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Old 05-27-2017, 08:57 PM   #1
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Onan 7.5 MDJE surging at no load

When I got back to the boat this year and fired of old Mr. Shaky I found that it would surge at no load. No surging under no load when starting, only after running under load and then shutting down the load. Seems to run fine under load. I've read the troubleshooting table and was hoping that one of my fellow trawler'ers might help me narrow the problem to this particular no load condition.

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Old 05-27-2017, 10:33 PM   #2
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I had a similar but not exactly same problem with my Onan 7.5. Turned out to be a points adjustment. Very little in the manual regarding the reason for the points, just how to adjust.

I ended up making a schematic diagram of the circuit to figure out how it worked. I don't have the diagram or manual with me and it has been a while since I had the problem. But I do recall it had something to do with the keep alive voltage for the shutdown solenoid. The points would arc on and off causing the 12vdc to the solenoid to go up and down resulting in the solenoid cycling and the genset to surge.

If you see the shutdown solenoid cycling check the points.

Let me know if you need more detail and I will go to the boat to look at the diagram and manual to refresh my memory.
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Old 05-28-2017, 01:06 AM   #3
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There is a gain adjustment on the governor. Lower the gain and it will increase stability. But if it is stable under load, might not bother with it.

Check that governor linkages and injection pump control lever are all free from binding. Any binding there reduces stability.
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Old 05-28-2017, 10:38 AM   #4
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There is a gain adjustment on the governor. Lower the gain and it will increase stability. But if it is stable under load, might not bother with it.

Check that governor linkages and injection pump control lever are all free from binding. Any binding there reduces stability.

You might try adjusting the gain linkage as Ski recommended above, but do so with caution. If you adjust the gain downward too much the generator will be low frequency at high loading levels.

That particular governor is a tricky one. Another trick is to look at theplate that the flyballs ride on. If the plate has a groove worn in it, you can turn it over to get a smooth surface.

On that particular generator I would probably leave it alone, and just put a load on it. If you think about it, having RPM stability with no load applied is vbery difficult as slight changes to throttle setting make a large rpm change.
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Old 06-01-2017, 10:11 AM   #5
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Thanks for all the possible fixes. I checked the points and found that they were out of adjustment by about .015. Adjusted them to the recommended .020. Didn't change the surging. At this point I'm going to let it go and see how Mr. Shakey does when out in the field. Just dock checks so far.

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Old 06-01-2017, 10:25 AM   #6
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Thanks for all the possible fixes. I checked the points and found that they were out of adjustment by about .015. Adjusted them to the recommended .020. Didn't change the surging. At this point I'm going to let it go and see how Mr. Shakey does when out in the field. Just dock checks so far.

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Points???

The MDJE is a Marine Diesel generator. 7.5KW
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Old 06-01-2017, 10:34 AM   #7
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Greetings,
Mr. ks. Yes, the Onan IS diesel BUT it it's anything like my 15kw there is a set of points associated with overspeed something or other. Overspeed cut off perhaps?
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Old 06-01-2017, 11:10 AM   #8
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Greetings,
Mr. ks. Yes, the Onan IS diesel BUT it it's anything like my 15kw there is a set of points associated with overspeed something or other. Overspeed cut off perhaps?
The only set of points on a ONAN DJ series generator are the starter cut off for automatic starting. They are located on the front of the generator on the accessory case and have no bearing on the operation of the generator.
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Old 06-01-2017, 10:08 PM   #9
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The Centrifugal Breaker Points are located under a plastic cap near the injector pump on MDJE 7.5 generators, it's part of the starter startup and run cycle. Needs to be set at around .040. Once set, you seldom have to check them again.
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Old 06-02-2017, 06:36 AM   #10
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Greetings,
Mr. E. They're the points I was talking about in post #7. Thanks. I'm not aboard and for the life of me couldn't remember what they did but I DID have a no-start situation when mine were not set correctly.
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Old 06-02-2017, 09:44 AM   #11
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The photos of the points are WONDERFUL.

But again, the points do NOTHING that affects how that engine runs.

You do not even need them.

In several engines I've worked on they were bypassed by some previous mechanic.

The ONLY function that they serve is to disconnect the starter motor once the engine starts.

The toggle switch that you press to start the generator does the exact same thing. When you release the toggle switch the starter motor disengages.
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Old 06-02-2017, 10:51 AM   #12
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The photos of the points are WONDERFUL.

