Your RV Generator and How It Works
How the Generator in Your RV Works
I had a guy ask me one day; "OK, I am not in the least a technical person, so how does a generator work? All I know is if it gets fuel, it runs and I get electricity out of it."
This article will give you a basic overview of the different designs of RV generators, how they operate, and some basic generator service tips.
It will also describe basic differences in gas-powered generators and diesel-powered generators and how they function.
This knowledge may one day help you determine the cause of any generator problems you may have.
A History of RV Generators
In the early days of camping in the USA, there were “rough campers.” For these people, "camping" included a tent, a sleeping bag, a backpack of dried foods, bottled water, a few matches, a snake bite kit, and not much else.
And you know what? I've been there, and I've done that!
It was fun! Especially when I was young, healthy, strong, and had very little money.
And, later in life, when I was first married, and we had kids, I "rough camped" for a while. We camped at resorts, at beaches, and in the mountains. We used larger tents, nicer sleeping bags, inflatable mattresses, a small cooking grill, several coolers for real food, and a few more accessories. And that was fun too, again, for a while!
After a few years, we purchased our first Tag-Along camper. It had rudimentary plumbing, electric lights, storage cabinets, a "porta-potty,” all more comforts for dealing with the vagaries of the weather, and then, of course, we had even more accessories to make our camping more fun.
Those first old campers of ours, way back then, had your basic accessories, such as electric lights, a water pump, a propane stove, but not a whole lot else. They did not have such things as television, air conditioning, hot water heaters, microwave ovens, and all of the amenities that you will find as standard equipment on the RVs of today.
RV and Motorhome Batteries
Our first couple of Tag-Alongs, way back then, did not have a generator, just a battery. This battery was usually a deep-discharge 12-volt DC battery, the same as what is used in an automobile.
It was usually tied down onto the front end of the Tag-Along, protected in a plastic case to protect it from the weather. You charged it up before you left home.
And, considering it only had to power a few 12-volt interior lights, and maybe a small water pump to provide water at the miniature sink, it was a more than adequate power system for a good week of fun.
Over time, as some campgrounds began to provide 115-volt AC at your site, camper manufacturers added small, efficient AC-to-DC converters, so you could charge your camper battery at the site.
And once you had an RV with AC as well as DC power, along came dual-mode light systems (110-VAC and 12-VDC), as well as connections for small appliances. RVs came to include showers, electric hot water heaters, larger water pumps, microwave ovens, TVs and multiple wall receptacles.
RV Battery Box
My motorhome batteries sit on a shelf that is open to the weather on the backside. They were getting dirty, so I found this great battery box on the web and put each of my batteries in one. The boxes keep road dirt off the batteries and (I hope) make them last longer.
Appliances for Motorhomes Created a Demand for Generators
The most important electrical addition manufacturers made to RVs and campers was extra electric receptacles, in the kitchen, bathroom, bedroom, and so on.
Once the power was accessible throughout an RV, people started bringing their favorite appliances from home, such as coffee makers, hair dryers, toasters, portable TV's and radios, and on, and on, and on.
RV manufacturers had to react by adding electric breaker panels (versus those old fuses) to handle potential overloads, and heavier wiring for the current all these appliances pulled.
Eventually campers became accustomed to using such appliances everywhere: in state parks, in the woods, alongside streams and lakes, and pretty much wherever they went.
So, the next "necessary item" for the typical modern camper was—you got it—a good electric generator. We all wanted those conveniences and therefore that power, everywhere, all of the time. At the flick of a switch!
Why Have a Built-In RV Generator?
Today, the average RV owner travels much further to reach their camping destination than even a decade ago.
On these longer trips, they often have to stop for the night at an interim campground or two along the way, or do the Wally-World thing (overnight in their parking lot?), or stop and sleep at a rest stop or other public parking lot. And the next morning they eat, pack up, and hit the road for the next part of their trip.
Of course, while on these longer trips they usually have a fridge that is packed with fresh food and beverages, and so they are able to eat very good meals and then sleep in that comfortable bed in the camper.
Myself, I have pulled into a rest area around 6-7 PM, fired up my generator, turned on the TV for local news and weather, eventually cooked a light dinner, and watched my favorite network TV shows, all with the AC running, and finally I'd go to sleep in my nice cool camper, on my own bed.
