Onan Generator Starts Then Shuts Down

Onan Generator Starts Then Shuts Down: Troubleshooting The Problem

The Onan power generator is a solid machine specifically designed to meet the energy demands of the average house owners as well as RV enthusiasts. If you want something to sustain your RV facilities/appliances or simply in need of a backup power source, this unit shall serve you well. That being said, the Onan model is far from perfect, there are many complaints about operations difficulties and so on.  Among the reported problems, the “Onan generator starts then shuts down” is considered to be extremely annoying to deal with. In addition to that, you could cause damages if you attempt to fix the machine without proper tools and knowledge.  

Onan Power Generator: What You Need To Keep In Mind

Fortunately, troubleshooting Onan generator is not a difficult task so as long as you could grasp the fundamentals, everything would turn out to be fine. Take a look at the information below and you could take care of the issue of “Onan generator starts then shuts down” in no time. As with other devices, call a certified electrician to fix the unit in case you don’t really have much confidence in your electronic skill.

The Working Principle Of Onan

Generally speaking, the control board regulates primary functions and the most important one is to keep the unit supplied with enough diesel (or LP gas).  That means the board would promptly stop Onan generator from working once it detects shortages of fuel and fuel pressure. If nothing goes wrong with the board, the regulator should receive field flash voltage and then preserve stable applications of varying loads on the stator. Depending on the current situation, the regulator increases and decreases the loads before transmitting them through the unit field circuits. Technically, the regulator works as a built-in protection against potential overloads of the machine.

In the case the Onan generator you are using happen to lack a regulator (notably NHE and BGE models), it would have a bridge rectifier. Upon starting, the generator control board would deliver currents through the + terminal of the rectifier.  A set of relays in the board open and close as the oil pressure change which determines the rectifier status. If everything goes smoothly, the rotor of the machine would start receiving DC voltages soon after the rectifier turns on. All things considered, the performance of Onan power generator relies a lot on the smooth cooperation between its various components.

Must-Have Tool And Gear

Without the right tool, it’s highly likely that the troubleshooting process would not provide you with a definite result. As a result, you have to make sure that you got proper tools and hears before assessing the Onan machine. Overall, you are going to need a functional multimeter around to exam and inspect essential readings of the generator components. Since you must disassemble the unit to reach the components, prepare some wrenches and screwdrivers. About the gears, a rugged pair of rubber gloves should be sufficient but to be in the safe side, wear facemask and goggles as well.

Onan Generator Starts Then Shuts Down: The Ultimate Troubleshooting Guide

Once you have everything on hand, it’s time to tackle the “Onan generator starts but won’t stay running” issue. 

Here are the potential culprits:

  • Sticky switches
  • Defective control board
  • Flawed regulator
  • Damaged wiring
  • Short or ground in the rotor  

1. Sticky Switch

Among the causes that lead to the “Onan generator starts then shuts down” issue, the sticky switch happen quite frequently. The oil pressure switch of Onan generator often sticks open in case the unit has not seen any use for an extended period of time. In many cases, the situation could be resolved by multiple start/stop cycles, some light tappings should work as well.

2. Defective Control Board

As soon as the Onan power generator is up and running, use the multimeter to check the AC output voltages. If there is no AC, proceed to check the field flash voltage through the field flash pin of the board. In the case field flash voltage is not present, the control board may be defective. If that happens to be the case, the “Onan generator starts then shuts down” issue could be taken care of by replacing the unit control board.

3. Flawed Regulator

If field flash voltage seems pretty normal, the regulator could be the source of problem which you could determine simply by running 12 volts through a diode to the regulator pin 9. In the case the generator starts producing normal or slightly low AC voltages after that, the regulator is definitely defective.

4. Damaged Wiring

Once you are certain that the regulator is functioning normally, there may be some connectivity issues with the unit wirings. You could address the issue by simply checking the wiring connectivity between the field flash pin and the regulator pin 7.

5. Short Or Ground In The Rotor

If you manage to rule out all causes above, the last place to check out is the regulator mating plug. Use the multimeter to check the resistance between pins 9 and 10 of the plug, the appropriate reading would be around 22 – 28 Ohms. If the reading is too low, there could be a short in the rotor. On the other hand, if reading is too high, it’s highly likely that there is a ground in the rotor .

Last Updated on November 30, 2018


  1. Richard Mcclintock July 2, 2021
  2. Stanley Morton July 1, 2022

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