We do stationary dock testing on boats and would like to set up some safeties on the engine to shut it down. The engine is a 5.7L gm running a 12 volt system.

We have done it in the past using a mechanical latching relay, and maybe that is the best way, or maybe there is a better solution out there.

We use an oil pressure dummy switch sensor and also one for temperature. For RPM we use a MSD rpm window switch. All of these devices provide the ground for the latching relay so if any one of them is triggered it will shut down the engine. The way we shut down the engine is by putting ground to the negative (tach) side of the coil via the latching relay.

The dummy sensors can be hooked directly to the negative side of the coil, but the rpm window switch causes an issue because when the rpm limit is hit, it puts out a ground killing spark to the engine and then when the rpm falls back below the limit it takes away the ground, so it acts like a rev limiter.

Does anyone have a better ideas for a solution? Ideally it would just be a box that i hook all my wires from the safeties to, and if it sees a ground it outputs a ground. and then I click a reset switch to reset it, or just turn the ignition off and on.




1 Answer 1


It's called "wired OR" and you get it basically for free:


simulate this circuit – Schematic created using CircuitLab

You can have any number of contact-closure sensors on the "Sensor Bus". If any of them closes, you'll trigger your latching relay to shut down the engine. Of course, before you can restart it, you'll have to clear whatever fault triggered the shutdown in the first place.

  • \$\begingroup\$ Your diagram doesn't show a way to unlatch the relay. Presumably a dual coil type with the reset winding connected to a push button. \$\endgroup\$
    – EinarA
    Oct 8, 2019 at 22:03
  • \$\begingroup\$ @EinarA: The OP said he already has a latching relay; presumably, he knows how to unlatch it! \$\endgroup\$
    – Dave Tweed
    Oct 8, 2019 at 22:14
  • \$\begingroup\$ So i could use a MK2KP-UA-DC12 wired like your diagram, and then add a switch to reset it. That cleans up my wiring a little bit. Would something like a MD-D1022A/12V also work? Or would that not be a robust solution? \$\endgroup\$
    – JustinSEI
    Oct 9, 2019 at 0:12

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