20KW Onan generator problem

[Bill Root]

The 20KW Onan generator in BUFFALO GAL has about 2500 hours on it. Several months ago it began shutting down shortly after starting. It would not restart from the bridge. When I went down into the gen room the "fault button" was out.

The fault button is connected to a breaker that shuts down the engine if there is a temp, oil pressure or overspeed problem. If I pushed the button back in the genny would start and run for a few seconds, then the button wouold pop out and it would quit. For the brief period it did run I could see that the oil pressure was OK and there was plenty of water pumping through the exhaust. I replaced the fault button switch and the genny started and ran fine up until yesterday when it started doing the same thing again; start, run for a few seconds, the fault button pops out and then it quit. I tried it half a dozen times with the same result. The oil pressure is fine and it's pumping water.

I would really like to fix this problem permanently but I'm stymied. Any suggestions?

 

[OBXTucker]

See section 3-4. I believe this is your model, correct?

http://www.cumminsonan.com/www/pdf/manuals/934-0124.pdf

 

[Jasper]

I had a similar problem until last week, when I changed the Sea Water Impeller!!
It has been two years this month since I first bought my Hatteras. The generator, an Onan 20kw, ran reasonably well and very occasionally cut out when I was running her or staying on board her in Stuart, USA.
After picking her up in Newcastle, Australia, I filled all tanks to the brim. The genset would run strongly for hours, while the mains were running or not, but then suddenly cut out. I suspected a fuel starve problem. Genset takes fuel via the port engine manifold.
I would prime the fuel pumps, as the generator is fed from the same manifold as the Port main, and the gen would kick back in for another hour or 5 minutes, no telling.

It gradually got worse, but every time I heard the generator begin to choke, I would turn on the prime pumps and she ran well: This happened on all the tanks. I assumed bad fuel/lines or both. I know the fuel is dirty, I have changed the filters (6 off) twice this year and it is clearing up)
Asked the local experts, they replaced the generator fuel pump. No improvement. Situation was getting so bad that part of my list to do before the summer season was to put in a new and separate fuel line for the genset.

However, while I was changing all the Sea Water impellors, including the generator SW one, I tested her and what do you know….no matter which tank I run off, she will not cut out, even while I start the Port engine, which I would expect, seeing as they are competing for the fuel. So question is: would an impellor make the difference? I am hoping that there is a sensor that checks water flow. BTW, there always seemed to be good water flow. The old impellor was in bad shape, missing a blade and misshaped.
IF this has solved my generator problem then it will be GREAT, I hope it is your issue as well, besides, checking an impeller is a 10 minute job and can only do good!

 

[Canuck Dennis]

I believe the answer is to disconnect ea. safety , run the set and see which one is causing the trip, could even be high cyl. head temp. Frustrating exercise if no easy immediate clues.

 

[Bill Root]

Thanks for the advice. It is a DL4 model. The impeller and belt were replaced about four months ago, so I doubt that's the problem. I'll look over the manual section that OBX sent. Sounds more and more like a bad sensor.

 

[bigbill]

bill, check the belt, belts do stretch and become loose. i know, i had the same problem

 

[SKYCHENEY]

Bill,
If you determine you have a bad sensor, I still have my old MDL4 in my barn and all of those were working when we pulled it out. Let me know if you need anything.

 

[GJH]

Bill,
If you determine you have a bad sensor, I still have my old MDL4 in my barn and all of those were working when we pulled it out. Let me know if you need anything.

Sky, you selling parts off that thing? I think I recall asking you once awhile ago about the sound shield, which you had available at the time. Anything else ?

 

[SKYCHENEY]

Sky, you selling parts off that thing? I think I recall asking you once awhile ago about the sound shield, which you had available at the time. Anything else ?

Sold the soundshield, raw water pump, and the heat exchanger. Most everything else is still available.

What do you need?

 

[GJH]

Sold the soundshield, raw water pump, and the heat exchanger. Most everything else is still available.

What do you need?

Uh, well.. those. How about the lift and injection pumps? Freshwater pump? Front end bearings? Starter and solenoids? Do you have any of the optional gauges and senders? I wouldn't mind having some working spares. How much do you want for the whole thing?

Refresh my memory, was there some particular reason you pulled it and put it in the barn?

 

[SKYCHENEY]

Uh, well.. those. How about the lift and injection pumps? Freshwater pump? Front end bearings? Starter and solenoids? Do you have any of the optional gauges and senders? I wouldn't mind having some working spares. How much do you want for the whole thing?

Refresh my memory, was there some particular reason you pulled it and put it in the barn?

Bad stator in the generator end. Quote was $1700 to rewind it and I just couldn't see putting that kind of money into it with the lack of support from Cummins.

The starter had been replaced by the previous owner with a 24v. It worked, but it occasionally would not engage the ring gear properly.

All of the control box is fine, the injection pump is still available, lift pump was sold. I'm sure the freight would kill you on the whole thing. I do still have the monel riser pipe.

 

[dottieshusband]

On mine, there is an oil pressure sensor, coolant temp sensor, and coolant level sensor... So any of those plus bad sensor or fault breaker. If its running hot, that should be observable, If it was me, I would jump the sensors one at a time to identify which one was shutting it down, then determine if it was really faulted, or a bad sensor.

 

[Bill Root]

Lots of very helpful support. Thanks to everyone. I'll be back on the boat next week. I'll post what I find at that time.

