Onan Water Pump

As a winter project, I've taken the water pump and heat exchanger off my Onan 7.5KW MDJE genset for some PM. The heat exchanger is cleaning up nicely. That's good, it's a $550-$700 part.

The water pump works, and really had no problems before I removed it. However, there's minor corrosion below the weep hole, even though I never saw any evidence of an actual leak. On the off chance the seal is leaking, I'd like to either rebuild it or buy a spare.

But, OMG they want $1,400-$1,700 for the replacement pump!

It's just a pulley-driven impeller pump. Theoretically, I could mount any old pump there, as long as the flow rate matched. I'd just have to drill and tap new mounting holes if they didn't match up.

Any reason this would be a bad idea? Anyone know a good shop on the East coast that does pump rebuilds?

CaptTom said:

Any reason this would be a bad idea??

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In your situation, I think your idea would work.

We have an MDJE in our 1973 boat. So far the only thing we've had to have fixed was the starter, which we had completely rebuilt by a local electrical shop that does this sort of thing.

It seem that MDJEs had at least two water pump configurations. On ours, the raw water pump is direct driven from the generator itself. It is mounted at the lower right corner of the control side--- injectors, intake air hot box, oil and fuel filters, etc. It is a flexible impeller pump but is hard-mounted to the generator.

There is a water pump that sits on a little platform near the top of the right side of the generator. This is a metal impeller pump, it's belt driven from the flywheel, and it's the coolant pump for the generator.

But it seems that some MDJEs have an opposite arrangement, with a metal impeller coolant pump mounted low and direct driven by the generator and the raw water pump mounted higher and belt driven from the flywhee.

If this is the arrangement you have, then I don't thing the generator's going to care what kind of pump provides the raw water as long as something does.

I think it is an Oberdorfer pump, can source elsewhere for around $500. Also, if bearings are good with no play or roughness, and no liquid leakage, leave it alone.

This applies to the circ pump, not the sea water pump.

The owner of the marine electrical shop we use who is a long-time Onan dealer gave us a piece of advice years ago regarding the belt-driven pump on the MDJE. And that is to run the belt fairly loose. Not loose enough to come off, but just tight enough to keep the pump going. This minimizes the side load on the pump and he said the seals and bearings will last a lot longer. So this is how we've run ours the last 16 years.

Leave the old working one alone until you have to replace it. In the mean time keep your eye out for a used one that is rebuilt or one that is rebuild-able to have as a spare when the time comes to replace the old one.

The corrosion you see could have come from the front cover weeping at some point. And perhaps has nothing to do with the seal leaking.

CaptTom said:

However, there's minor corrosion below the weep hole, even though I never saw any evidence of an actual leak. On the off chance the seal is leaking, I'd like to either rebuild it or buy a spare.
But, OMG they want $1,400-$1,700 for the replacement pump!

It's just a pulley-driven impeller pump. Theoretically, I could mount any old pump there, as long as the flow rate matched. I'd just have to drill and tap new mounting holes if they didn't match up.

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The coolant pedestal pump on the platform is indeed an Oberdorfer circulation pump. Very easy to rebuild with simple tools and a vise. the rebuild kit isn't too expensive either. The pump brand new price is $600 at Pump Masters.

The Saltwater Onan gear driven pump on the side of the Onan Gen. engine is the ridiculously expensive $1500 rubber impeller pump. When it goes out consider replacing with a Jabsco pedestal impeller pump or an electric pump.

If you decide to change the Oberdorfer out with another brand of pump, keep in mind the Oberdorfer pump is a "slinger style" centrifuge pump designed to circulate hot water and not a rubber impeller pump. So a rubber impeller pump won't do! But it is far better and simpler to just rebuild it. You can order the rebuild kit on line.

If you need the Onan manuals for the MDJE, PM your email address to me and I will send them too you.

Edelweiss said:

The Saltwater Onan gear driven pump on the side of the Onan Gen. engine is the ridiculously expensive $1500 rubber impeller pump. When it goes out consider replacing with a Jabsco pedestal impeller pump or an electric pump...

If you need the Onan manuals for the MDJE, PM your email address to me and I will send them too you.

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I have the manuals, thank you.

I should have mentioned, it's the raw water pump, not the circulating water pump. Both are belt-driven off the flywheel.

I can't seem to find the flow rating for the existing part, Onan 170-3172, or its replacement, 131-0257.

I've seen the Depco site, I may call them Monday just to see what they say.

$1500 for a new pump is out of the question. I'm interested in the idea of an electric pump. I'm actually surprised that isn't done more, given how unreliable and problematic rubber belts and impellers seem to be.

I also think those who say to just clean it up and use it, since it's showing no other signs of failure, may be onto something.

Ok, sounds like somebody changed out the original gear driven pump then? I haven't seen a MDJE from the factory with two belt driven pedestal mounted pumps, but never say never!!

I replaced my Onan gear driven saltwater pump, so it's the same as yours is now. I removed the old Onan 3/8" pump, fashioned a cover plate and gasket to cover the hole. Then bought a standard Jabsco 3/8" output brass pedestal pump with a rubber impeller, which I mounted on the platform opposite the Oberdorfer pump, resized the belt and all was good. It actually flows a little more water than the old pump, but that doesn't seem to matter. I changed it out 5 or 6 years ago and hundreds of hours later still going strong.

