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Onan MDJF not charging the start battery

Discussion in 'Technical Discussion' started by JoeQ, Jul 25, 2008.

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  1. JoeQ, Jul 25, 2008
    #1
    JoeQ

    JoeQ New Member

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    I have an Onan MDJF generator that starts and runs fine. It produces AC and does what it is supposed to do as far as AC is concerned. The problem is that there's no DC output to charge the starting battery.

    It has a 10 amp ammeter that shows a small negative deflection when the genny is running.

    Any onan experts or anyone with a wiring diagram or workshop manual? Any ideas?

    Thanks,
  2. K1W1, Jul 25, 2008
    #2
    K1W1

    K1W1 Senior Member

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    Hi,

    I take it that the Battery Charging is doe with a separate Alternator mounted on the engine somewhere- Is this the way it is setup?

    If your only way of seeing that the thing is charging is the installed ammeter, put a voltmeter on the battery lugs when starting, observe the voltage with the engine off, crank it and see if the voltage figure goes up or remains the same/drops when running.

    There could be a number of reasons why you get a negative figure when running, the DC Exciter( on the AC Alternator bit) may be battery fed or there may be other powered things fed by the battery when the genset is running.

    If there is no change or it drops then you probably have an alternator or voltage regulator problem, not knowing the exact arrgt I would suggest that if you are somewhere civilised you remove the battery charging alternator and take it to an Auto Electrician who can run it on a test bench to see what the score is.

    Most modern Battery Charging alternators have built in regulators.

    Another point to look at is- Have you had any electric welding done onboard recently, or any big electrical surges?
  3. JoeQ, Jul 25, 2008
    #3
    JoeQ

    JoeQ New Member

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    Thanks for the response. There isn't a separate alternator for battery charging. I'll check with a volt meter as you suggested the next time I'm aboard.

    As for welding, yes. There was a contractor who attempted to weld the exhausts on the main engines. Would this have caused a problem?
  4. K1W1, Jul 25, 2008
    #4
    K1W1

    K1W1 Senior Member

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    Hi,

    Ok, It's one of these things that winds up and then charges the battery, does the alternator also provide the starter service? I have only seen 1 or 2 of these Onan sets and it was more than 20 yrs ago so my memory is a bit vague.

    Electric Welding may well be the root cause of your problems, the side effect of welding with non isolated alternators is often to blow the diodes.

    You might save yourself a lot of grief and trouble if you get a competent electrician to have a look at it for you.

    I have had a bit of a look at a couple of sites to see if anyone has manuals etc. I see you have posted this same question in a few other places. Hopefully someone who can help you out with that will read your post.

    Good Luck in finding a speedy solution.
  5. JoeQ, Jul 25, 2008
    #5
    JoeQ

    JoeQ New Member

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    No, it's got a separate starter motor. If I could lay my hands on a wiring diagram I'm sure I could trace the problem myself. Thanks for your assistance.
  6. Capt J, Jul 28, 2008
    #6
    Capt J

    Capt J Senior Member

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    It sounds like the diodes or voltage regulator in the alternator is bad.

    Welding definately could have caused the problem.

    Try the Onan website, or call them or a local dealer. They are very helpful and should send you a wiring diagram without an issue
  7. JoeQ, Jul 28, 2008
    #7
    JoeQ

    JoeQ New Member

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    Thanks for all you suggestions, including the private messages. I've contacted Onan and will hopefully have a resolution very soon.

    It does appear that welding may have been the problem.