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Motorhome Generator Maintenance

Originally Published in MotorHome Magazine

There are many advantages to owning a large Class A motorhome but to most people the ability to have instant access to 120-volt AC power at all times is perhaps one of the greatest assets. Some motorhome travelers like to camp in wilderness areas where power or other connections are not readily available. Others spend long weekends at motor sports events where it’s necessary to make your own power. The benefit of a built-in generator during these times is priceless. Even if you don’t rely on your generator for hundreds or thousands of hours per year, you still need to perform scheduled maintenance procedures for those times when shorepower is not available.

Generators come in various types and sizes and, of course, run on different fuels. The most common fuel source is gasoline or diesel due to the ability of the engine to share the fuel tank with the chassis engine. Although there are some motorhomes equipped with LP-gas powered generators, they are less common. While the focus of this article will be on gasoline- and diesel-powered generators and how to perform basic maintenance on them, some of these tips will also apply to LP-gas generators.

Just like the chassis that propels you down the road, a generator also has an engine and that engine (depending on its type) has a crankcase, an oil filter and an air filter as well as fuel filter(s) and spark plug(s) if it uses gasoline. Gasoline-powered generators can be fuel injected or carbureted and if they are carbureted they too will require servicing, which is best left to the pros. Diesels and some gasoline generators are liquid cooled so they also have a radiator and require coolant/antifreeze servicing as well.

According to most experts we talked with, one of the most common problems with RV generators is lack of use. According to Onan, the largest supplier of gas and diesel generators, all generators should be exercised for two hours per month with a 50 percent electrical load. This exercise ensures that the entire engine and electrical windings are brought up to full temperature, which burns off moisture that can lead to corrosion and other problems.

If you can handle changing the engine oil and filters in your automobile or other powered equipment around the house, you can likely handle this project. Because there are so many different sizes and types of generators, the service requirements will vary depending on the generator. Check your owner’s manual for your brand and model to see which items need to be done.

Any time you are working on power equipment and under a motorhome there is some danger, so be sure to chock your wheels and if you are raising your motor­home using the built-in jacks it is also important to add safety stands. Before starting to work on the generator you need to disable the auto start feature if your model is equipped with one.

Finally, remember to collect all fluids that you drain, such as the used engine oil and/or engine coolant, and dispose of them properly.

Also, before you begin servicing the generator, start the genset for a few minutes to slightly warm the engine and circulate the oil and contaminants to ensure they are drained and removed properly.

This is a fairly lengthy project because of the many steps involved in heating up and cooling down the generator several times and could take two to three hours depending on how hard and fast you work. But none of the steps is particularly difficult and this is a project that can be completed by the average do-it-yourselfer in your driveway, just as we did.

If you are not so inclined you can always seek out your local Onan service center and have it service the generator. Normally when a service center services the coach it also performs a full electrical load test that requires specialized equipment and it is also capable of removing the exhaust spark arrestor and cleaning it as needed.

A full DIY service like this is not very expensive but it could make a big difference in your comfort level when you’re off the grid and need to make your own power. So don’t overlook this item when it comes time to service the rest of your coach.

Step 1. After warming the engine for two to three minutes, locate the oil drain plug and oil filter on the generator. Place a large catch pan under the generator and remove the drain plug, then the oil filter, and allow all of the old oil to drain.

Step 2. On this Onan Quiet Diesel 7500 generator the oil filter is located behind the access panel, which must be opened in order to reveal the oil filter and the diesel-fuel filter.

Step 3. Accessing the oil filter on some models can be difficult but using a rubber strap wrench allows it to be removed fairly easily. Expect some oil to spill when you remove the old filter.

Step 4. Before installing the new oil filter, lubricate the filter gasket with fresh oil to allow it to be installed and removed more easily.

Step 5. Next, install the new oil filter and tighten 1⁄2-3⁄4 turns past hand-tight. Most generator manufacturers recommend using OEM oil filters but in many cases aftermarket filters are available such as the Purolator PureOne model we used. Other high-quality brands include NAPA, WIX and Mobil 1. Cross-reference charts are available online or at most auto
parts retailers. Of course, if you prefer genuine OEM brands they are available at Camping World and other RV-parts stores.

Step 6. After draining the oil, removing the old oil filter and reinstalling the oil filter and drain plug you are ready to refill the crankcase with new engine oil. This generator specifies 3 quarts of a 15W-40 diesel-rated oil. We selected Shell Rotella T Triple Protection.

