Onan 7500 QD Generator surging RPMs

[Jeff Ramey]

Ok guys, I'm scratching my head on this one. Any advise/direction would be helpful.
Background:
On our recent trip to Florida in our '00 Beaver Patriot Thunder, we had the generator running as we drove down the road in order to use the roof airconditioners (our dash A/C wasn't working at that time). Somewhere around Alabama, we were passed by a pickup driver pulling a boat at 75 mph. The problem was all the other traffic was flowing at 65 mph. The driver, realizing (too late) their excessive closure speed to the cars in front of them, hit the brakes. HARD. The boat on their trailer decided it was no longer satisfied with just being "along for the ride", and decided it was time to drive. Swerving, skid marks, and tire smoke and dirt clouds ensued. The driver managed to careen off into the median and get the rig to a stop. In order to avoid all this chaos, I immediately hit the jake brake and stepped hardish on my own brakes bringing the rig to a respectable and controlled stop at a safe, but too close for my comfort, distance from the vehicles in front of me.
Problem:
Sometime after this at one of our rest stops, we noticed the 7500QD generator surging. A rhythmic Vroom, vroom, vroom could be heard from the generator side of the coach. I shut it down and restarted it several times. I checked for fault codes but got none. I did a lot of research over the next week trying to figure out what needed fixing. I suspected that some trash in the fuel tank found its way into the generator fuel line.
Attempted repairs:
 I disconnected the fuel line from the generator and ran a new clean one from the generator into a clean fuel can with fresh diesel and Seafoam in it hoping to clean injectors?) or the carb (didn't see one). NOPE, NOT IT. I changed the fuel filter. NOPE, NOT IT. Then I changed the Fuel pump. Whew! That's a real pain! (my genset isn't on a slide). NOPE, NOT IT EITHER. Any ideas? The surging is very prevalent and I'm pretty sure I'm not producing any clean power because of it. Could it be trash in the line prior to the fuel pump? It seemed to flow pretty darn easy up my arm and onto the ground when I disconnected the line! I see fuel trickle out at a "drinking straw" rate from the return line when the genset was running.
The oil/filter/air filter were changed about a year ago.

[Gerald Farris]

Jeff,
Your 7.5 Onan Quiet Diesel generator is not a constant speed generator. It is a DC generator with an inverter that supplies 120 volt AC power to the coach. So you need to determine what your power output is over differing loads to determine if you even have a problem instead of throwing parts at a problem that may not even exist. Also, the reason that you could not find a carb is that they do not exist on a standard diesel engines.

Gerald
 

[Jeff Ramey]

Thanks! Once again, Gerald to the rescue! I should really just start a monthly allotment to you for all the advice you've provided me over the last couple of years.
I really feel like a fish out of water with diesel engines; automotive, and aircraft, much more comfortable for me.
Aside from just sticking my meter leads in one of the outlets, what would be the best way to determine the genset power output?
I understand that the genset is a DC generator, now that you've told me, but how does it generate frequencies?
Does it have a square wave, or sawtooth generator circuit to mimic 60hz at 120vac?
Generating voltage seems easy enough, but I know "dirty" frequencies kill sensitive electronics.
The genset constantly revs from idle (750 rpm? to about 1100 rpm) according to my sensitive calibrated eardrum.
That can't be good for the generator. I know it drives me crazy just listening to it for 5 mins.
Anyone experienced anything like this before?

[Bill Lampkin]

Jeff, Do you see any codes on the gen display on the gen itself? Could be a two digit code that may lead you in the right direction. Do you have the manual? Try google Onan 7.5 QD manual to get one. Your generator should not 'surge'. Not normal.

[Bill Lampkin]

from another on line post:

Chboone/Koop: As algoma mentions above, the QD7500s and above are inverter based generators. The RPM of the engine will vary from idle to 3525 rpm based upon the load/demand on the AC side. The QD5500's and below (with one exception) are conventional generators and will attempt to sustain a fixed rotor speed regardless of load.

