ONAN 20 kw Over rev shutdown

[Passages]

Oh no, it's an ONAN.

During last weekends trip, I gave the generator a good workout. Ovens, AC, hot water heater, chargers, all the usual stuff.
After about 8 running hours funny things began to occur. First, the forward AC began making a clicking sound like a auto turn signal. Then the fridge light began flashing while the trash compactor was operating. Then there was a sound of the generator reving up followed by an immediate shut down.

I let it cool for about 2 hours, with all loads off, pushed the fault reset button and it started up. Turned on just the MSTR AC (it was still hot in Miami last week) and it ran great all night. Next day I restored normal loading except hot water heater and still ran great for a few hours. Early evening it began acting up and heard the over rev followed by auto shut down.. Restarted it and ran ok for a few hours. Curious, I began shedding load and heard the genny begin to over rev again.


OK, what I suspect is happening is whatever regulates the engine speed to the demand is going funky. I think as devices switch on an off, the gen motor is not reacting to the reduced load and going into over speed fault shut down.

So, I'm looking for suggestions on how to diagnose and fix. Any ideas?

 

[Pascal]

It could be the Governor not adjusting. See if you can reproduce the fault while monitoring frequency. RPM directly affects frequency not voltage

I would avoid using any sensitive electronic controlled aplliances and compressors. Try with the water heater, a non Induction cooktop, non digital oven, electric grill etc.

If you need a pro there is a really good Onan specialist here in Miami, he s American so he won’t have any issue venturing north of the border aka Miami Dade county line

 

[Avenger]

Sounds like a faulty governor to me.

Pic of the injection pump would help so we can see what it is and what kind of governor it has.

 

[Passages]

Sounds like a faulty governor to me.

Pic of the injection pump would help so we can see what it is and what kind of governor it has.

Not good a posting pictures but it's a Stanadyne DB2-4086 rpm is 1800

 

[dottieshusband]

The governor on the oh no is electronic, mdl4. There is an over rev sensor. To me, what you discribe is a generator running close to one of it's shut down parameters. Check the simple things first... like the impeller, and obstruction in the strainer.. if it's running hot, the thermal exhaust or coolant temp could shut it down, but mine acts up before the auto shutdown trips. Your auto shutdown tripped, so I'm guessing it's your impeller or exhaust temp sensor. Replaced both of mine in the last few weeks, easy fixes. Onan will tell you the exhaust temp sensor is unavailable, but there is a substitute.

 

[Avenger]

The governor on the oh no is electronic, mdl4.

The pump spec shows up as mechanical. Would they have been retrofitted? Seems an expensive proposition for a generator in a Viking.

 

[Pascal]

The governor on the oh no is electronic, mdl4. There is an over rev sensor. To me, what you discribe is a generator running close to one of it's shut down parameters. Check the simple things first... like the impeller, and obstruction in the strainer.. if it's running hot, the thermal exhaust or coolant temp could shut it down, but mine acts up before the auto shutdown trips. Your auto shutdown tripped, so I'm guessing it's your impeller or exhaust temp sensor. Replaced both of mine in the last few weeks, easy fixes. Onan will tell you the exhaust temp sensor is unavailable, but there is a substitute.

The voltage regulator is electronic but the Governor, on the engine by the injection pump, is mechanical and regulates RPM to 1800 Two different things

If the engine is reving up it s not an overheating Or water flow issue. And if there was an overheating issue, it would happen under load, not reducing loads. There would also be tell tale signs like sound and steam.

 

[doc g]

like everyone else has said it sounds like a governor issue . The AVR senses voltage and adjusts the excitation voltage (in DC) to keep the AC voltage in the proper parameter, lower DC excitation voltage lower AC output. The governor keeps the engine running at 1800 rpm regardless of load. That determines your frequency. When all is right the no load frequency should be 62-63 HZ. Fully loaded it's ok to be 59 or so. The ac out voltage should be 240 volt or close to it. (there is an adjustable pot to adjust the AC voltage on the AVR. This all assumes it is a 240 v unit which is what Hatteras used in most cases. I hope this helps! I have learned a whole lotta OH- NO control stuff this past year . I believe the AVR also can induce a shutdown if the governor does not keep the operating RPM - 1800. IRRC it would take out the CB2 breaker at that point ....Pat

 

[Passages]

Yes, when things were going funky this weekend, output voltage was 240v.

