at my campground or storage yard?
Yes. Diagnosis, fuel system service, filters, plugs, oil and most electrical faults are mobile work done where the RV is parked, anywhere in Florida. Call 888-759-1438 with the model and spec from the data plate and the hours on the meter and we can tell you a good deal before we arrive.
My generator has not been run in three years. What should I do first?
Do not simply crank it. Old fuel in the tank and the carburetor is the likely problem and running it on that fuel spreads the contamination further. The sensible order is to have the fuel condition assessed, service the fuel system, then start it. Repeated cranking on stale fuel usually ends with a flat battery and the same generator.