Jerry: In the majority of cases with a QD hunting around at different RPMs unrelated to load you will find it's fuel delivery related. In the majority of those cases it is the fuel filter that is needing replacement, but it can be cracked fuel lines allowing air in the lines or a failing fuel pump. Cummins/Onan has a recommended fuel delivery test as follows:

FUEL PUMP/Fuel Flow Test

1. Disconnect the fuel return hose from the line to the supply tank and point the end into a quart (1 liter) or larger container that has volume graduations. There are 2 fuel lines entering the unit. 1 is the supply, 1 is the return line. Make sure all testing is done from the RETURN line which will be clearly marked on the cover.

2. Prime the genset by pushing the Start/Stop switch to STOP and holding it there for the duration of the test. Flow rate into your container should not be less than 200 ml (0.4 pints) per minute.

3. If flow is less than specified, replace the fuel filter. Re-test. If that does not help, look for other issues in the fuel system such as wet or leaking fuel lines especially at unions and connections. Last, replace the fuel pump as the pump is not serviceable.

If the unit passes the delivery test, I use a separate powered small 12V fuel pump and a 1 gallon container of fresh fuel. Using my own supply/return lines I'll run the unit on my "test" pump and fresh fuel to see if the problem still exists. If the problem clears up then I'll look at the 12v wiring to the pump and/or replacing the fuel pump.
__________________

[Gerald Farris]

Jeff,
The Onan DC generators just run fast enough to create the power needed for the inverter to power the 120V AC loads. The inverter creates a pure sine wave 60hz 120V power supply for your coach just like the inverter in your coach does from the house batteries  The 7.5 and 8.0 Onan generators are both DC generators that operate at a variable RPM. However, the 10 and 12.5 KW are not inverter generators and therefore they most maintain a constant 1800 RPM speed to provide a 60hz power supply.

Check you generator with a load like both A/C units running off of the generator and see what your voltage reading is. The load should stabilize your generator engine speed. If it still surges badly with a load, you may have a problem, but I doubt it.

Gerald

[Jeff Ramey]

FOLLOW UP REPORT:

You might not believe this, but it’s working like a champ now!
I had scouted the internet and this forum for solutions (all good advise too!) but none of it solved my surging issue. The coach couldn’t run both ACs because of the surging issue and generator would shut down eventually. For the longest time it didn’t throw any useful codes. Then recently, it threw a code 25. Ah ha! Finally something beyond “33”. I looked for governor related issues (as I suspected originally) and was getting nowhere. No big surprise there. Then, I bowed my head and prayed. Really. I asked God if he would show me what was wrong with this generator. Then, I saw the strangest, most oddball thing. Under the black cover that hides the governor, you can find the rotor and stator secured by a spring (which I already checked for proper tension.) securing this stator is 4 long threaded screws. They were all almost completely backed out! They were sticking up about 1” and wobbling around in the holes they fit in. This allowed the stator to wiggle ever so slightly. About 1/16 of an inch. I didn’t think this had anything to do with my problem as it was still held pretty tightly in place, but I figured as long as I was here, and those obviously weren’t supposed to be loose, I’d tighten them down. That was easy said than done. I had to be careful not to snap them off as they were seized in there pretty good. I got them out and WD40’ed the screws and holes really good. Then slowly, I cranked them all back in one by one. This required screwing them in a quarter turn till it squeaked, backing it in and out awhile and spraying more magic oil. Finally after about 2 hours, I got them all in a screwed down proper. I replaced the cover and for giggles, cranked it up, fully expecting to hear it surge again. SURPRISE! It runs smooth as a kitten purring. Who would have ever in a million years thought of something so off the wall as the 4 governor stator screws causing my troubles?!? I was just about to take it in later today and pay thousands to have a tech try and fix it! God is good, and an awesome Onan technician!