 

[dottieshusband]

If you'll Google radionerds images stanadyne, it will bring up the manual for the db2 pump on the mdl4. The governor isindeed internal to the pump. Although I had the exact symptoms a few months ago, irregular running, irregular rpm's voltage variation, shutting down etc... pumping water just fine. Initially replaced the exhaust temp sensor, which fixed it for a while, but ultimately it turned out to be a deteriorating impeller, and blades obstructing flow to the heat exchanger.. Hey, by all means, pull the injector pump first..

 

[doc g]

If you watch the Hertz meter and it changes the rpm has changed . It could sound as if it’s revving if it’s not getting enough water too . Also the low oil pressure, over heat, exhaust temp shutdown will take one of the breakers and over rev will take out the other ....Pat

 

[bigbill]

does it have a mechanical fuel lift pomp? i had an older onan 15 kw that the fuel pump leaked fuel into the crankcase and made it over rev because the oil was full of fuel

 

[Passages]

Good info guys. I agree, try the simple stuff first.

I noticed the RACOR sight bowl seemed dark so I went and replaced the element. Plan to replace the impeller today. It's only 1 and half year old with 150 hours, seems to spit out a good amount of water, but hey, lets give that a try as well.

I was going thru the PO's spare parts box an saw a fuel filter marked 20kw. It's not a RACOR so it must be a secondary. A little aluminium retangular boxy thing. I didn't even know it had a secondary fuel filter. Hmmm.....

Can fuel starvation cause an engine to race?

 

[doc g]

rectangle box thingy is secondary fuel filter and fuel starvation will cause the engine to bog down and lose rpm not over rev. If you look at the service/operators manual it will tell what faults cause what breakers to open........Pat

 

[Passages]

Replaced secondary. Now I can't get fuel to prime. Said heck with that and began to replace impeller. Now the new impeller is in the pump but the pump is not marked input/output and of course I did not make a note of proper orientation while pulling it out.

So, is there an easy way to tell which raw water pump port is input? Right now I have it input rw, short span with the bronze compression piece, then ouput. My gut tells me it should be the other way but I'm not sure.

I needed to keep repeating "Boating is FUN"

 

[Passages]

Looks likes I had the pump right to begin with.

Sometimes, when you're sweaty, greasy and frustrated, you just need to take a step back, pour yourself a cold one and do something else for a while, lest you over think things and screw it up even more.

 

[doc g]

on mine the upper hose connection is in and the lower is out to the heat exchanger. It really does not matter which way you put the impeller in anyway .....Pat

 

[Passages]

Had the PO over to the boat yesterday. Told him about my over rev, shut down issue. He immediately knew the problem and the solution.

The primary RACOR is shared by 2 generators. If the inactive generator is not valved off, the other genny will, after time, begin to suck air, over rev and auto shut down.
Said it happened to him several times before he figured it out.

Lesson: Always try to stay on good terms with the PO.

 

[Pascal]

Not sure how air being sucked in could cause an over rev... I also fail to see how air could get sucked in from the other gen fuel return, injectors, injector pump secondary and the lift pump. I guess I ve seen weirdest glitches but it seem a bit far fetched

 

[doc g]

that means you are sharing a fuel source for both generators, not a good idea IMO, if you have a fuel issue with one you have it on both.If you had a air leak in the return line of the non operating generator I suppose it could suck air into the operating one. Also the return line usually dumps into the top of the tank if the level in the tank is low enough yeah you can suck air . I would assume Viking did not set it up that way. Are you finding air in the system and having to reprime the gen before it will restart?? Again , fuel starvation can't cause over rev ....Pat