I have a mower with an expensive potted ECM that appears to have gone bad. I have bypassed it successfully for starting the mower. It controlled safety switches, which I have figured out how to put back in the starting circuit with a few wires.

It also ran a relay that sent a 12V signal to trigger a solenoid to shut off fuel to the engine. This worked off the mower's key. The solenoid is only supposed to draw current while shutting down the engine, not while the engine is running. If it receives current while the engine is running, it will shut it down.

I need to be able to make 12V go to the relay COIL for a maximum of 10 seconds when I turn off the key. I don't know how much current the relay's coil draws, but this is a small relay similar to those found under car hoods, so it can't be a lot. Probably something like 50 mA, if other solenoids are any guide.

I don't need to store energy to run the solenoid. I can run it off the mower's battery via some existing hot wire or other.

I need a simple circuit that draws no current whatsoever except when running the solenoid, except that it doesn't matter if it draws a tiny amount while the mower is running. The circuit has to be turned on when a hot 12V wire from the key goes dead. It has to shut off as soon as the solenoid has done its job.

Can someone point me in the right direction? I am somewhat familiar with electronics. I can put a circuit together, but I don't want to get deeply into designing this circuit with my limited knowledge.

I could shut the mower off with a momentary switch or a new linkage to the shut-off lever, but I would rather have it work the way it originally did.

 

Can someone point me in the right direction?

I believe this might work for you.
Initially when first connected the relay will activate for the designated time then drop out.
When the mower is started +12 volts is applied to point A discharging capacitor C1 through transistor Q1 and diode D1.
This prevents the relay from activating while the mower is running.
When the key switch is turned OFF, Q1 shuts OFF and C1 will start charging and activate the relay coil through mosfet M1 again for 10 seconds.
The circuit draws a appx 1.3ma while the mower is running.
C1, R3 and the specs on the relay coil determine the time the relay will stay activated, may need to change the value of C1.

 

I really appreciate you taking the time to suggest that. You may have saved me a lot of aggravation. I will check it out.

 

Another option that would not require a timing circuit, but instead stay active as long as the engine was running, after the key was switched off.It could use a very short time interval timer that was reset with each ignition pulse, and would time out when there were no more pulses. The benefits would be, first, it would keep the fuel flow off until the engine stopped turning, if for some reason that took over ten seconds, and second, it would reduce the battery drain if the stops were quicker.

 

Thanks for the suggestion.

If I can get this thing to shut the valve for three seconds, it should be 100% reliable. I picked 10 as an excessive figure to remove all doubt.

My transistors arrived, so when all my other problems give me time, I will breadboard this and see what happens.

 

The time delay can be reduced by lowering the value of C1.