Often, in a rest area, I will leave the generator running all night, along with a couple of roof fans, recharging my house batteries and providing just enough background noise to drown out most of the outside noise from the comings and goings of other vehicles, campers, and truckers in the rest area.
I will get up in the morning, make fresh coffee, toast a muffin, and boil or fry an egg for a hot breakfast, maybe even take a hot shower, all before I shut down my generator and continue my trip.
Now I have a motorhome, with a built-in generator that is designed especially for the appliances in my RV. It is ridiculously easy to operate. And, often, parked right beside me, might be a Class-C, a Fifth-Wheel, or a Tag-Along, all with their own built-in generators, providing those creature comforts we are all spoiled with. Ain't camping life a rough life?
Portable Generators Came First, Then the Built-In RV Generator
Before camper owners started buying their RV with a built-in generator, many of them used portable generators.
They were convenient, and reliable. But they took up a lot of room when it came to storing them.
And not many models were quiet enough for use at night in a campground.
Because by this time the RV manufacturers knew that they had to do whatever as necessary to make their motorhomes more competitive than the others, they all started installing built-in generators in the larger motorhomes.
So, Now We Can Talk About Generators and Motorhomes
Built-in generators are so popular today that it's actually hard to find a motorhome that doesn't come with a nice generator built into it.
Why only in motorhomes, you might ask, and not in towable campers?
Well, a motorhome already has a fuel source for its engine, so the generator can be fueled by the same tank as the engine.
You rarely see a towable camper with a built-in generator, because to install a generator the manufacturers would have to install a fuel tank as well.
Troubleshooting: Reasons Why Your Generator Might Not Start Up
1. Fuel Supply
Every motorhome manufacturer today that includes an electric generator in their design also includes one little innovation: that is, they no longer attach the fuel line for the generator to the bottom of the RV’s main fuel tank.
They attach it now about a quarter of the way up from the bottom of the tank, so that it will only pick up fuel if the main fuel tank is more than 1/4 full.
This redesign was intended to avoid the situation where a camper out in the middle of nowhere runs the generator until it uses up so much fuel that there isn’t enough left to drive the vehicle back to a gas station and fill it up again.
So, if you are out somewhere rough camping and your generator won't fire up when you turn it on, check that your fuel tank is more than 1/4 full. And if your rig is parked on a slant, the tank may have to be even more than 1/4 full for the fuel level to reach the line to the generator.
2. Oil Supply
Many built-in generators won't start if the oil is down to one quart or less. And that's one reason why you should check the oil in the generator often.
3. 12-Volt Power Switch
To get started, your built-in generator needs power from the 12-volt Coach battery system in your RV. If the power switch to the coach's 12-volt accessories has been turned off, for example to prevent power drainage while the vehicle is in storage, the generator can't get power. Many RV owners don't know where this cut-off switch is. Find it and check it.
4. Fuel Pump or Fuel Filter
If the generator won't run, or runs sluggishly, and there are no fuel, oil, or connection problems, the generator itself may be bad.
But the problem is usually the fuel filter (which can become clogged, especially when using diesel). Fuel filters are common culprits but they are pretty cheap to replace.
Understand How Your Generator Works!
If you use an electric generator, you should have at least a general understanding of how it works, so that you can make sure your generator is there for you when you need it.
First of all, it has to start, easily and quickly. Then it has to run, smoothly, automatically, and efficiently.
Here are some basic facts about generators.
There are four basic functional sections of an electric generator. Of course you can tear a generator down to smaller and smaller working pieces, but I am not trying to teach you to be a mechanic, rather I want you to know how one functions from a high level.
That way you will understand the importance of those pesky preventive maintenance parts in a generator, and why they need to be monitored and even replaced occasionally.
The four functional parts of a generator are:
- a fueled mechanical motor
- a fuel supply system
- an electric starter motor
- an electric power generator
1. The Fueled Motor
Generators are run by a fueled motor of some kind, usually gasoline or diesel, as I have mentioned.
The motor uses the combustion of the fuel to keep the crankshaft of the motor turning.