Step 7.
After refilling the generator start it up for a few minutes and then shut it down and check for leaks around the drain plug and oil filter. Also verify the oil level on the dipstick before moving to the next item on your maintenance list. Onan suggests that oil and filters be changed every 150 hours on most of its gas and diesel engines. But these intervals vary so check your model.

Step 8. The next thing to check is the condition of the air filter. Though Onan only suggests that the air filter be changed every 500 hours, it also suggests you check it more often if
you operate in dusty areas. So the next step is to locate your air filter housing.

Step 9. After removing the wing nut and cover, the air filter is exposed and after removal of the second wing nut and air filter cap it can be easily removed and inspected. This one was in good condition so it will be reused. Now you will need to reinstall the filter, covers and wing nuts.

Step 10. A generator has to have good electrical connections to the chassis for a ground and good connections taking the power from the generator to the transfer switch and main breaker. As you can see, this ground connection was slightly dirty and in need of cleaning.

Step 11.
We sprayed the connection with battery-post cleaner and allowed it to remove the corrosion and then brushed away the dirt.

Step 12. After cleaning, the connection is ready for service. If your ground is really bad it may require removal and more aggressive steps to remove corrosion.

Step 13. Locate the power terminals from the generator and clean them just as you did with the ground strap. You can even add a protective material such as battery corrosion preventative spray. They should look like this when you are finished.

Step 14.
Of course, diesel engines don’t have spark plugs but gasoline models do and those plugs need to be replaced every 450 hours, according to Onan. Most of the common gasoline generators used in motorhomes have a V-twin engine, therefore they have two spark plugs that need to be removed and replaced. OEM replacements are available from RV-parts stores. As you can see in this photo there is a big difference between a new plug and an old, worn plug, and this will affect your generator’s ability to operate efficiently. Check the owner’s manual for your generator to find the location and proper gap for your spark plugs.

Step 15.
Now it’s time to flush the cool­ant, which is a diesel generator service item and should be tested every 500 hours and flushed and changed every 1,000 hours. First you need to locate the coolant drain on the generator and open the drain and the radiator cap to allow a complete drain. Of course, make sure the radiator is cool before opening the cap.

Step 16. After the initial coolant drain (4.2 quarts total on this model), close the drain and refill the radiator (this one has a hidden radiator cap that has to be uncovered to access it) with distilled water. Replace the radiator cap and run the generator again until warm. Allow the generator to cool and drain com­pletely and repeat one more time. This ensures all the old coolant has been removed.

Step 17. After you complete the final drain of the radiator, reinstall the drain cap and this time fill with your choice of coolant/antifreeze. Because the radiator and hoses already have some residual water remaining inside you will need to add exactly half of the rated system-cooling capacity with 100 percent undiluted coolant. In our case that means 2.1 quarts. Then top off with distilled water until it is completely full. This ensures that the coolant will be a 50/50 mix of water and coolant. Reinstall the radiator cap, run until warm and recheck the level one more time. If needed add coolant until completely full. Also, drain and refill the overflow tank with the same 50/50 solution. We chose Final Charge Global Extended Life Coolant/Antifreeze, and we use the same product in the chassis engine as well. This prevents carrying around two different coolants.

Step 18. The final item(s) in a routine service is the fuel filter(s). Some coachbuilders, such as Tiffin, install an extra fuel filter (shown here) in line with the generator in addition to the generator’s own fuel filter. Onan calls for various schedules depending on the type of generator, but in general they range from 250 to 500 hours between filter changes. Here we have located the inline filter installed by Tiffin, which can easily be replaced using a screw­driver to loosen the hose clamps.

Step 19. Because diesel engines and fuel-injected gasoline engines use high-pressure connections for the generator’s fuel filters, special tools may be required  to access the special fittings. In some cases flare-nut wrenches are required. If you don’t have the right tools this may be best left to the pros. On this generator the fuel filter is in the same access panel as the oil filter and it has two flare nuts that have to be bled and relieved of pressure and then disconnected. There is a single mounting nut that has to be loosened to allow removal of the filter. For this service we are not replacing the filter as it only had 250 hours’ use and it is rated for 500 hours. After changing the filter on a diesel or a fuel-injected gasoline engine you will have to manually prime the fuel system several times (see owner’s manual for instructions) to restore fuel pressure, otherwise the generator may not restart properly.

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