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Need idea to replace unavailable shutdown breaker in Onan MCCK generator

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Nov 2, 2012
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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
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.

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

edited version of the photo that is brighter than original.
P9222719 (1).JPG
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It is easy to get a one minute delay from a 555 IC.

You'll need a DC power supply, to power a 555 circuit. Can you find some point where you can tap (either AC or DC) to do so?

To replace K7, it seems to me you want the relay to stay on all the time the generator is running. You also want the relay to energize immediately on startup.

I'm not sure if the relay's default state should be contacts closed. Suppose the contacts open in an emergency. As soon as the generator stops running, the 555 would lose power, the relay contacts would close, and the generator would want to start running again, wouldn't it?

Therefore I believe the relay's default state should be contacts open.

Choose a 12V relay that is capable of carrying the contact current, the ammeter should tell you this. You have a 12V battery, you need a 555 timer then the only thing to ascertain is the polarity of the LOP signal, is it +12 for good or 0V? I would think that the timer is for starting, so when the engine is cranked the supply remains for 1minute (seems too long?) to give the engine time to get up to running oil pressure, then if the oil pressure drops, it cuts the engine immediately.

The K7 relay switch is passively closed all the time, until it trips open from a LOP condition. (low oil pressure)
It is a an automatic off switch that trips off on a heat up of an internal metal piece that gets hot form the LOP condition, so like a thermal breaker, but only one side is a thermal breaker, other side is a contact switch.

I have an idea of using a 2 pole DC breaker maybe 10 amps. The circuit as it exists functions similar to a 2 pole breaker would function, since there are 2 separate circuits for the emergency relay.

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.

My idea preserves the original functionality. You want the crank limiter to about a minute. Any condition that drops the oil pressure, it was designed to shut down after a minute.
so if it cant start or runs out of gas or looses oil or overheats, after a minute of crank time , it completely shuts down.
Overheats kills the ignition, goes open circuit immediately, so it stops firing, LOP condition then exists, it will crank for a minute then shutdown.

You need the minute delay so that when cranking to startup, it has time to fire up. And a delay say, if it is sputtering to a start due to it priming the fuel bowl, it has time to crank to do that and get going.
So you need the delay on starting and stopping.

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