Today, there are many different designs of motors, from the simple to the complex. An example I can describe here is a simple, cheap gasoline-powered lawn-mower motor with two cylinders.
Each cylinder contains a piston which is connected to a crankshaft. Each cylinder has a fuel input port and an air input port, as well as an exhaust port. The two cylinders are designed so that when one cylinder is firing, the other one is getting fuel and air to fire.
This whole assembly is mechanically designed so that you will get continual firing and exhaust, and the crankshaft will turn from all of this combustion action in the cylinders.
In a diesel motor, there is no spark plug; the combustion is achieved via high compression of the diesel fuel.
One of the most important things to watch is your generator motor's lubrication. Your generator motor uses oil to keep all of those mechanical parts running smoothly as they rub against each other. You should always make sure you have enough oil in the motor, and always change the oil filter on the motor as scheduled by the manufacturer.
Some electric generators are air-cooled. But a larger generator will often have a cooling system similar to an automobile engine, using a water-and-antifreeze cooling liquid.
If your generator is one of these, you need to check the level of the coolant reservoir regularly, and assure the level is kept within the indicated limits. And, as with an automobile, you need to consider the amount and strength of antifreeze appropriate for the season and region where you are using the generator.
2. The Fuel System
Gasoline Combustion: A Simplistic Explanation
A stand-alone gasoline generator gets its fuel from its own fuel tank. The gasoline is usually fed via gravity to the carburetor on the motor. In the carburetor, the gasoline is mixed with air (or oxygen) and injected into the motor's cylinder where a spark is provided via a spark plug.
This spark causes the mixture to explode, and forces the piston in the cylinder to slide open, which moves the crankshaft of the motor. All of this is done via mechanical timing, which keeps the motor turning.
In a coach with a built-in gasoline generator, the functions are the same, but the gasoline comes from the coach's main fuel tank.
Diesel Combustion: A Simplistic Explanation
A stand-alone diesel generator also gets its fuel from its own fuel tank. The fuel is fed via a fuel injection system directly into the cylinder of the motor, and mixed with oxygen there. Then the piston moves to compress the diesel fuel to the pressure necessary for it to explode.
No spark is needed, nor is there any carburetor on a diesel motor. When the fuel mixture explodes, it forces the piston to move to the open position in the cylinder, thus turning the shaft of the motor. This cycle repeats itself via mechanical timing which keeps the motor turning.
Fuel System Service
These parts of a generator's fuel system will need occasional service:
- The fuel filter
- The carburetor fuel jets (on a gasoline motor) or diesel fuel injector jets (on a diesel motor)
- The spark plugs (on a gasoline motor)
If the fuel filter gets dirty or blocked, even partially, then too little fuel will be provided to your motor, and it may not run. If it runs, it may miss or turn off sporadically.
In a gasoline generator, the spark plug will, over time, need replacement. It can become fouled with carbon or dirt, and eventually the contacts will wear out.
If the fuel jets are blocked, then the motor may not run for lack of fuel. If they are worn, the wrong mixture will be provided, and the motor will not run properly.
You can replace a fuel filter, and maybe even a spark plug if it is placed conveniently for you on the engine, but when it comes to replacing the fuel jets you really want a trained mechanic to deal with that.
3. The Electric Starter Motor
Your generator has to be started by turning the motor until it goes through several firing cycles of several cylinders. Once the motor gets going, the starter is no longer needed, as the motor will run on its own.
The starter motor is electric and requires voltage to turn, and in your coach, this voltage comes from your house batteries. It takes a significant amount of current to turn your generator motor, even more with a larger-sized motor/generator combination.
You should also know that because a diesel motor uses a higher level of compression to force combustion, it is harder to turn, and thus its starter requires significantly more current than a gasoline motor to get it to turn at a high enough speed.
The reason you need to know this is simple. There are some hefty wires coming from your house batteries to the generator of your coach. And you should check occasionally to make sure the following things are true:
- Your house batteries are in good shape, and hold a strong charge.
- Your connections to your batteries, as well as to your generator are clean, without corrosion, and attached tightly at each end.
- When your starter turns, it doesn't make weird sounds, and is actually turning the generator motor's shaft.
4. The Electric Power Generator
Here’s a simple description again, this time of the part of your generator that actually generates the electricity.
