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Old 11-12-2019, 20:39   #1
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Onan diesel generator

I have a 9.0kv onan diesel generator.....after running all night it stopped running...I think I might have fouled up the filter with sand and grass....anyway now it starts but no water is flowing through it to cool it...(filter has been cleaned) ..I have been told it might be the water pump impeller....
Question does the generator have a temperature gauge that might not want to open? Like I said I have no clue about generators....help...
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Old 11-12-2019, 21:14   #2
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Re: Onan diesel generator

It'll have a thermostat on the fresh water side, but that has zero effect on raw water flow. Maybe something jammed in the through hull or a gunked up heat exchanger? Failing that, yes it could very well be an impeller issue.
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Old 12-12-2019, 07:24   #3
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Onan diesel generator

Way this often happens is that something clogs the strainer or thru hull, that leads to the impeller overheating and breaking apart.
So even if you replace the impeller, be sure the raw water side isnít blocked before you start it again, or you may lose another impeller.
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Old 12-12-2019, 09:34   #4
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Re: Onan diesel generator

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Way this often happens is that something clogs the strainer or thru hull, that leads to the impeller overheating and breaking apart.
So even if you replace the impeller, be sure the raw water side isnít blocked before you start it again, or you may lose another impeller.
Yes I took the strainer apart and cleaned it very well....the other question I have...one of my mechanics said that the strainer was kind of small and might be a good idea to change it for a bigger one....has anyone done that? I kind of like the idea...being in Florida we have a lot of shallow waters and I can see how that would foul up the strainer and choke off the generator....
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Old 12-12-2019, 09:38   #5
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Re: Onan diesel generator

I have the MDKBM model which is either the same as yours or perhaps one model higher but with the same engine block assembly. Mine goes through a lot of impellers so I've gotten better at changing them out. Since the cooling pump impeller is directly attached to the engine there will always be raw water flow through the engine regardless of temperature. Most likely you need to replace the impeller. If the tines are all gone, then it is a good idea to add 3-4 hours more work and clean out the heat exchanger, since that is where those bits of impeller end up and block the water flow there. The detailed shop manuals for the generator are on my website, if you need them.
Putting a new impeller on is not easy, it can be easier to remove the whole pump assembly from the side of the generator and then replace the impeller there; I just saw that done and I believe it is a lot faster and easier than just replacing the impeller - since that is hard to get into the pump receptacle at just the right angle to let it slide in.
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Old 12-12-2019, 10:30   #6
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Re: Onan diesel generator

I also have an Onan genset. Perfect in every way except the impeller (as Zanshin says). A quick internet search will find lots of information. You'll see some people install an AC air conditioner water pump ahead of the impeller and wire it to go on with the genset.

There are many reports of an impeller lasting less than 100 hours. If you haven't changed the impeller in the last year, do that first and see if the problem goes away. Even if there's another problem, the impeller needs to be changed anyways.

Zanshin is also right that the impeller is a pain-in-the neck to change because of the pump design. I've done it, but usually leave it to a mechanic. When the impeller is removed you always count the number of missing vanes and then fish that number out of the heat exchanger (this part isn't hard).

I'd also caution you on running the genset all night unless the AC is running steadily. You need to keep a good load on the genset or it carbons up and you have big repair bill.
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Old 12-12-2019, 10:41   #7
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Onan diesel generator

I gave up on generator water pumps, I think them foolish but understand why they are there itís for ease of installation.
I removed my water pump and instead use a March AC electric water pump that is used for air conditioners, in that use they run for years, likely tens of thousands of hours and never require any kind of maintenance whatsoever.
They must however be installed in the hull below the water line, just like they are for air conditioners.
I wired it direct to the generator output, so when the generator is making AC power, the pump is running, this means that there is maybe a 5 sec time from engine start until there is water flow, but that hurts nothing of course, but it does make it so that excessive cranking cannot flood the engine because there is no water flow until there is AC power.
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Old 12-12-2019, 14:15   #8
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Re: Onan diesel generator

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I gave up on generator water pumps, I think them foolish but understand why they are there itís for ease of installation.
I removed my water pump and instead use a March AC electric water pump that is used for air conditioners, in that use they run for years, likely tens of thousands of hours and never require any kind of maintenance whatsoever.
They must however be installed in the hull below the water line, just like they are for air conditioners.
I wired it direct to the generator output, so when the generator is making AC power, the pump is running, this means that there is maybe a 5 sec time from engine start until there is water flow, but that hurts nothing of course, but it does make it so that excessive cranking cannot flood the engine because there is no water flow until there is AC power.
Interesting thought about changing to a March AC ..... do you have a model number? Will it work with my Generator.....if it does indeed remove the issues of the impeller...I would be willing to give it a try.....by the way I have a 46í phantom ..... gen is below water line....
thank you...
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Old 12-12-2019, 14:29   #9
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Re: Onan diesel generator

I used the March pump that is used for my 16K air conditioner, that way the same spare covers both. I think you need to find out the water flow required by your generator manufacturer and then at least match that, in my case I exceeded the water flow but it doesnít seem to cause any problem as the motor is running before the water flow starts,and the water flow ends just before the engine stops so it seems to blow out any water in the exhaust system as the engine coasts down.
Sorry but Iím afraid my water flow requirement may be different than yours as mine is a small generator.
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Old 13-12-2019, 16:23   #10
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Re: Onan diesel generator. FIXED...

Generator fixed......one fin on the impelled broken.....humn I wonder how much it is going to cost me....

Thank you all for your great input...
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