Onan 4000 not producing power but runs great!

Lots of possibility, get it checked for sure. Could be brushes worn out, stator, rotor, fuse, brakes, transfer switch, loose wire. The list goes on...

Look up trouble shooting for sure.
Or let the local Cummins throw parts at it. But you would not be here if you wanted to take it in.

Onan has been building generators since 1925. Which one do you have.

The older gen's like the 4KY units they work on a power in / power out basis, if you put in 12 volts you get out 120 volts.

They have circuitry that 'clips' excess voltage, I.E. if you put 13 volts in you will only get 120 out, not 130, but there is no way to boost low voltage. To protect anything you plug in, and to prevent overload the system merely shuts off power output if it's too low. So if your house batteries are low, say 10 volts, it's enough to crank and light up the motor, but too low to make power safely so it just shuts itself back off as a safety.

First put a voltmeter on the house batteries at rest, then get someone to start the Onan while you continue to monitor the voltage. If levels are good, then you may have a little surface rust on the brushes or just a poor connection.

Often just running it with the button held down for a few minutes, or several start / stops will clear it up.

By all means reply with Model and Spec. For example, our Model Number begins in "4KY" then a string of characters that in our case ends in "K," so we have a Model 4KY Spec K.

We had several generator techs on this forum, but most gave up on trying to help us because "we" (as a group) failed to provide that simple information. You may be able to get more help at www.smokstak.com but you've got to be specific.

When generator starts but doesn't provide power you should check the fuse/breaker on the generator - if that's OK then maybe your transfer switch is stuck.

Generator operating from switch located on generator but not the remote might indicate that you need to clean the contacts on the wire harness - also might indicate you have problem with switch.

Hope this helps.

You give no model number so the best I can do is direct you to the troubleshooting guide at
http://www.flightsystems.com/pdf/onan-rv-troubleshooing-guide.pdf