Picture a coil of wire wrapped around a shaft. This coil of wire spins inside the opening of a larger outer coil of wire. Visualize that the shaft of your generator is attached to the shaft of the fueled motor and turns when the motor turns.
Leaving all of the electro-mechanics out, if you connect a voltage to the ends of the inner coil of wire, and spin it inside the outer coil of wire, and then attach something like a charger or inverter to the ends of the wires on the fixed outer coil of wire, you will get AC voltage from these output wires.
Again, I have left out all of that complex design stuff like the magnetic metals used, the complex ways that the wires are wound, the contacts used, and on and on. I just want you to know that there are parts in here that sometimes require maintenance.
Luckily, the generator itself is designed for minimal maintenance, and if there is a problem with yours, you need to see a trained generator mechanic.
Even though I have owned several motorhomes, I purchased this book so I could see how other brands are wired. I found information on several different RV electrical wiring systems that I wasn't aware of and I have been able to help other RV owners who had electrical problems.
Generator Parts That You Can Check and Service
I will now list the things that can and should be serviced periodically. If you want your generator to last for decades, replace the parts below at the intervals suggested by the manufacturer. Running the generator once a month or so is a good idea.
The fuel pump and fuel filter are the parts that most commonly give trouble. Here's an article with pictures on how I replaced a fuel pump and fuel filter in my Onan 5500 generator.
If any of the terms or abbreviations in use here are new to you, check out my article on electrical terms. It might help.
Tips for Generator Maintenance
With a gas generator, I suggest keeping your old spark plug after you replace it, in case you need one in an emergency.
Keep a spare, just in case, especially if you use your generator a lot.
If the generator is running rough, sometimes you can remove and clean the air filter for better performance, as a temporary solution.
Only a trained mechanic should service this.
Only a trained mechanic should service this.
Check for wear or leaks, especially if you smell fuel.
DC and AC wiring
Inspect wiring periodically for wear, torn insulation, and loose connections. If necessary, get a trained mechanic to replace wires.
Check oil level and change as needed, or when suggested by the manufacturer.
These are easy to change. Follow the manufacturer’s instructions.
Use the manufacturer’s suggested coolant (anti-freeze) mixture. Monitor the level regularly when using the generator, and keep the level within the limits marked on the reservoir.
Comments Are Welcome
Hopefully, I didn't lose you with my attempt at simplifying how an electric generator functions.
If you experience a problem with your generator, check the comments below for troubleshooting tips, and feel free to post a comment about your own issue!
Basic RV Generator Maintenance
RV Generator Troubleshooting
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
We just bought a new, never used Holiday Rambler Diesel Navigator 38F. The generator is a Cummins, and it won’t turn on, it just ticks. The batteries are brand new and show that the voltage is good, and I have the motorhome plugged into our home. Can you tell me what is wrong?
Your generator starter is powered by your coach batteries.
Make sure your Cut-Off switch is in the right position to allow power to the interior equipment, and make sure the batteries are fully charged.
Also, make sure the batteries have water in them.
If you just hear a clicking sound and the generator is not turning, then your battery voltage is most likely your problem.Helpful 2
I have an RV bus with an onan 7500 gen. The generator starts and runs all of the coach except a/c units. When I turn either a/c unit on the front or back it shuts the gen down in about 5 min. Gen repair shop are telling me my coach batteries are bad. Will that make gen shut down when a/c are turned in?Helpful 1
Does a Onan Microquiet 4000 have a fuel filter?
Yes, the normal configuration of an Onan generator in an RV is to always have a fuel filter on the input fuel line. They are often configured with the fuel filter screwed onto the end of the external fuel pump.
Can you run an Onan generator with the engine off?
Yes, you can run your motorhome's generator when the engine is OFF.
Your Electrical system in your RV is designed for your generator to support all of your electrical equipment while you are camping at a site where there is no electrical hookup.Helpful 1
My Generac seized up, so I'm shopping used Onans. Do you think I need to buy the Onan install kit to put it in?
I would recommend that you get the install kit. Otherwise, if you have problems in the future, you will probably suspect your custom wiring as your culprit when things go wrong.Helpful 3
© 2010 Don Bobbitt