You need 12 volts from somewhere to power the fuel pump and the battery-powered (as opposed to magneto) ignition system. If you've disconnected the voltage regulator then the Onan isn't making the 12 volts, it's coming from the battery and the Onan will run until the battery gets low.

My Onan has not had a voltage regulator for many years. The 12 volts it needs to run comes from the power converter / house battery (I use the rear-mounted house battery as the generator battery on my 1974 GMC).

If you are using the house battery to start the Onan, the house battery will be providing the 12v for the fuel pump and ignition system. If your battery charger is on line when 110 is present, it will keep the battery up. If your battery charger is off line when the Onan is running, you will run down which ever battery is starting the Onan.
There are many Onans out there running quite happily without the 12v Onan regulator installed.
Tom, MS II

Someone does not know what they are talking about and it is NOT you.
The Onan needs 12 DC volts to get started, run the fuel pump, and the ignition. That 12 volts comes from the house batteries or the Onan starting battery.

How those battery(s) gets charged / recharged is a different topic and varies depending on what year your coach is.

There is 30 VAC voltage from the internal alternator and it is used to tell the board that the Onan is running. The 30 VAC is there even when the voltage regulator is disconnected. My voltage regulator has been disconnected for years.

I thought you said that with the pins jumpered 5 to 9 that it starts but does not stay running with the jumper removed. If so that means that the battery was powering the ignition and fuel pump with the jumper installed. I'll have to go back and reread this thread.

As far as the voltage regulator goes, it is only used for charging the starting battery. Sometimes when the VR goes bad it will shut down the 30VAC from the alternator and cause the board to sense that the engine is not running. This is the prime reason why we disconnect the VR when charging of the separate Onan starting battery is not needed.

Some people with separate Onan starting batteries have disconnected the VR and installed a combiner from the house battery(s). This then allows the separate Onan starting battery to be charged whenever the 110VAC is working through the converter or when the Main Engine is running through main engine driven alternator (through the isolator). Doing it this way, the Onan starting battery should always be freshly charged even if you have not run the Onan for many months.

I hope that I have answered you question.

Ken B.

Yes, on a 1973 with an separate Onan starting battery is should run with the VR disconnected. On a 1973 with the VR disconnected there is no method of charging the battery and it will eventually run the battery down.

That bring up the question. How did you disconnect the VR? There is one pin on the VR that has two wires on it. When disconnecting the two wires from the YR, the wires still need to be connected to each other even though they are no longer connected to the VR terminal. If they are not connected to each other you will not get power to the ignition and fuel pump. Could this be the reason you are not getting power to the ignition and fuel pump with the 5 to 9 jumper removed?

I disconnected the regulator from mine a number of years ago not because it wasn't needed but because it was the source of a phantom drain on my battery. So if you do decide to reconnect it you should check and make sure it isn't draining your battery down while the coach is in storage.

I am sure that your favorite GMC parts supplier could provide you with an Onan VR from their collective pile of Onan Parts machines. Jim Bounds used to have a fair collection of machines.
Tom, MS II