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Servicing an Onan Diesel Generator

This auxiliary source of AC power requires regular maintenance.

Download a PDF of this article that includes step-by-step photo instructions.

By Mark Quasius, F333630
February 2017

With the generator's slide-out extended, the unit's service points are accessible.The RV generator, an important component in a motorhome’s electrical system, provides 120-volt-AC power (or, depending on the model, 120/240-volt AC power) for dry camping or when the coach is being driven. Onan, a product line of Cummins Power Generation, is the most popular brand of RV generator.

This article concentrates on the procedures necessary to service generators in the Cummins Onan Quiet Diesel (QD) Series, with an emphasis on the servicing procedures for larger models (10 kilowatts and 12.5 kilowatts). Even within the Cummins Onan product line, service intervals and procedures vary depending on the generator size and date of manufacture; while some of the basics may apply to other diesel generator brands and models, be sure to check the owners manual for specific instructions.

The generator is housed in a sound-deadening enclosure and can be mounted to a slide-rail mechanism, which allows the generator to be extended from the front cap for easy access. Depending on the year and model, a Cummins Onan QD generator may be equipped with either an Isuzu or a Kubota water-cooled diesel engine, which are among the most reliable in the industry. The engine is directly coupled to the generator, unlike belt-driven units found in many gasoline-powered generators.

Cummins Onan diesel generators range from 3.2 kilowatts to 12.5 kilowatts. A four-pole generator normally must be operated at 1,800 rpm to provide 60 hertz of power, but the smaller sizes use inverter technology to allow variable-speed operation that raises the engine rpm as the load increases. Larger generators — 10-kilowatt or 12.5-kilowatt — do not use inverter technology; they run at a constant governed 1,800 rpm regardless of how much load is applied.

The Cummins Onan QD generator has an operation switch on its enclosure. A second remote switch usually is mounted inside the coach. Holding the switch in the “Run” position turns on the glow plugs to preheat the engine. The switch may have to be held a few seconds before the engine will crank. This is normal; the time varies depending on how cold the ambient temperature is. Once the glow plugs  have warmed the engine enough, the starter engages to crank the engine. And once the engine starts, the switch can be released. Most switches have a red pilot light that illuminates when the generator is running. To stop the generator, hold the switch to the “Stop” position for a second or two.

If a generator runs out of fuel or its fuel filter is changed, the unit won’t start without first priming the fuel system to bleed out the air. Hold the operation switch at the “Stop” position and continue holding it. In some cases, you may be able to hear the generator’s electric fuel pump run, but, regardless, the red pilot light will illuminate. Release the operation switch after 30 seconds; the fuel system should be primed. If the generator fails to start, repeat this procedure. Note that this procedure can be performed at the generator or at the remote switch in the motorhome.

Cummins Onan QD generators contain extensive diagnostics to detect system faults, which are displayed and retrieved from the status light on the operation switch. When the light flashes once, then rests and repeats, it indicates a shutdown due to high engine coolant temperature. If the light flashes twice, rests, and repeats, it indicates a low oil pressure fault. When the light flashes three times, rests, and repeats, it indicates a service fault; in that case, a second step is required to retrieve the specific fault code.

To retrieve a service fault code, you may first have to wake up the control by pressing and holding the “Stop” switch. Then tap the “Stop” switch once. This displays a two-digit service code, which consists of two sets of flashes. The first set of flashes represents the first number of the code; after a pause, the second set of flashes indicates the code’s second number. After a long pause, the sequence repeats. For example, fault code 36 appears as blink-blink-blink — pause — blink-blink-blink-blink-blink-blink. Press the “Stop” switch to halt the flashing code. A complete list of fault codes appears in the back of the operators manual or can be found online in the Cummins Onan RV Generator Handbook, To watch a Cummins Onan video that demonstrates how to retrieve a fault code, visit

Basic Requirements

It’s very important to exercise a Cummins Onan generator regularly, because generators that sit unused have more problems. Cummins Onan owners manuals state that the generator should be run at least two hours every month if use is infrequent. The generator should be operated at approximately half power to drive off moisture from inside the generator and to remove oxide from electrical contacts and generator slip rings. Running the unit at light or no load won’t accomplish that. Exercising the generator also protects your warranty. If an issue develops later and the hour meter shows that the unit hasn’t been run much, warranty coverage may be denied.