 

[Nonchalant1]

If it's like the 15KW, some of the sensors are connected in a string to the power wire for the reset button that keeps popping, but the oil pressure sensor may be connected direct. Since the reset button pops after a few seconds of getting power, you can find the various sensors and disconnect each one individually until you find out which one is bad and then replace it.

Also check the oil pressure sensor delay system which lets the engine run until there is enough oil pressure to keep from activating the sensor. I believe it's a centrifugal switch near the fuel governor. If it isn't working right, the low oil pressure upon starting will shut it down in a few seconds.

 

[Bill Root]

Got into the generator shut-down problem with my mechanic yesterday. We determined that the overspeed switch was shutting the engine down at 1900 rpm. Proper shut-down speed is 2150. The engine spikes to 1950 at start, then settles back to 1900, so the switch was tripping at every start attempt. After cleaning and adusting the switch, generator runs perfectly.

 

[Bob Bradley]

God bless you. I gave up on my 7.5 and put in a Norpro 9.0 and haven't looked back. Best $6k I've spent since I bought her 10 years ago. I tran that genny for 6 straight days while the power was off during and after Sandy. Can't say enough good about it.

 

[ashefrin]

I recently purchased a 1980 50 Hatteras convertible and the Generator has had some problems
when i purchased it I was told there were 900 hours on it and that is what it showed on the meter,
i was also told a relay was not working As the boat was being moved for me the captain called me and said the gen quit and the wiring harness was smoking Now the generator does not work and the yard suggest replacing to the tune of more than $20k does anyone have suggestions how to confirm the generator is reparable ...Thanks for any advice

 

[Boatsb]

It can be fixed. New harness, some parts and pieces I'm sure but it can be repaired..

 

[52CMY]

I recently purchased a 1980 50 Hatteras convertible and the Generator has had some problems
when i purchased it I was told there were 900 hours on it and that is what it showed on the meter,
i was also told a relay was not working As the boat was being moved for me the captain called me and said the gen quit and the wiring harness was smoking Now the generator does not work and the yard suggest replacing to the tune of more than $20k does anyone have suggestions how to confirm the generator is reparable ...Thanks for any advice

First Welcome Ashefrin. I'm a very new member of this forum as well--Lot's of great info and advice on this forum--sadly I'm afraid may be too new to my own issues with my Onan 20 MDL4 (Took me a while just to find the Model number plate hidden by an engine room (ER) bulkhead and ER Fireboy firebottle on my 1990 52CMY.
What model GenSet do you have? Sadly I have just recently found with the MDL4 I have, after Onan was bought by Cummins in the early 1990s, Cummins stopped supporting the MDL4 with parts and factory support just as soon as legally permitted, and that was a very long time ago. There is another forum called SmokeStak that has an valuable and very active sub-group specific to old Onan diesel GenSets. Some components are still available from a Cummins retailer--(I just bought a new Fault Reset Circuit breaker switch from Cummings in Dallas last week. turns out that was not causing my Fault CB button to pop each time my MDL4 is trying to start--now I have a spare shunt CB).
I got into my Control Box and can disable the "2B" lead to the CB and the GenSet runs like a top--I'm still tracking down which sensor (LOG, HEGT, OSpeed, or HCT or the time delayed Oil pressure after start sensor is bad or I have a bad ground somewhere)

My plan is to not put any serious money into repairing this archaic machine as it's just not reliable or supportable enough to be my sole source of AC power.
So I am considering several options:
1) add a large bank of modern Lithium House batteries (LiFiPro batteries) and redundant modern Invertor/charger system, plus extra alternators to my Detroits and solar.
2) add a small 6-7.5kW new genset as my primary generator, keeping the old Onan as my backup when I need more power.
3) give up on the Onan now and replace with a 12-16kW--I really don't need 20kW.
4) keep dealing with my Onan for as long as possible--maybe buy a portable honda gas generator (EU1000 or EG2800 for <$1500) out in a vented box in the cockpit as an "emergency" option to save the day when the Onan dies out on the water to get back to a marina to fix.

I must say, our big-boat sailboat friends and off-grid solar house folks have really made huge strides in electrical system design that are becoming pretty darned attractive to me to incorporate on my Hat. The more I've learned from various youtube bloggers and websites, the more likely I'm investing in an options where i'm not dealing with my generator loudly and wastefully running all the time when I'm not at the marina!

 

[Captain Ralph]

I recently purchased a 1980 50 Hatteras convertible and the Generator has had some problems
when i purchased it I was told there were 900 hours on it and that is what it showed on the meter,
i was also told a relay was not working As the boat was being moved for me the captain called me and said the gen quit and the wiring harness was smoking Now the generator does not work and the yard suggest replacing to the tune of more than $20k does anyone have suggestions how to confirm the generator is reparable ...Thanks for any advice

I'm assuming you are talking about the DCvolt wire harness around the gen-set engine that is smoking. That is not good but easly repairable. I feel that is just a visual result and not the whole picture of failure.
However, if the engine overheated and not shut down because of a bad relay or the engine (sized) because of no oil pressure and not shutting down because of a bad relay OR a few other things that this un-defined relay was supposed to control is a boo boo on you.

Now, you are asking this forum to read the the yards mind as to what really happened and how to compare repair estimates??

Can you offer any thing else the yard said is wrong with your gen-set that may help us, help you?

Model number of the gen-set sure would help also.