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Rich Biebel · 02-16-2016, 09:16 PM

I have not run across one that ever did that.

I will look into it for you tomorrow.

I am an Onan repair tech and have worked on many of those....

A 7500 diesel should be a variable speed inverter/generator......Yes?

Unfortunately the inverter section is a sealed unit and the control is part of that inverter section....NOT serviceable....

I will look at a machine's electrical print and trace out the starter circuitry but it is not a good scenario since you have changed most components already.....

If you can give me the Model Info off the I.D. label it would help.....

Looking in the Service Manual for a typical Cummins-Onan 7500 diesel inverter-generator this morning did not show me anything that is likely to be a simple fix for this machine.

The starter is controlled directly by the Controller PC board located inside the Inverter section and as I said not serviceable...

The starter should be de-energized when the engine reaches 800 rpm and the controller is always monitoring engine speed from output of the generator end......

Unless a wire is damaged, shorted, or possibly output is incorrect.......your Inverter-Controller is likely bad.

Not an uncommon issue on those units and since it is 20 years old.....anything is possible......

I would start the unit with the covers removed and pull the starter solenoid B+ wire off the starter and put a voltmeter on it to see if it is actually re-energizing after the unit is running.

I would also monitor output power and see if there is anything changing there at the same time.

The Inverter-generators can do some very difficult to diagnose things...

I had one that would "full field" the power end during cranking. The starter could barely turn the engine over due to the high field strength.....You think the starter or battery/cables are bad but it was the Inverter-control unit....

02-16-2016, 09:16 PM	#2
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help I have not run across one that ever did that. I will look into it for you tomorrow. I am an Onan repair tech and have worked on many of those.... A 7500 diesel should be a variable speed inverter/generator......Yes? Unfortunately the inverter section is a sealed unit and the control is part of that inverter section....NOT serviceable.... I will look at a machine's electrical print and trace out the starter circuitry but it is not a good scenario since you have changed most components already..... If you can give me the Model Info off the I.D. label it would help..... Looking in the Service Manual for a typical Cummins-Onan 7500 diesel inverter-generator this morning did not show me anything that is likely to be a simple fix for this machine. The starter is controlled directly by the Controller PC board located inside the Inverter section and as I said not serviceable... The starter should be de-energized when the engine reaches 800 rpm and the controller is always monitoring engine speed from output of the generator end...... Unless a wire is damaged, shorted, or possibly output is incorrect.......your Inverter-Controller is likely bad. Not an uncommon issue on those units and since it is 20 years old.....anything is possible...... I would start the unit with the covers removed and pull the starter solenoid B+ wire off the starter and put a voltmeter on it to see if it is actually re-energizing after the unit is running. I would also monitor output power and see if there is anything changing there at the same time. The Inverter-generators can do some very difficult to diagnose things... I had one that would "full field" the power end during cranking. The starter could barely turn the engine over due to the high field strength.....You think the starter or battery/cables are bad but it was the Inverter-control unit.... __________________ Rich Biebel S/C 1479 Last edited by Rich Biebel; 02-17-2016 at 09:48 AM.