A good prestart checklist begins with extending the generator slide and removing the side access cover. Then check the engine oil level and look quickly for anything out of place, such as fuel or coolant leaks. (Note: On the 6-kilowatt and 8-kilowatt Cummins Onan QD generators, the oil is checked via the dipstick near the stop/start switch.) Obviously, if you are driving and decide to use the coach’s air conditioners, you won’t make these checks right then, but don’t ignore them. Make it part of your routine to check the generator regularly at a campsite, depending on how often it has been used.

Refer to the generator’s service schedule to see what is required, and then obtain the correct filters and fluids. If parts must be ordered, the generator model number and spec revision will be needed. This information appears on a placard mounted to the generator near its circuit breaker and main operation switch. The placard also indicates the oil capacity of the crankcase as well as Cummins Onan part numbers for the most common filters.

The most common service interval is 250 hours or one year, whichever comes first. It consists of a basic engine oil and filter change. Servicing of fuel and air filters comes at 500 hours or two-year intervals. However, a brand-new generator requires oil and filter changes after 50 hours to remove contaminants from the oil during the engine’s break-in period. Refer to the operators manual for the specific requirements of your generator model.

Engine Oil And Filter

To change the oil and oil filter, a 7/8-inch socket and ratchet, a drain pan, and an oil filter wrench are needed. A pair of rubber gloves and some rags or disposable shop towels also come in handy. Cummins Onan generators are known for having extremely tight oil drain lugs when they leave the factory. To loosen the drain plug the first time, use a large breaker bar, possibly with a pipe for more leverage. Many owners also find that the factory has installed a smaller-diameter oil filter; don’t worry about the difference in size between the initial break-in filter and the replacement. However, your oil filter wrench may not fit the smaller filter, so a large pair of channel-lock pliers or some other tool may be needed to remove that filter.

Begin by warming up the engine. Run it long enough to heat the oil and allow it to flow through the system, ensuring that any sludge is dissolved and suspended in the hot oil. Shut off the engine and switch off the circuit breaker on the generator. Also be sure to disengage the Automatic Generator Start module, if the motorhome is equipped with one, so that it cannot start the generator during service. Once everything is set, extend the generator and remove the service access cover from the driver’s side of the generator, if you haven’t already.

Before removing the drain plug, remove the oil filler cap or the dipstick to allow the crankcase to vent. This minimizes splatter when draining the oil. Remove the drain plug and let the old oil flow into the drain pan. Oil capacities range between three and six quarts, so be sure the pan is large enough.

While the oil is draining, wipe off the drain plug and inspect the gasket. If the gasket has a rubber sealing insert, check for tears or rips, which could cause a slow leak. Replace the gasket if necessary. This is also a good time to inspect the fan belt and the engine.

Check the operators manual for the correct engine oil. It should be rated for use in diesel engines. Whether the generator is equipped with an Isuzu or a Kubota engine, it most likely requires API class CF oil or better. It isn’t as picky as the emissions-controlled engine in your motorhome, but in most cases, you’ll want to use the same oil as in your coach to keep things standardized. You may also want to consider a synthetic oil for the generator. A synthetic oil’s viscosity is typically 5W-40; in comparison to 15W-40 nonsynthetic oil, synthetic oil flows better at low temperatures, which can be a big help when starting the generator in cold weather.

