This valuable power source needs “exercise” in order to perform properly.
By Mark Quasius, F333630
December 2014
The generator is a key component of a motorhome. When shore power is not available, the generator powers many electrical devices, from the microwave oven that heats our meals to the rooftop air conditioner that cools the coach. And, yet, the generator is undoubtedly one of the RV’s most overlooked items. This article focuses on a gasoline-powered Cummins Onan generator. That brand is one of the most popular found in RVs, but the principles of operation and maintenance apply to all gasoline models.
Like any engine, a generator needs regular maintenance. It also requires “exercise,” but that task often is neglected. In fact, a generator works best when it is run frequently; not using it actually does more harm than good. Cummins Onan recommends exercising your generator by running it at a minimum of 50 percent load for two hours every month. The reasons are as follows:
- The generator’s windings can accumulate moisture. Applying a load for a reasonable period of time allows these windings to heat up and cook the moisture out of the generator.
- Running the engine helps to distribute oil around the crankcase, thereby lubricating critical parts. And that helps to stop rust and to prevent seals from drying out.
- Most important, exercising the generator keeps the fuel system in good working order.
Gasoline degrades rather quickly; its shelf life is several months in a clean, sealed container. After that, it may leave behind varnish and gum that can plug and destroy fuel systems, especially carburetors. The shelf life of gasoline that contains ethanol is even shorter. If the fuel varnishes, it will begin to plug up or restrict the minute galleries and passages inside the carburetor, causing the engine to surge, starve out, or even fail to run.
Properly exercising the generator ensures that the gasoline flushes through the system and does not degrade in the carburetor. It’s important to bring the unit up to operating temperature and to run it nonstop for at least those two hours every month, as Cummins Onan recommends. Short-cycling an engine by operating it for just a few minutes will promote unwanted condensation. You want to apply an adequate load to your generator and run it long enough to create sufficient heat. Typically, air conditioners and heating elements, such as an electric water heater, offer enough amps to place a good load on the generator to create that heat.
I recently assisted full-time RVers who weren’t aware of these exercise requirements. Their generator hadn’t been run in more than a year, because they were plugged in to campground power and hadn’t moved. Their generator cranked over but just wouldn’t start. I suspected the problem was fuel-related, but I ran through the usual tests just to confirm. I removed and inspected the spark plugs, which were soaking wet. The spark tested good, and there was plenty of compression, so that pretty much pointed to the fuel.
I removed the air filter so the carburetor was visible. I then held the on-off switch down to the prime position. I heard the fuel pump kick in, and I waited as gasoline poured out of the carburetor inlet. This confirmed that the fuel pump was working fine and that the fuel filter wasn’t plugged. It also indicated that the needle and seat were stuck in the open position, which explained why the spark plugs were soaked. So, the basic problem was not a lack of fuel, but rather too much of it.
I disconnected the fuel hose, governor linkage, and wiring harness from the shutdown solenoid; removed the carburetor from the engine; and set it aside. I then removed the solenoid valve, which also held the float bowl on, and I inspected the carburetor. Green gunk had formed at the bottom of the float bowl, in the main metering jet, and on the float’s needle and seat. I am used to rebuilding carburetors, but EPA regulations render such carburetors not serviceable. No rebuild kits are available; the float pin is pressed in place and not removable.
I tried a quick fix by spraying carburetor and choke cleaner into the metering gallery, around the needle and seat, and into the bowl. The needle and seat eventually opened and closed, but I never got anything really clean. I reassembled the carburetor, reinstalled it, and started the engine. It ran erratically. I constantly had to work the choke by hand to get it to stop surging and starving. Obviously, the damage was too extensive, and the carburetor needed to be replaced.
The crew at the nearest Cummins-Onan service center said they had a good supply of such carburetors, because many motorhome owners fail to properly exercise their generators. So, after spending $242 for a new carburetor, we were ready to finish the job. The new carburetor came with a new shutoff solenoid, but it had a different fuel inlet that eliminated the screw-on secondary filter previously attached to the carburetor. As a result, a longer hose was needed. We got that, along with hose clamps, from a nearby auto parts store. Constant-tension clamps were chosen rather than standard worm-drive clamps, so as to provide extra security on a pressurized fuel line.
After replacing the carburetor intake gaskets on each side of the heat isolator, we installed the carburetor and connected the new fuel line. While we were at it, the fuel filter and air cleaner element were replaced. After holding down the priming switch for a few seconds, I started the generator. It ran smoothly. After it warmed up, we shut it down and changed the engine oil and filter. The remainder of the year-old gasoline was depleted by taking the motorhome for a long drive. Then the tank was topped off with fresh gas.
The owners intend to travel more often with the motorhome this year, add STA-BIL fuel conditioner to the gas, and exercise the generator every month. A $242 lesson learned.