By: Author John Cunningham. Published: 2021/06/29 at 9:38 am
When the power goes out, I’m always surprised by how many things I can’t do. Even my workshop door is electric! So when you need a generator, you need it right now!
You are in the right place; a minute from now, you’ll know how to get the juice flowing again.
The top two common causes of a generator that won’t start after an oil change include:
- Oil level low
- Oil level switch fault
In this post, you’ll learn more about the top two causes of a no-start and how you can fix them quickly. I will also cover some other less likely but possible causes.
1 Incorrect Oil Level Will Prevent Start
I know incorrect oil levels might seem too simple an answer, but for most, this is the problem. All generators I know of are fitted with a low-level oil switch.
The switch is a fail-safe feature, preventing engine failure. Meaning that if the oil level is low, the engine won’t start.
The very first step towards diagnosis is checking the oil level. Although we suspect the oil level may be low, too full is also bad.
How to Check Engine Oil
An overfull oil level won’t necessarily disable the ignition system like a low oil level. Nevertheless, it may cause plug fouling, which may result in a no-start.
Check engine oil level as follows:
Place the generator on level ground & locate the dipstick
Remove clean and refit dipstick
Remove the dipstick and hold upright to read level
Some dipsticks can be a pain in the ass to read; try pressing the dipstick against a flat sheet of kitchen towel to highlight the level. Dipstick types and level indicator symbols vary.
Dipstick markings are as follows:
Oil level correct
Full level – the upper mark is always the full level and the optimal oil level. Commonly marked by the letter “F” or “Max” or simply notched
Oil level low
Low level – it’s the lowest mark dipstick, commonly the letter “L” or “Min” or a notch. This is the danger area, and as you now know, it’s where the oil switch kicks in and prevents the engine from starting
Hatched area – a hatched area between the low level and full level indicates an acceptable oil level. It’s neither optimal nor critical. Adding some oil here would be nice but not strictly required
Horizontal engine – Many generators are fitted with horizontal engines. Dipsticks on horizontal engines are a little different.
Full – To adjust the oil level on most horizontal engines, fill with oil until it flows out of the dipstick hole. That’s it, it’s full, now refit the dipstick.
Oil Type
Adding oil is, as you already know, easy. It is also easy to add too much oil, and you know that’s not good either. These small engines don’t hold a ton of oil, and they use a splash lubrication system, meaning too much oil prevents the splash paddles from distributing oil throughout the motor. A case of more being less.
Most generator engines like 5W30 or 10W30 engine oil, and the average size engine will hold approx.1 quart from empty.
Mechanics tips for adding oil:
- Avoid if possible mixing oil grades
- Use a funnel to add oil
- Add a little oil at a time, stop and recheck the oil level
- Use a siphon to remove excess oil
2 Faulty Oil Level Switch
The second most likely cause is not as simple to fix but is simple to bypass. While bypassing the switch is not ideal, it does at least get the juice flowing quickly. More on that below.
The low oil level switch is a simple on-off component fitted to the oil pan of the engine. It’s wired in parallel to the ignition system. When the oil level is low, the switch will ground and prevent voltage from reaching the spark plug.
How to Diagnose and Bypass Generator Oil Switch
Diagnosing and bypassing the oil switch is the one action. Meaning that by simply disconnecting the oil switch from the ignition system, we are eliminating it altogether.
If the switch is faulty (or the oil is low), your generator will start right up. On the other hand, if she still refuses to fire, we may have a different problem on our hands, and I’ve added a couple of troubleshooting links at the end of this page to help you with that.
Bypass / diagnose oil switch as follows:
Locate oil switch on the engine pan
Follow wiring to locate wiring terminal (maybe inline or at the switch)
Disconnect terminal by pulling apart or removing the fastener
Attempt to start the engine
Result
If your engine started and you are confident the oil level is correct, – you have successfully diagnosed a faulty oil level switch. Leaving it disconnected is a solution, but it is better to replace the faulty switch. It’s covered below.
If you choose to leave it disconnected, tape off the raw wiring to prevent both grounding and corrosion.
Alternatively, diagnose using a Voltmeter
Alternatively, you can use a voltmeter to diagnose. With a voltmeter set to resistance check for continuity, you’ll need to drain the oil to check low-level operation. See below.
Good sensor with oil level correct
Good sensor with oil level low
Note: All oil switch wiring will have a terminal for disconnecting. The thing is, they may not always be easily accessible. For the purpose of testing, it’s OK to cut the wire in a convenient place. (leave plenty of wire on either side of the cut)
Remember, if your generator is fitted with a Honda engine, it is most likely the module that has failed, rather than the switch itself.
To replace the module, remove the fastener and disconnect the two conveniently located wiring terminals. Rebuild in reverse order, easy, love Honda!
Honda module failure is more common
A warning for those who decide to disconnect the switch as opposed to repair it. Keep a tight eye on the oil level (check with every gas fill), as your engine will now start no matter what the oil level is. As you know, an engine won’t last long with a low oil level.
How to Replace Oil Switch
Some models are easier to replace than others. Many Briggs & Stratton engines are a dream, and then there’s the Honda engine.
I love Honda motors; they are really good motors and easy to work on, but replacing the oil switch requires an engine strip down. That said, and to be fair to the Honda, it is more likely to be the oil switch module that fails, and it’s easy to replace.
Replacing the Honda oil switch module is as follows:
Disconnect the wiring and remove the module fastener
Replace in reverse order; the wiring only fits one way so you can’t get it wrong.
Replacing a Briggs & Stratton type oil switch is as follows:
- Remove oil (drain)
- Locate oil level switch on the oil pan
- Remove sensor wire
- Remove switch fasteners (usually two)
- Place shop rags under the switch & remove the switch
- Remove old interface gasket, O-Ring, etc.
- Clean oil pan interface
- When fitting a new switch uses a new gasket, O-Ring, etc.
- Fit both fasteners by hand before using socket and ratchet to snug
- Refit sensor wire & test
Less Common Causes of No-start
There are, of course, lots of other possible causes of no start after an oil change. While these other causes may seem remote. Let me just say, as a mechanic with twenty-five years of experience, I’ve seen some really odd, seemingly not connected, remote possibility crap happen.
Anyway, here are a few things you can check quickly:
- Gas tap On?
- Fuel level OK?
- Switch on OK?
- Choke On?
- Consumers plugged out?
- Fuel quality OK?
- Plug wire on securely?
- Try starting after removing the air filter – sign of a wet air filter often happens by tilting the machine incorrectly (carb side should always face up)
- Try starting after removing the gas cap – a sign of faulty cap
- Try cleaning the spark plug – repeated starting attempts foul spark plug
If none of this helped, then we’ll need to remove the spark plug and check for spark. You’ll find a spark test tool here on the “Generator maintenance tools page” and a video here on spark testing.
If that checks out okay, we’ll need to check the carburetor bowl for dirt. Check out this video – Carburetor bowl clean.
I’ve included the following links. They may not be for generators, but the engines are often identical, and the test process definitely is.
You may find these other generator posts useful:
The generator starts then dies
- About the Author
- Latest Posts
John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.