The oil filter is easily accessible on the side of the engine. Removing the filter will allow some oil to dribble down the side and possibly make a mess. Fortunately, most engines have a small sheet-metal chute that directs the oil away from the engine. Hold a small container beneath the spout when removing the filter and you’ll avoid the cleanup work. Apply a light film of fresh oil to the new oil filter’s gasket before installing the filter to prevent binding as it is tightened. Tighten the filter by hand according to the directions.

Fuel Filter

At first glance, changing the fuel filter looks to be a challenge. The generator must be compact to fit into a motorhome, so everything is packed fairly tightly inside the generator enclosure. The fuel filter, buried inside the enclosure, can be reached through the service access opening, but it is very difficult to access the fuel lines that attach to the filter. The best method is to remove the nut that holds the filter to the mounting bracket. You’ll need some basic tools, such as a 10-mm wrench or socket set and a set of open-end wrenches ranging from 7/16-inch to 11/16-inch. A bit of fuel will dribble as you remove the old lines, so keep some rags or disposable towels handy.

Detach the mounting bracket by removing the two bolts that hold the bracket to the sidewall; the bolts easily can be accessed with a 10-mm wrench or socket from the outside of the generator housing. The bracket can then drop free of the filter. The fuel lines are long and flexible enough that the filter can then be pulled out through the access opening and the lines can be removed. After the lines have been attached to the new filter, the assembly can be placed back into the generator housing and the bracket reattached. Once the filter has been replaced, you’ll need to purge air from the fuel lines. Do this by following the priming procedure detailed earlier.

Cooling System

The Isuzu and Kubota dry-sleeve diesel engines in Cummins Onan QD generators do not require supplemental cooling additives (SCAs) or organic acid technology (OAT) coolants like most of the larger wet-sleeve Cummins engines that power a motorhome. The Isuzu and Kubota engines need only a basic green heavy-duty coolant that is mixed with distilled water in a 50-50 ratio to prevent damage from freezing temperatures. However, the engines can take a higher-tech coolant with no ill effects. If your motorhome uses Final Charge OAT, for example, this same coolant also can be used in the generator to help standardize your spare supplies inventory. Just be sure to drain the previous coolant before replacing with new.

The radiator’s plastic reservoir tank contains excess coolant that expands when hot. Once the engine cools, the coolant is drawn back into the radiator via the two-way radiator cap. The level in the transparent tank can be observed from outside the generator through a sight glass type of opening. Under normal generator operation, the coolant level can be monitored and coolant can be added without having to open the service access panel. However, if the system is quite low, such as during a coolant drain or bad leak, the cooling system must be filled via the radiator cap. The radiator cap and filler neck are connected to a long hose. The filler neck and cap slide into a bracket beneath the coolant reservoir tank and can be removed easily without tools. The filler neck is then lifted high so that it is above the engine; the cap can then be removed and new coolant added.

The coolant should be flushed and replaced every 1,500 hours or five years, whichever comes first. It’s highly unlikely that you’ll accumulate that many generator hours, so the odds are that you’ll change coolant after five years.

When it’s time to replace coolant or you want to use a different coolant, it’s not hard to do. Other than some rags and a funnel, all that’s needed is a 3/8-inch hex key wrench. Slide the filler neck out of the bracket, raise the filler neck as high as it will go, and remove the radiator cap. Place a drain pan beneath the generator, remove the 3/8-inch hex Allen plug from the bottom of the radiator, and allow the coolant to drain.

If you feel it is necessary to flush the system after the coolant has finished draining, use a radiator flush solution. When finished, apply pipe thread sealant, such as Teflon tape, to the drain plug and reinstall the plug. Tighten the plug just enough to prevent it from working loose or leaking; overtightening could result in damage to the soft aluminum radiator.

Finally, refill the system with a preformulated coolant or a 50-50 mix of pure coolant and distilled water. To prevent mineral buildup in the cooling system, tap water is not recommended. Keep the filler hose high above the engine when filling. Once the cooling system is full, replace the radiator cap and place the filler neck assembly back into the bracket beneath the reservoir tank. Then add enough coolant to the reservoir tank to reach the “full-cold” level.

