Gear Illustration


Understanding the inner workings of a motorhome generator and how it should be maintained can save time, money and trouble


If you’ve ever endured a blackout, you understand how easy it is to take reliable, accessible power for granted. You stumble around lighting candles, and even after hours of living in relative darkness, you still find yourself reaching for light switches in vain. Though frustrating, you know the electric company will fix the problem and life will eventually return to normal – but in a motorhome, it may not be that easy. While it’s true that the 12-volt DC system will keep your lights on (for a while), other modern conveniences that run on 120-volt AC power (like the TV, air conditioner, microwave and other appliances you may bring) won’t be available if the generator goes down when you’re boondocking. Your best defense against such a scenario is regular generator maintenance.

Most of us wouldn’t think of buying a car, driving it a few miles each summer, and then ignoring it for the rest of the year, but that’s essentially what a lot of motorhome owners do when it comes to their generator.

A generator is comprised mainly of an engine, one that shares a lot of similarities with other engines including (in the case of a gasoline generator) a carburetor, air and oil filters, belts and other components. You may have grown accustomed to that power plant roaring to life whenever you push the start button, but all it takes is a few months of neglect and the generator can leave you in the dark – literally.

To get some expert advice on generator maintenance, we visited Smith Powerhouse Inc. in Bellflower, California, a factory-authorized service and warranty center for Cummins Onan, Honda, Kohler and Generac generators. Owner David Voloshin and shop foreman Matt Rudametkin are certified master generator technicians who have nearly 50 years of experience between them, and they’ve seen it all.

By far the most common motorhome generators in use today are the 4,000-watt Cummins Onan (known as the MicroLite, 4000, MicroQuiet 4000 and the RV QG 4000), 5,500- and 7,000-watt Cummins Onan (known as the Marquis Gold 5500 and 7000 or RV QG 5500 and RV QG 7000) and the Cummins Onan Quiet Diesel generators, so these will be the focus of this article. While the most frequent types of maintenance, such as oil/filter and spark plug changes shouldn’t prove too challenging for most DIYers (and can save some money), Smith Powerhouse recommends against more advanced maintenance procedures for two important reasons: One, you will likely do more harm than good if you don’t know what you are doing; and two, there are many ways to seriously hurt yourself. When it comes to major scheduled maintenance, it’s best to leave the heavy lifting to experts like Smith Powerhouse.

With all that said, let’s take a closer look at what’s inside those familiar green (or black) boxes.

Cummins Onan MicroLite, MicroQuiet, RV QG 4000

generators-0046 GeneratorLead
[1] This is what most of us are familiar with when we open the generator compartment door: a plastic box that tells us little or nothing about its inner workings. But the popular Cummins Onan 4,000-watt gas generators are actually very easy to service. Start by turning the two black levers to release the front cover. [2] The Cummins Onan 4,000-watt generator is a single-cylinder, air-cooled engine designed to run on gasoline. Above the start/stop switch (A) is the carburetor (B), which is fitted with a mixture screw (C) at the bottom to allow limited adjustments for altitude. The cover emblazoned with “Onan OHV” is the valve cover. Bottom right is the yellow oil filler/dipstick (D). Note the large brush assembly (E); this acts as a gasket to compartmentalize the engine components when the cover is in place, promoting proper airflow from the fan (at left, not visible) over the critical engine components. It is for this reason that you should never run the generator with the cover off, even in very hot weather. Make sure you have the ID tag info (F) on hand when ordering parts.
generators-0053 generators-0048
[3] Checking the oil is similar to any other engine: Unscrew the cap and note the level. If the generator has been properly maintained and the oil has been changed at the recommended intervals, it should be brown like this – not dark brown or black. [4] Before changing the generator’s oil, run it for at least 10 minutes to get it up to operating temperature, which helps the oil drain easily and completely. Motorhomes factory-equipped with a generator like this one (or even generator prep) will have an access panel underneath the generator compartment. Simply remove the two screws and the drain plug will be exposed.
 generators-0089 generators-0056
[5] Remove the plug, and allow the oil to drain into a suitable container. The Cummins Onan 4,000-watt generators only hold 1.7 quarts of oil, so this process goes pretty quickly. This generator doesn’t use an oil filter, so all you have to do now is replace the drain plug and access panel, and refill the crankcase with the recommended oil. [6] The air-filter housing is also clearly labeled, and the element is easy to inspect/replace. Simply remove the wing nut on the side of the housing, and then remove the wing nut that holds the filter in place. The filter can then be pulled free. This one still looks in good condition; a filter in need of replacement will be a dark color. When replacing the filter, be sure to reinstall both wing nuts, not just the one on the housing, or the filter will not be seated properly and will not clean incoming air.
generators-0063 [7] To the right of the yellow oil filler is a gray (in this instance) or black spark-plug-cover boot. Pull it down, and the end of the engine’s single spark plug will be revealed. The plug can then be removed with a common spark-plug wrench and inspected/replaced.
[8, 9, 10] These generators also come equipped with a small fuel filter that should be replaced periodically according to the maintenance schedule. Remove the fuel-hose clamp (A) first, then pull the hose off the filter barb. Next, pull the rubber gasket out of its groove to gain access to the filter. The filter assembly is threaded and can be easily removed with a 9/16-inch deep socket.
generators-0075 generators-0078
[11] An inexpensive upgrade Smith Powerhouse recommends is a second fuel filter (located inline in an accessible location) to further ensure against dirt and grit entering the fuel system. [12] The most common cause of hard starting (or not starting at all) in a motorhome generator is lack of use. Smith Powerhouse recommends running a generator every four weeks for two hours under load (such as running an air conditioner) to keep it properly exercised. The discoloration at the bottom of this carburetor float bowl means that fuel sat there for several months, and the deposits clogged the tiny orifices in the carburetor. Once allowed to deteriorate to this point, you’re looking at a $300 bill for the carburetor, plus removal/replacement labor. If running the generator regularly isn’t an option, Smith Powerhouse recommends installing a shutoff valve in the fuel line before the generator. Turn the valve off, then run the generator until it is out of fuel. Adding a fuel stabilizer (Onan OnaFresh) is also recommended.
[13] Don’t forget that the generator is more than just an engine – it’s a power-generation system. Letting it sit for long periods can cause the brushes to stick and the slip ring (shown) to oxidize, causing any number of power-delivery issues. This damage is not easily repairable and will more than likely require the attention of a professional. generators-0093
 generators-0084  [14] The Cummins Onan 4,000-watt generator is an overhead valve engine, complete with valve springs and rocker arms. Valve clearance is adjusted in similar fashion to many other engines, with a pair of wrenches and a feeler gauge. However, neither Smith Powerhouse nor Cummins Onan recommends you try this yourself. For one thing, valves that are too tight or loose will cause engine damage. And, an accidental bump of the starter can mean serious injury.
 [15] Wherever you may take your generator for service or troubleshooting, Smith Powerhouse recommends you ask the shop personnel if they have a load bank. Similar to a dynamometer for cars, a load bank can load the generator to different percentages to make sure it’s running correctly and producing the correct frequency, amperage and voltage for a given load. Smith Powerhouse conducts this test with every generator it services. generators-0088


