What causes my Guardian RV 6500 generator to bog down when under a load?

SOURCE: Generac Guardian 15kW Generator

That sounds just about right, if the relay contacts get welded together it sometimes takes out the driver on the printed circuit board. As you found out, 12vdc from 23 to 194 makes the transfer switch move to the Generator position, relay not energized makes the ATS move to the Normal position.

You can make a nice test function by installing a 10 amp contact switch in series with either the N1 or N2 terminals. When the switch is open the generator thinks the Normal power has failed.

Regards,
Carl

SOURCE: Attempted to start Generac Guardian 15 kw air

I work on these every day. Its either gonna be the rotor or stater, the voltage regulator, but most likely its a bad stepper motor which is the throttle motor, or control board. Thats all that could be causing that so its one of those. If its the rotor or stater thats bad it causes the generator to RPM up because it thinks its not producing enough power due to that. I had the same problem with the same one we just fixed and it was the slip rings on the end of the rotor so bad rotor.

SOURCE: my generac guardian RV ran for a few hours then

Check for low oil or over temp.

SOURCE: I have a 6500 lp onan marquis generator in my rv.

ONANS ARE PRETTY QUIRKY AS IT IS NOT CONSUMER FRIENDLY. THE GENSET IS DESIGNED FOR ROUTINE AND SIMPLE MAINTENANCE PERFORMED BY THE OWNER, AFTER THAT, U WILL NEED TO TAKE IT TO THE NEAREST DEALER OR CUMMINS DIESEL REPAIR SHOP.

SOURCE: RV Onan Emerald 1 Generator

Check all the ground wires and make sure their not broken or loose. If the wires are good next check the neg. Wire to the coil. Too check the swiitch make sure you have continuity too all the post when the switch in the on position, this requires removing the switch.