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Old 09-25-2015, 12:28 PM   #1
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Genset Puzzle

I have a Onan genset, model MDKAU with 10 years experience but only 220 hours. I am the 4th owner of the boat and as far as I can tell the POs did a good job with maintenance.
The problem: genset runs 20-30 minutes then cuts off. When running it has robust water flow and doesn't have a problem under load.
When it shuts down I get a "service needed" code followed by a secondary problem code #58 which is the "high exhaust temperature" code.

What I have tried to fix problem: replaced exhaust line from engine to outside, removed seawater pump and serviced, replaced impeller, cleaned heat exchanger, replaced all gaskets, new thermostat and gasket, new inlet and outlet hoses, and replaced corroded wires and terminals on pressure switch.

Still having same problem. Suggestions??
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Old 09-25-2015, 12:47 PM   #2
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The switch itself?
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Old 09-25-2015, 12:51 PM   #3
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Its possible the exhaust mixer elbow is corroded and/or clogged. They are almost always cast iron and have a finite life.

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Old 09-25-2015, 02:08 PM   #4
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Peter...I bypassed the switch with same results but have not tried replacing.

Ken....I forgot to include that the mixer elbow was replaced as well when I reworked exhaust. That was one of my first thoughts as well and the original was in bad shape.

Frustrating. I'm wondering if the on/off switch on the unit (not the remote) is faulty.
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Old 09-25-2015, 02:15 PM   #5
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check strainer, seacock and inlet.
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Old 09-25-2015, 02:34 PM   #6
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Thanks. strainer clean and I observe plenty of water being pumped out. I have not had a diver check the outside hull strainer but would guess it is clear.
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Old 09-25-2015, 03:25 PM   #7
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Curious whether the fault codes were present and the same before your servicing or perhaps a recent result of it. Also any idea at what temperature threshold the shutdown is triggering?If after your servicing then perhaps a bad contact on the temp sensor bad sensor or an air bubble in the cooling system. If before maybe a broken vane from an impeller or two stuck downstream from the water pump. You might want to try and test the sensor off the engine with a meter to see what temperature it wants to shut off at before replacing it.


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Old 09-25-2015, 03:57 PM   #8
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Air bubble in the coolant tank? It happened to me, but after a coolant change.
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Old 09-25-2015, 04:15 PM   #9
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Greetings,
Mr. BR. Have you tried something as simple as Goggling "Onan MDKAU stops"? I just did an got a pile of hits.
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Old 09-25-2015, 04:20 PM   #10
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What is Code #58?
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Old 09-25-2015, 04:41 PM   #11
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I would check the temperature with a IR thermometer to see if it's really overheating. If the temp is normal, you might have a faulty exhaust temp sensor or circuit board.
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Old 09-25-2015, 04:46 PM   #12
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Cafe. #3 code showed up during winter decommissioning last year. But the follow up sub code #58 did not. The assumption at the yard was that normal servicing (replacing engine zincs, oil and fuel filter change, impeller change) would satisfy the code. This spring it ran fine the first two trips but then started shutting down after 30 minutes. That's when the code #58 showed up for the first time. That's also when I started doing all the things I listed in my opening message.
Your message did trigger a thought for me. When I replaced the thermostat I noticed that the coolant tank on top of the engine was extremely full but the remote overflow tank wasdry, even though the engine had just run for 20 minutes. No leaks so I'm wondering if there is a bubble or kink in the overflow hose. Don't know if that would cause a sensor to think overheat but it's something else to check.
I'm trying to do as many things as I can before paying the yard for labor charges on stuff that I can eliminate. Already paid a bunch with no fix.

Thanks for the suggestions.
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Old 09-25-2015, 04:50 PM   #13
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Mr. RTF. Thanks. I haven't tried that approach. Duh. Sometimes it's the logical stuff that I overlook. I just tried the Cummins web site which was not much help b
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Old 09-25-2015, 04:52 PM   #14
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Mahal. Thanks. Another good idea.
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Old 09-25-2015, 05:06 PM   #15
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I would certainly check the exhaust/water mixer to make sure it is not clogged. If you have a digital pyrometer, you can measure the temperature of the mixer and exhaust hose when running. I don't have the specs for this generator, but I suspect it should not be more than 130F or so. My 4KW Kohler runs about 105F in 80F, a 25F temperature gain.

You can troubleshoot the sensor with a multimeter and a hair dryer. Heat the sensor until the contacts close and measure the temperature with the pyrometer. Continue monitoring as it cools and check the temperature when the contacts open. This will give you an idea if it is functioning or not, and at what temperatures.

From the Owner's Manual for the Onan MDKAU:

HIGH EXHAUST TEMPERATURE—CODE NO. 58
(Exhaust temperature exceeded design limits)
Corrective Action:
1. Open the sea cock all the way.
2. Close the sea cock and clean the sea water strainer. If the strainer is above the water line, fill it with
water to assist priming. Secure the strainer cover and reopen the sea cock.
3. Reconnect or replace any disconnected or leaking raw water hoses (Page 3-7).
4. Remove any blockage from the strainer on the through-hull fitting on the bottom side of the hull.
5. Replace the raw water impeller (Page 3-10).
6. Disconnect connector P1 (black) from controller A1 (Page 5-1) and check continuity between Pin 11
and B– (ground). If open, check for a missing, bent or corroded pin, faulty wiring or open exhaust temperature switch S5 (Page 5-6). Repair as necessary.

HIGH EXHAUST TEMPERATURE SWITCH S5
The high exhaust temperature switch is bolted to the
ear on the side of the water/exhaust mixer (Figure
5-5). The switch is accessible by removing the
enclosure back panel.
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Old 09-25-2015, 06:42 PM   #16
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Quote:
Originally Posted by Bill Ranson View Post
Peter...I bypassed the switch with same results but have not tried replacing.
If you bypassed the switch electrically, and it still shutdown on high exhaust temp, then it has to be the control circuit. How exactly did you bypass it?
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Old 09-25-2015, 06:48 PM   #17
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Have you replaced the coolant? 10 years old it will be shot. I don't want to get into this again (arguments) but bad coolant causes cavitation around the cylinders and the result can be local overheating and corrosion. Might be as simple as that?
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Old 09-25-2015, 09:07 PM   #18
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I've got some coolant from my old International Scout, 1980 model, diesel, that I would give you. It's originall but was working fine when removed for a lower radiator hose replacement. The truck even has a sticker that says "lifetime coolant, do not replace" so It's good. (couldnt resist, all in fun). Seriously tho, I do however have some parts for the MDK series gensets.
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Old 09-26-2015, 09:48 AM   #19
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I was speaking of the pressure switch. Bypass was probably incorrect term but I removed it from the electric circuit thinking that it could not effect the shutdown if the genset started with it disconnected. It checked out fine with a multimeter.
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Old 09-26-2015, 10:15 AM   #20
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You mention checking a pressure switch. Are you checking the raw water pressure sensor or the exhaust temp switch? Has the genset overheated before? I had a genset (not an Onan) overheat on a previous boat and it shutdown. The temp switch had to be replaced before it would run properly again. The dealer told me that an overheat usually ruins the temp sensor.
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