Hi There: I have a very old Guardian Model #0041094, Serial #3515822. I recently serviced it and found the battery split on one end with all the acid gone from that cell. After servicing the generator and replacing the battery, the generator will start and run for a short time and shut down with the "overspeed" light showing. I first thought I had a control board problem but decided to look a little deeper. When I start the generator the AC voltage at the generator circuit breaker is either 304 volts or 31 volts. The measured hz is 58 to 59 at 304 volts or 60 hz at 31 volts. Checking each pole to ground shows exactly 1/2 of each voltage reading. When I first checked it I thought my meter had gone bad but it is a UEI DM393 True RMS Multimeter and although nearly 20 years old performs well. The red LED on the VR lights when the engine is started however it goes out just before the engine shuts down. We live in Northern Arizona so corrosion is not much of a problem but I disconnected all the wires on the VR just to check. They all looked clean and tight and there was no change in behavior. The LED is noticeably dimmer at 31 volts and brighter at 304 volts. The engine hour meter shows a total of 645.3 hours of operation. I purchased it new I believe in 1999 and personally installed it so I know it's always been serviced properly and has been reliable until now. Any guidance as to how to proceed with further diagnostics will be greatly appreciated. We live in a remote rural area of Northern Arizona with no service people within 75 to 100 miles. This generator's primary function is backup for our water well submersible pump so when our APS power fails (not infrequently) we run out of water soon. I am attaching photos in case they are needed.

Gerry [ATTACH=JSON]{"data-align":"none","data-size":"small","data-attachmentid":58926}[/ATTACH]
I'm brand new to this forum so keep that in mind. In my limited experience I have found that voltage regulators tend to die before other components on these types of generators. So, If it were me, I would try replacing that first just to see if it solves your problem. Short of that, I hope an expert can jump in and give you better info!