Onan thermal shutdown device circuit

Discussion in 'General Electronics Chat' started by sdowney717, Sep 22, 2015.

  1. sdowney717

    Thread Starter Active Member

    Jul 18, 2012
    I was given an old Onan MCCK 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 pressue (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 "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.
    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.
    Last edited: Sep 22, 2015
  2. sdowney717

    Thread Starter Active Member

    Jul 18, 2012
    Thinking about this emergency shutdown relay, it functions in a way like a 2 pole DC breaker.
    The question is determining how much current the control box circuitry draws from the 12vdc battery.
    Current usage would be some relays and the ignition coil.

    I have another running MCCK, I can measure the amps flowing through the emergency relay.

    Then maybe get a 2 pole breaker, then when oil pressure drops, send some current to the other pole of the breaker to trip it off.

    Time to trip say if you had a 10 amp breaker and ran 12 amps, how many seconds to trip off? etc... need to figure it out.

    So am I on the right track or is there a better idea?
  3. tcmtech

    Distinguished Member

    Nov 4, 2013
    If its an Onan I would not worry about the delayed shutdown ability. They usually self destruct well before the shutdowns take effect.

    If it was me I would just refit a more modern low oil detection device from a newer type of engine like a Honda to the Onan.
  4. DNA Robotics


    Jun 13, 2014
    They use CB115 now except that it only has 3 terminals. The pigtail wire goes to the oil pressure switch and overheat switch. Maybe you can make it work.

    Gen controls CB115.jpg
  5. DNA Robotics


    Jun 13, 2014
    The part number for CB115 is 320-1706-11. No info on the R115 resistor, but I thought it was inside the fault breaker.