AC Generator Service

Generator lead - picture


Proper care and maintenance are critical for optimal performance of a motorhome’s auxiliary power plant


If you are a DIY kind of person, the annual service of your motorhome’s AC generator should be on your to-do list. In many ways, the generator engine is just like the main coach engine, only smaller. Servicing a generator consists of many of the same items as servicing the coach engine, but of course it is a lot easier due to its size and location.

On many Class A diesel motorhomes, the generator is placed in a front-mounted slide tray that allows easy access to the engine. A gas-engine motorhome will usually have a gas generator and a diesel-engine motorhome will usually have a diesel-engine generator because the generator usually shares the same fuel tank as the chassis.

For this article, we are specifically covering the annual service of an Onan Quiet 7500 diesel generator. However, even if you have a slightly different diesel generator or a gasoline- or LP-gas-fueled generator, many of the service items will be similar. Service intervals vary by make and model, so it’s important to refer to the owner’s manual for the generator in your coach to make sure you are servicing the correct items at the correct time. Most of the service intervals for motorhome generators are expressed in terms of hours of usage (with a minimum of annually) so use your hour meter as the guide in order to decide which items need to be serviced.

One of the most important things you can do to keep your generator functioning properly is to use it regularly. Onan suggests that a generator should be run under a 50 percent load once a month for at least two hours. So even if you don’t ever boondock, you should operate the generator as stated above to keep the generator in top condition.

Since this procedure involves being under the motorhome and handling fluids such as used engine oil, there are a few safety precautions. First, set the parking brake on the coach AND chock the wheels to prevent the motorhome from rolling. On most rear-engine diesel coaches, the front generator slide can be opened with the motorhome at normal ride height and you can service the generator without using any jacks. If you decide to use the leveling jacks to make the generator access easier, you will also need to use safety jack stands as well.

Another important aspect of this procedure is dealing with the drained oil and/or coolant. Used oil should always be collected for proper disposal at a recycling center, as should the coolant, with both in separate and sealed containers. Many engine coolants are attractive to animals, so never leave a drain pan where it can be accessed by pets, etc.


After referring to the service manual for your motorhome’s generator, you will need to purchase the various filters and fluids for your project and then gather up a few hand tools. The owner’s manual will have specifications for fluids, such as the engine oil and coolant. Remember, diesel engines and gasoline engines require different fluids, so pick the right one for your generator.

The only other thing you will need is a catch pan and an hour or two of your time, and you are ready to tackle the annual generator service and save yourself a lot of money in the process. Let’s take a look at the steps we took to service our Onan diesel generator.


  1. REF: Onan suggests that a generator should be run under a 50 percent load once a month for at least two hours –

    I just looked up what Onan / Cummins has to say about their Diesel Generators and reading their current owners manuals it says that,

    “If you own a diesel generator,
    regular exercising can help reduce internal condensation,
    and helps keep seals lubricated”.

    There is NO recommendation of running under a load any amount of time.
    Remember this gen set uses an Alternator and a Inverter and the brushes of an alternator carry very small loads compared to old fashioned generators. So running a diesel engine and alternator for extended periods just wears it out.


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