Cummins Onan Marquis Gold/RV QG 5500 and 7000 Generators

[1] Another very popular motorhome gas generator is the 5,500- and 7,000-watt Cummins Onan generators, typically found in larger gas Class A motorhomes. It is a V-twin engine oriented on its side, with the valve covers facing up (the right cylinder is visible). Though it is a completely different design from the Cummins Onan 4,000-watt generators, many maintenance procedures are similar. Note, for example, that the oil fill/dipstick is in the same location as the 4000. generators-0109
generators-0112 [2] The air cleaner uses a paper element that is readily accessible from the front of the unit. Simply unsnap the clips and the cover comes right off. This filter element still looks good.
[3] Draining the oil on the 5500 and 7000 is simple, since it uses a petcock instead of a drain plug. Warm the engine for about 10 minutes, then put a pan underneath and open the petcock; the oil is routed underneath the coach through a rubber hose. generators-0113
generators-0114-crop [4] Unlike its smaller brother, the twin-cylinder 5,500- and 7,000-watt generators do have an oil filter that looks like a small automotive filter. It is accessed through a hole in the bottom of the generator compartment. Both the replacement filter and handy wrench are available through Cummins Onan dealers.
[5] The Marquis Gold has its spark plugs oriented on the sides of the cylinders (A), which can make them a challenge to reach. A swivel socket does the trick here. Removing the left (or No. 1) cylinder spark plug requires the removal of the air-filter assembly first. generators-0115-crop
generators-0118 [6] Depending on the year and model, the 5,500- and 7,000-watt generators may actually have two fuel filters: this secondary one, near the carburetor, and a primary one at the fuel pump. Be sure to inspect your particular generator to see if it has two; Smith Powerhouse says a lot of home mechanics don’t know about the primary filter, so they don’t replace it and wonder why the generator still doesn’t run properly.


Cummins Onan Quiet Diesel

[1] Diesel generators are typically located in the nose of the motorhome, so most of the items you’re likely to maintain in Quiet Diesel generators are located here, such as the oil fill/dipstick. generators-0096
generators-0099  generators-0100
 [2, 3] On Onan Quiet Diesel 6,000-, 7,500- and 8,000-watt generators, the radiator cap is located underneath a plate to the left of the coolant fill. Remove the fastener that secures the plate, then lift up on the plate and pull it toward you. You can then pull out the filler neck and remove the cap to check the coolant level in the radiator. Only remove this cap when the engine is cold; removing it when hot can result in severe burns.
[4] The coolant fill is just to the left of the oil fill/dipstick. It is important to note that this is an overflow bottle, similar to the one next to the radiator on your automobile. Its level goes up and down depending on generator temperature, so just because it is full does not mean the radiator is full. generators-0098
generators-0101 generators-0102
[5, 6] The air-filter cartridge is located underneath the unit at the front. It also uses two wing nuts – one for the housing, one for the filter, so don’t forget to replace both.
[7] Cummins Onan Quiet Diesel generators are so reliable that people sometimes forget that they need maintenance. Allowing an air filter to get this filthy not only reduces performance, but can admit dirt into the engine that will cause damage over time. Just an FYI: Replacing a generator like this one is about $8,000, plus tax and labor. generators-0103
generators-0106 [8] The oil-drain plug on Quiet Diesel generators is located directly underneath the unit.
[9] The oil filter (A) is accessed through an inspection plate underneath the unit. An oil-filter wrench may be required to remove it, and you’ll have to replace it by feel, but it’s not difficult. generators-0105



Smith Powerhouse, Inc. | 562-633-1390 | www.smithpowerhouse.com
Cummins Power Generation | www.power.cummins.com



Please enter your comment!
Please enter your name here