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Onan MDKBT 27kw oil press senders

Discussion in 'Generators' started by Pascal, Sep 27, 2016.

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  1. Pascal

    Pascal Senior Member

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    getting this odd glitch on BOTH 27kw Onan MKDBTs (2009 with 1500 and 2200 hours)

    After running for longer time periods, at least 24 hours, oil pressure readings gradually goes down to the point where it will pre alarm on the Onan panel as well as the alarm on the boat Isis monitoring. Eventually, a few hours later generator will shut down with oil pressure fault.

    I don't see any oil pressure switch in the parts book or wiring diagram, it seems that Onan uses the sender output to trigger a shut down.

    Oil level good, changed the oil and filters on both units over the past few weeks but still happens. If anything the drop seems to happen more rapidly now, after 12 to 18 hours.

    Both gensets show the same symptoms.

    I have already ordered two sensors so I can replace them when we get back home on Thursday but I find it odd that would take so long for pressure reading to come down.

    anybody seen that failure mode?
  2. Pascal

    Pascal Senior Member

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    Forgot to mentions that along with great pressure drop, pressure also fluctuates fairly rapidly by 6 to 9 psi

    Also, I checked the obvious including making sure the wire on the senders were tight but I found a disconnected wire next to the senders... White like the sender wire, same ring terminal but disconnected. Sender only has one terminal. Same loose wire on both gensets
  3. Capt Ralph

    Capt Ralph Senior Member

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    That wire was used for an original two wire sender. No telling what vendor Cummins/Onan is using this week or if it will have one or two studs.
    If you OHM it out, it should short to case ground. Ah, a thought, in the box is a stud for all signal grounds, logical ground and controller ground. Separate to the ACv green, & white wire stud.
    If loose from vibration and heat, could cause a bad connection.

    Questions; Is your oil level increasing or smell like fuel? Are you using a synthetic oil?
  4. Pascal

    Pascal Senior Member

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    Oil level is good, no fuel getting in. No smell. Using rotella 15/40
  5. Capt Ralph

    Capt Ralph Senior Member

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    This is good.


    Not good; I can not find a second oil p sender in the parts lookup.
    Usually there are two, one for a gauge the size of a shot glass, and a oil p switch, that usually shuts the motor down.
    Sometimes Td together, sometimes on different parts of the engine.

    Oh, that extra white wire is also used for isolated grounded units.
  6. Capt Ralph

    Capt Ralph Senior Member

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    Have you measured to oil going into the engine?
    Those rubber stopper dip sticks can fool you.
    While your waiting for those senders to come in, try lowering the oil level a tad.
    Foaming oil can cause some headaches like this on small engines.
  7. Pascal

    Pascal Senior Member

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    Level is mid dipstick so foaming not an issue. No sign of foaming when I pull the stick after shut down. I ll check the stoppers but paint markings would have been obvious.

    I double checked th diagrams, no additional pressure switch. Only the sender which the electronics use to display pressure on the digital panel and to shut down At a preset value.

    Problem is getting worst... Now last 10 to 12 hours before alarm is triggered. Second night in a row the Isis monitoring alarm in our room goes off around 5am :( good,thing we re on way back:)
  8. Lepke

    Lepke Member

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    I would put in a tee and a mechanical gauge and verify the pressure. I'd also like to know the oil temp. Years ago I ran into a similar problem that finally was traced to a bad batch of oil pressure relief valve springs the oem got from their supplier. As the engine and oil got hotter, the spring would relax more and more and allow the relief valve to release more and more pressure.
  9. Capt J

    Capt J Senior Member

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    I'd also switch from Rotella to Mobil delvac. I've seen the rotella not make nearly as much oil pressure after it gets 50-100 hours on it.
  10. Capt Ralph

    Capt Ralph Senior Member

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    I have witnessed this also, but on larger engines. AND, very long ago. Anything is possible.
    Any history or logs from previous captain?
  11. Capt J

    Capt J Senior Member

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    But both of them doing it......I know they're the same brand/size etc. But seems like the issue would pop up at different times.

    Wait a second, what brand of oil filters are you using on them? Perhaps it's an oil filter issue.
  12. Capt Ralph

    Capt Ralph Senior Member

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    Hopefully not Frams...

    On the 844L block, The OhNo or NL filters are only to use.

    And again, a bad vendor batch of oil pump reg springs could effect both engines. Assuming serial numbers are probably close in series.
    Lots of use on this block but you never know.
  13. Pascal

    Pascal Senior Member

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    No history or log. Used both Onan and purolator filter (all I coudl get) but that made no difference. First oil change was with Cat brand 15W40, second with rotella. I ve used rotella for 8 years/ 7000 hours on northern lights / 3000 hours on 3412Es with no issue...

    Will check oil temp.

    Going to replace the senders when I get to maimi and see what happens.
  14. Capt J

    Capt J Senior Member

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    I would start with the senders. But once in Miami I would hook an analog oil pressure gauge in to see what actual oil pressure is. It sounds to me that it's a heat related issue (temperature inside sound box), possibly a wire going bad or something like that, but it's both......perhaps the oil pressure senders only go so long.....
  15. Capt Ralph

    Capt Ralph Senior Member

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    Cat uses the same block for the 2.2 gen sets. Just deeper oil pan.
    Good oil. Good block.
    I would also advise to ensure the case breather is clean. Throwing the mini turbo on changed the CCV setup.
    Don't think it's an issue here but usually overlooked maintenance.
    Skippy J has a good idea, your there at the fitting, put a T in and an ole mechanical gauge.
    Make it pretty and permanent.
  16. CSkipR

    CSkipR Member

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    On a Westerbeke the sender are called an oil, temp, exhaust switch.
  17. Pascal

    Pascal Senior Member

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    Looks like it turned out to be the senders. Replaced both of them and added a Tee with mechanical gauge. Ran 36 hours earlier this week and pressure held steady, no alarm.
  18. Capt Ralph

    Capt Ralph Senior Member

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    Outstanding news. Thx for the update.
    What kind of pressures are you observing on the mechanical gauges?
  19. Pascal

    Pascal Senior Member

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    Around 50/55... Not as precise as the digital but close enough.
  20. Capt J

    Capt J Senior Member

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    That's great news. You might want to keep a spare sender on the boat. I've had other various safety sensors (raw water flow), coolant temp, etc. go bad and always at the wrong time.