Onan generator MCCK emergency shutdown device replacement?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Sep 22, 2015.

  1. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I was given an old Onan MCCK marine generator.
    The K7 relay-emergency device is crushed so beyond repair, and these parts are long ago out of production.

    What I would like to do is design something that would function in a similar way.
    The way the device works is if low oil pressure (LOP) switch S4 conducts from loss of oil pressure, power flows to a thermal time delay device (K7) connected to ground , (functions like a thermal breaker), after 1 minute, the device trips open cutting power to the generator off-on-off switch S1, so it turns off like you flipped the switch to off. the k7 device is in circuit before the S1 switch with one end connected to B+ battery voltage.

    The "emergency relay" K7 device has 4 wires connections, two for the heater circuit to trip it off, two for contacts that cut the power. A red button pops out and after it cools this can be manually reset.

    I took good pictures of the wiring diagram and schematic, originally drawn up in 1968 https://picasaweb.google.com/lh/photo/d81O9tFegpnW2gk3QSMGI9MTjNZETYmyPJy0liipFm0?feat=directlink
    If I bypass this shutdown control, and the gen ran out of gas, or overheated, or had low oil pressure, it might kill the ignition, but it would crank till the battery went dead.

    I was thinking maybe a timer circuit instead of the heater circuit, say after a minute of time it could cut power to the generator at the S1 off-on-off switch?
    What I need is someone to look here and tell me how I can do this.
    Thanks
    K7 relay shows up in lower left of picture, S1 switch lower right on the wiring diagram.
    If the timer then kills the power to the gen , perhaps this could be made so the reset would simply involve moving the off-on-off switch off and then on again? Instead of a button that pops out?
    And if a relay gets involved, in an error condition, you would not want it consuming power.

    So I need some ideas on how to make something else work here.

    link to picture of schematic, it can also be downloaded from there. At top, click more - download photo, edited is brighter than original.
    https://picasaweb.google.com/lh/photo/d81O9tFegpnW2gk3QSMGI9MTjNZETYmyPJy0liipFm0?feat=directlink
     
  2. philSweet
    Joined: May 2008
    Posts: 2,226
    Likes: 139, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    ... or until the starter died or caught fire. Both happen. I was a customer of remote USFS water pumping systems that would do this (Onan powered, w/ solar battery maintenance). No water - USFS comes out in 2 weeks and adds oil and decides battery is dead. Comes back in 2 weeks with battery. Comes back in another two weeks with starter. This happened about twice a year for five years.:rolleyes:

    You definitely want that mechanical latch cutout if there in any remote or autostart feature (off, on, auto SW; not off, on, off).

    It looks like you will need two devices, a plug-in DC current or voltage sense relay; and a mechanical latch with manual reset. It's getting harder to just find these things in the local electrical fix-it shop. You'll need bases for the relays, and perhaps a couple resistors.

    Wholesale prices for the lot would be about $5, but good luck getting the stuff in hand for less than $50. I'd keep trying to find a replacement.
     
  3. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I have an idea of using a 2 pole DC breaker maybe 10 amps.

    One side runs the gen and other side takes current from the LOP condition.

    Run the oil pressure circuit LOP through a 40 amp cube relay, this preserves all the exiting gen wires and relays.

    Take power from the start battery to cube relay, through a 1 ohm resistor and to the breaker to ground. After about a minute of time delay the 2 pole breaker will trip off ending the cranking.

    so a low oil condition sends power to the cube relay which sends power through a current limiting resistor to the 10 amp breaker, after a minute or so, the 2 pole breaker trips off, killing the gen power.

    I plan to measure the real current amperage of the control box when gen is running, If lower than I think, might be able to use a 5 amp 2 pole breaker. All the current flows through K7 emergency relay breaker. except for starter, maybe the coil, I need to check that..
     
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