Onan 6500 genset can't put out rated power, output fluctuates

Davi Rozgonyi

Hey all, it's been a while.... hoping y'all are doing great! And hoping someone has some knowledge of my generator problem.

It's an old (30 y/0) Onan 6500 marine genset in my amel super maramu. Symptoms are: gennie holds loads fine up to about 2000 watts of demand, but much above that (but well below 6500) it will cut out the production of power for a minute, the engine speeds up as the load comes off, then in a minute or even less, it cycles back to engine under load and the watts coming back online. So it works to a point all day long (at half load or less) but then will cut totally out and back if I put too much load on. The engine side has been serviced/rebuilt last year, and never stops running, just the revvs change as the output fluctuates from zero watts to 2000 or 2500 or so. It sounds like the engine is not the problem. 

What can I look for to see why it can't handle even half it's rated load? Mech/fuel side possible? The filters were changed last year too. Voltage regulator? Motherboard thingy? The local mechanics want to pull the whole thing to fix whatevers wrong but that's a huge job here and I'd rather wait or replace it if that's the case.

Any thoughts would be MUCH appreciated!
All the best
SY Wake

Craig Briggs

Just a shot in the dark, but could it be an AC circuit breaker failing? Try either replacing them or, to diagnose, temporarily hard wire across them and see if it holds. 

Craig Briggs - s/v Sangaris / SN68  Tropic Isle Harbor, FL

Bill Kinney


If the engine speed is changing, I would guess that the likely problem IS with the engine side. The engine speed is mechanically governed, and shouldn’t vary more than a few percent under any load.  As the engine speed varies, the frequency of the power will change as well. This can put some of the plugged in equipment at risk of damage.

I think it is unlikely that the engine power output could be half normal without other problems. But DO check fuel availability.  A weak primary fuel pump, or clogged fuel filter could be the issue by choking the engine when it demands more fuel to maintain speed under higher load. If there is sufficient fuel, then the governor and/or injection pump are the next candidates.

It is also possible that there is something wrong with the electrical control of the generator that is overloading the engine, but I can’t think what that might be, assuming that the voltage stays in specification before the engine starts to struggle.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

Craig Briggs

Bill, David -
Good points Bill.  I think I may have read David's original post a bit differently -  not that the engine speeds up or slows down a lot or for very long, but that it is simply "unloading" as the output drops off and then it is "lugging down" as the load then comes back on. That is, it is just a normal lag of the governor which one would see if you, say, flipped on a very large load. RPM's will momentarily drop as the governor quickly - but not instantaneously - feeds more fuel to keep the engine at 1500 (or 1800) rpm, the governor being a mechanical device with a discrete response time.

Perhaps David can shed more light on this that might help home in on engine vs. generator.
Craig Briggs - s/v Sangaris / SN68  Tropic Isle Harbor, FL