Onan 4000 Generator Won’t Stay Running (Why and How To Fix)

When motors break down or refuse to run all the time, there is a myriad of problems that could be the actual source. The trick is to be able to get to the actual issue as quickly as possible. That way the generator will run like it is supposed to.

One of the main issues that could be the source for your Onan generator failing to stay on is the carb. If it is dirty or in need of rebuilding, then you will have trouble keeping the generator running all the time. Cleaning or rebuilding the carb is the only solution here.

To learn more reasons why your generator won’t run all the tie, just continue to read our article. It has the information you want to know about so you can fix the problem and have power when you need it.

Why Does My Onan 4000 Generator Keep Shutting Off?


Space won’t allow us to present every issue that will create this situation. One source may be that the oil tank is low. Onan generators are supposed to have a low oil shut-off mechanism that will turn the engine off when it detects that the oil levels are too low. Fill the oil tank to remedy this problem.

Next, check your fuel filter. This can get dirty and cut down the flow of fuel to the motor. While you are at it, check the fuel line to see if it has sprung a leak. Both of these sources are easy fixes as you can plug the holes with duct tape or replace the fuel line. Or you can replace the fuel filter or clean it.

After these two areas check out or are fixed and you still have the problem, check the air filter and the spark plug. If neither has been changed in a while, they may be the source for your inability to keep the generator running.

Finally, check to see if the generator is overloaded. Many modern generators are now designed to shut down if they are overloaded. Lessen the load and you should be able to use your unit with ease.

In the end, if these simple fixes do not work, contact an Onan technician to have him help you diagnose the problem.

Onan 4000 Generator Will Not Stay Running

The previous section only lists some of the problems that can cause your generator to malfunction. This section will add to that list so you have plenty of areas to look at to figure out what caused the problem.

  • 1. Sticky switches - this can take place when you have not used your unit for a long time. The switch that does it the most is the oil pressure switch as the oil can get sticky when not heated all the time.

Multiple starts and stops should be able to loosen up the sticky situation and get the generator working again. Or do some light tapping in the right place to help loosen the sticky oil.

  • 2. Defective control board - to diagnose this problem you will need a multimeter. You will want to monitor the AC output and then use the field flash pin to check the field flash voltage. If neither brings results, then the control board may be bad.

The only thing that can be done in this situation is to replace the control board.

  • 3. Flawed regulator - if the control board checks out, then the source may be the regulator. To see if this is the problem run 12 volts through a diode to the regulator pin 9. If you see voltage coming from the generator then the regulator is bad and needs replacing.
  • 4. Damaged wiring - wires break, corrode, loosen, and more. Always check the wiring to make sure they are in top shape, tightly connected, and not corroded. Also, using regulator pin 7 will help you check their connectivity
  • 5. Rotor short - the last place to check would be the regulator mating plug. Again you have to use your multimeter to check the resistance between pin 9 and 10 and you should get a reading between 22-28 Ohms if everything is okay.

If the reading is below 22 then there is a short in the rotor. If the reading is above 28, then there is a ground problem in the same part. Just repair the problem when you find it.

Download The Onan 4000 Generator Manual


Cummins is the company that makes Onan generators and they have placed a PDF version of the manual online. You can reach it at this link. That is just one option you have. There are a lot of options that pop up if you use the subsection title as your search words.

How do I Adjust The Carburetor On My Onan Generator?


There should be an altitude adjustment knob on the newer models. Just turning it will adjust the air to fuel ratio and keep your generator running like a clock. The drawback to this newer system is that every other adjustment you could have made on older models is not possible anymore. You need a technician to do it for you.

On older models, you will need to remove the bowl, clean it and then put it back in place. Then look for the fuel adjustment screw and turn it in the direction that gives you a smooth-running motor.

There may also be a low and high-speed set of screws on the generator. You will need to have the generator up and running to turn those screws and adjust the carb.

Some Final Words

If these easy fixes do not do the trick and get your generator running continuously, then call a technician. The more modern the device the harder it is for DIY mechanics to fix the problems. Not everyone has the right tools for this task either.

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