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I need a circuit to disengage power to the start switch which is separate from key switch when engine has started so there is no chance of accidentally engaging the starter with the already operating engine. I have the separate start switch and know how to wire it with a relay. You turn on the key then use to start button. But then the button must electrically disconnect so there is no chance of accidentally pressing it and engaging the starter when the motor is running.

I have repeated myself as I seem to get the same answers in how to wire up the start button! That I know how to do.

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  • How do we know if the engine is running? We'll need a signal back from the engine to create the interlock. – Transistor Jun 3 '16 at 22:54
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    you could connect a normally-closed oil pressure sensor in series with the start button - the start button would only operate if there was no oil pressure (=engine not running). – Peter Bennett Jun 3 '16 at 23:04
  • See if this answer works for you. It was meant as a security measure, but should work for you ... might need slight modification, though. I think the idea is sound and what you are looking for. – Pᴀᴜʟsᴛᴇʀ2♦ Jun 3 '16 at 23:27
  • What kind of vehicle is this on? – Ben Jun 3 '16 at 23:33
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Only issue I found is the Oil Pressure switch must be changed for an adjustable one of around 30-50psi as a normal 5psi will not allow proper starting when engine is warm. I used the ground from the oil pressure switch for a relay to operate the starter through the push button. The light on the start button comes on once the key is turned to on. Once you press the start button and the engine fires, then the light in the start button goes out and there is no chance of accidentally engaging the starter.

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