But again, the points do NOTHING that affects how that engine runs.

You do not even need them.

In several engines I've worked on they were bypassed by some previous mechanic.

The ONLY function that they serve is to disconnect the starter motor once the engine starts.

The toggle switch that you press to start the generator does the exact same thing. When you release the toggle switch the starter motor disengages.
In addition to disconnecting the starter the points also provide a keep-alive voltage to the shutdown solenoid. I believe this is a safety feature to shut off fuel if the engine quits spinning.

I ended up tracing every wire and drawing a schematic diagram to figure it out.

The output from the points, 12vdc nominal, also goes to the shutdown solenoid. If the 12vdc is lost the solenoid will shutdown the genset.

In my case the 12vdc was dropping to various voltages around 8vdc due to the points not being adjusted properly. This caused the shutdown solenoid to activate and de-activate rapidly, resulting in the genset surging.
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Old 06-02-2017, 11:07 AM   #13
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In addition to disconnecting the starter the points also provide a keep-alive voltage to the shutdown solenoid. I believe this is a safety feature to shut off fuel if the engine quits spinning.

I ended up tracing every wire and drawing a schematic diagram to figure it out.

The output from the points, 12vdc nominal, also goes to the shutdown solenoid. If the 12vdc is lost the solenoid will shutdown the genset.

In my case the 12vdc was dropping to various voltages around 8vdc due to the points not being adjusted properly. This caused the shutdown solenoid to activate and de-activate rapidly, resulting in the genset surging.

Something is not making sense there. This is coming from memory, and allot of generators were modified by owners because the ponts were problematic.

The points open when the engine reaches a speed just above cranking speed.
Since the points open the circuit they cannot carry the solenoid current.

The solenoid current passes through the oil pressure sender and the temperature sender. If either of those senders opens the solenoid dropps out, the plunger moves foreward forcing the injection pump to zero fuel.

The only thing I can think of in your case is that the installation must have used a toggle switch that you just put to the "on" position to start and run the generator. If that was the case then the points would be in the start circuit, and them opening and closing would engage and disengage the starter motor. That I suppose could cause DC voltage to swing based on the large starter current.
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Old 06-02-2017, 12:43 PM   #14
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The Centrifugal Breaker Points are located under a plastic cap near the injector pump on MDJE 7.5 generators, it's part of the starter startup and run cycle. Needs to be set at around .040. Once set, you seldom have to check them again.
Interesting. My manual clearly shows .020 under the illustration. They seemed to be set to .040 and I changed them to .020. My genset is a MDJE 1978 and has a metal cover over the points.

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Old 06-03-2017, 10:24 PM   #15
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Interesting. My manual clearly shows .020 under the illustration. They seemed to be set to .040 and I changed them to .020. My genset is a MDJE 1978 and has a metal cover over the points.

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That was from my manual which is a 1976 model Gen set. I checked the online manual and it shows .040 too. I don't know that it matters as long as the generator starts before it cuts off the starter voltage.. It is a metal cap, I was thinking of something else. Not something I look at every day.
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Old 06-03-2017, 10:39 PM   #16
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Greetings,
Pull the fuse on the control panel before you take that metal cover off OR have a spare fuse handy. Don't ask me how I know.
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Old 06-04-2017, 08:34 AM   #17
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Greetings,
Pull the fuse on the control panel before you take that metal cover off OR have a spare fuse handy. Don't ask me how I know.
Now you tell me!
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Old 06-04-2017, 08:13 PM   #18
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Something is not making sense there. This is coming from memory, and allot of generators were modified by owners because the ponts were problematic.

The points open when the engine reaches a speed just above cranking speed.
Since the points open the circuit they cannot carry the solenoid current.

The solenoid current passes through the oil pressure sender and the temperature sender. If either of those senders opens the solenoid dropps out, the plunger moves foreward forcing the injection pump to zero fuel.

The only thing I can think of in your case is that the installation must have used a toggle switch that you just put to the "on" position to start and run the generator. If that was the case then the points would be in the start circuit, and them opening and closing would engage and disengage the starter motor. That I suppose could cause DC voltage to swing based on the large starter current.
No toggle switch. I'm certain of that.

However, this is also coming from my aged memory as well. So I'll have to dig out the diagram I made the next time I go to the boat to satisfy my curiosity and report back.
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