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Old 07-07-2016, 07:58 PM   #1
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Onan 3MDJA Won't Run

I am new to the forum, and have searched, but not found a thread dealing with this problem so here goes. We have a 1980 Mainship 34, Sirius,with an Onan 3MDJA generator. The last time the generator ran it was on light load (the battery charger) for about 10 minutes, then the decompression solenoid de-energised and it stopped.

So far I have checked the cooling system in case it was an overheat, all clear. Have checked the high air, high water temperature cut outs, and the oil pressure switch, all good. I have inspected the brushes and they all look good to me. Trying to start it, the oil pressure switch closes, the decompression solenoid energises, the engine fires, but stops when the start switch is released, because the decompression solenoid de-energises.

I found the engine would run if I jumped across the current reversal (charge) relay. The jumper keeps the solenoid energised from the battery rather than the generator (while the generator is running), but there is no output from the generator, 12 volt or 120 volt.

Can anyone identify the problem, and its possible solution? Any help much appreciated.
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Old 07-07-2016, 08:50 PM   #2
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When it starts up, it the AC section making volts?

You really need a full schematic to troubleshoot these things, and they are not easy due to the gen end and starter functions being combined in one.
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Old 07-07-2016, 09:12 PM   #3
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If mine acts up I hold the shut down solenoid from activating for about 5 or 10 seconds and it usually clears up the problem.
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Old 07-08-2016, 12:09 AM   #4
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The start/stop solenoid is often a problem, only sure fix is a new one. A new one from Cummins/Onan was $ 350 AUD(made in China),I found one ebay from China (but with local stock) for $60. Mine would not pull in, but if I pulled it across it manually it would hold and run. They have 2 separate functions, an initial pull in, and then a hold.
Is the decompression solenoid a different solenoid, or is it the start/stop solenoid?
No ideas about the lack of output, but I found a manual online for my MDKD and it had a trouble shooting section.
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Old 07-08-2016, 09:09 AM   #5
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This gennie is not very similar to the two and four cyl MDJ series. It is a single and the run stop is controlled by a solenoid that trips a compression release on the valve gear, no connection to fuel system. Also no starter motor, the starting function is embedded in the generator end. It has a series of relays that send DC into the gen end, the comp release stays on to make it easier to spin, once oil pressure comes up the comp release disengages and engine fires. Once running and start released, the relays change position and the gen end starts making AC. Lots happens for these to work.
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Old 07-08-2016, 01:11 PM   #6
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Thanks for the replies.
When I get the engine running with the current reversal relay jumpered there is no generator output, ac or dc.
The solenoid doesn't seem to be the problem, it will energise in the starting cycle when the oil pressure comes up, but once the engine starts, it doesn't get energized from the generator.
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Old 07-08-2016, 03:00 PM   #7
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May be an issue with the AC side of things, not making AC to latch the relay. Can't help much beyond that.
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Old 07-16-2016, 12:19 AM   #8
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i HAVE AN MDJA

I run my decompression solenoid on a its own switch. When I start up my generator I get it revolving quickly and then flick the decompression solenoid and start it running.
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