[Rich Biebel]

It should have shut down and lit a fault code. Secondary code should have been a 12.

Start the machine and open the circuit breaker on the machine itself. You need to read the voltage right at the input side of that breaker. It’s hard to get to but with proper clip on leads you can do that. Input side is bottom of breaker usually. A 2 pole breaker and if it’s a 240 output and I believe it is you should see 240 across both.

There is a sensing transformer and I’ve never found one bad. I don’t recall why they use that but the transformer is wired between the breaker and the control harness.

Erratic voltage output is going to be tough to diagnose without a known good control board. The wiring from the breaker to the control transformer and then to the control board connector has to be checked. That is usually not a problem though.

What year is the truck or coach ? 09 ? 00 ? How many hours on the generator ?

The control part number is 300-5961 from Cummins-Onan