Air Filter

For 10-kilowatt and 12.5-kilowatt models, the air filter should be replaced at 500 hours or every two years, whichever comes first, but it should be checked more frequently. If the generator is used mainly to power air conditioners while the motorhome travels on interstate highways, the filter probably will not have to be replaced for two years; at that point, the paper filter element is more likely to fail because of age rather than dirt accumulation. But when a generator is run often at rallies, or while the motorhome is driven over dusty campground roads, airflow beneath the generator is likely to stir up dust that can accumulate in the air filter, requiring more frequent changes.

Like the fuel filter, the air filter is packed tightly inside the generator housing. Some filters are easy to reach, but for others it’s best to remove the three bolts on top of the housing that hold the coolant reservoir tank in place; this allows the tank to be moved off to one side. A 10-mm wrench or socket is needed to remove the bolts. Then unsnap the two metal clips that secure the cover to the housing and remove the element from the housing. Replacing the filter is simply done in the reverse order. It may be necessary to loosen some of the air cleaner mounting bolts to make it easier to operate the top-most snap latch for the air cleaner cover.

Spark Arrestor

The Cummins Onan generator is equipped with a spark arrestor so that the unit can be operated legally at U.S. Forest Service sites. The spark arrestor is a filter screen on the muffler that traps burning embers or soot that could cause a spark and start a grass fire if allowed to exit the exhaust ports. While this is more of a necessity on a portable gasoline-powered generator, it’s not as pertinent to a large diesel generator. But the Cummins Onan QD has a spark arrestor, so it is a point of service.

According to the service schedule and Forest Service recommendations, the spark arrestor should be serviced at 250 hours or one year, whichever occurs first, the same interval as the engine oil change. Again, refer to your operators manual. The procedure is to remove a small pipe plug from the muffler and then run the generator at a good load for five minutes to eject any soot from that area. Shut down the engine and allow the exhaust area to cool before replacing the pipe plug.

The vast majority of people report that the muffler discharges no soot because the generator runs so clean. In fact, I waited seven years before pulling the spark arrestor plug on the Cummins Onan 10-kilowatt generator in my previous coach. The muffler was clean and expelled nothing. However, soot may build up and restrict the exhaust of an older generator that has been operated for many hours, or if the unit is frequently used for high-altitude camping. In 7.5-kilowatt and 8-kilowatt Cummins Onan generators, it’s fairly easy to get to the spark arrestor plug, which can be seen from underneath the generator. But on 10-kilowatt and 12.5-kilowatt Cummins Onan generators, the entire top of the generator housing must be removed to get to the spark arrestor plug.

Final Tasks

Once all service tasks are complete, clean up spilled fluids and close up the housing. Run the engine and allow the cooling system to warm up to ensure that any air in the cooling system has been purged. Then shut off the generator. Allow everything to settle and cool and then recheck the fluids.

The oil level should be right at the full line on the dipstick. Excess coolant from the reservoir will be drawn back into the radiator, so the coolant should be at the “full-cold” level. When the generator is running and hot, the coolant should be at the “full-hot” level. If air was purged when the system was running, the reservoir tank will be low, so fill it to the correct level.

When you are finished, record the date and generator hours on the service logs so you’ll know when the next service is due. The logs also can help substantiate a warranty claim, if necessary. If you have an electronic management system, such as a SilverLeaf Electronics product or a Cummins Onan Energy Command 30 Automatic Generator Starting (EC-30 AGS) system, reset the “Service Due in xxx Hours” display. The Cummins Onan diesel generator is now ready to provide many hours of reliable service, and you have the satisfaction of a job well done.

Further Info

Cummins Power Generation
1400 73rd Ave. N.E.
Minneapolis, MN 55432
(800) 888-6626