Edelweiss said:

Ok, sounds like somebody changed out the original gear driven pump then? I haven't seen a MDJE from the factory with two belt driven pedestal mounted pumps, but never say never!!

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Our MDJE is stock and has the pump arrangement you describe. Only one belt-driven pump up above on the little platform for the engine coolant.

......bought a standard Jabsco 3/8" output brass pedestal pump with a rubber impeller, which I mounted on the platform opposite the Oberdorfer pump, resized the belt and all was good..

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Great to read your description of what you did. I'm going to write it out, print it, and stick in our Onan manual so if or when the day comes the raw water pump craps out I'll have your idea to act on. Thanks.

I second touching base with Depco Pump. I have had several pumps rebuilt by them. They are reasonable and fast. They will also sandblast the pump and paint it to match the engine paint. Great folks there. Howard

Figured I'd post a follow-up to the saga above. Over the winter, I bought an March electric pump and installed it last weekend.

The hardest part was finding where on the genset I could take off 120VAC to power it. That solved, I lowered the sea strainer so the cover is just at or below the waterline (no losing prime), got a new belt to fit the circulating pump now that the raw water pump is gone, new hoses all around, and fired it up.

I love it! There's a good, strong flow of cooling water out the exhaust now. I test-ran it for an hour under load, no issues.

Edelweiss, thanks for the suggestion!

The raw water pump bearing went out on my MDJE 7.5K last week while we were at the Isthmus on Catalina Island. The pump configuration is both pumps are up top, raw water on the left and coolant on the right both driven by the same belt. Luckily the over-temp sensor shut the generator down.

Since we were staying on the island for a week I purchased a shorter belt from the island warehouse to drive just the coolant pump then I rerouted my livewell pump to the raw water strainer for the generator and plumbed the strainer direct to the heat-exchanger. I had to top of the anti-freeze since it boiled over but I keep two gallons on reserve. Sounds kind of busy but only took about an hour and we were back in business. We ran the generator as needed for 5 days under this configuration.

Now that we're back in port I need to source a new raw-water pump. I'm thinking about going with the Jabsco Pedestal with 1/2" ports but would be interested in seeing what electric pump CaptTom used.

Greetings,
Mr. TG. Impressive improvisation!

TG I think we need to rename you MacGyver. Nicely done.
If you want to go to an electric pump, I think an Oberdorfer 104 should be about right. It's not self priming so it would need to be mounted below the waterline.

Electric pump, no question. Runs when the generator starts, no belt issues, stops when the generator stops. Put the pump anywhere you want, in your galley if you want to watch it!

I just purchased a Jabsco 1673-1001 1/2 inch Pedestal mount pulley driven on ebay for $125.00. New price is around $245 on up.

There's a guy on ebay parting out never used military surplus water filtration units that incorporated these pumps.

I like the Jabsco because of the parts availability.

TG said:

Now that we're back in port I need to source a new raw-water pump. I'm thinking about going with the Jabsco Pedestal with 1/2" ports but would be interested in seeing what electric pump CaptTom used.

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I used a March LC-3CP-MD pump:


Very happy with the results so far. And if it fails I can cannibalize one of the air conditioners.

TG said:

Now that we're back in port I need to source a new raw-water pump. I'm thinking about going with the Jabsco Pedestal with 1/2" ports but would be interested in seeing what electric pump CaptTom used.

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1/2" or 3/8" will work fine. I replaced mine MDJE pump with the 1/2" Jabsco long ago and it works great. But I did have to switch to a larger diameter belt pulley as it pumped a whole lot more water than the original. If I had it to do over again I would probably go with the smaller pump. The larger pump was a bit of an overkill.

Hi capt. Tom. I am considering converting to the same electric pump set . Can you tell me how to or Where to hook up the wires ? And if you ever bad any problems if the the water flow is to much. Thanks, for. Capt Ted

No problems at all. It's been one of my favorite fixes. Be advised I put relatively few hours on the genset, an hour or (rarely) two per day when I'm at anchor, which was a lot last year, not so much this year.

I don't think you can really have too much flow. The pump can only run when the engine AND generator end are running, and I can't imagine it being able to overcome the flow of exhaust gasses.

As for where I actually hooked it up, I don't recall. I do recall opening a couple of wiring box covers and spending a bit of time scratching my head until I figured out where to connect to. I did take some pictures, but unfortunately no notes:


The small green and white wires are from the pump.

Thanks CaptTom, I will be installing electric pump today. I am far from being an electrition and your picture helps a lot . Looks like the green and white wires connect together to the same source.? Thanks again...Capt Ted

Keep your eye out for a used take out generator on craigslist. I found one for $500 and it saved me several thousand in new or rebuilt parts over the years. Plus I still have a power plant waiting to be installed.

Onan raw water pump

I installed the march Lc-3cp-md pump .. I was concerned this pump would have to much water flow but looks perfect, maybe just a very little more flow then the origina,l which I'm very happy about.. Thanks Capt.Tom. ?