Electronics help needed for Diesel fuel cut off control!

[Crazyrabbit]

Hello Crazyrabbit here,

I want to automate the operation of the fuel solenoid so that any time the power is set for idle, and the engine speed is above idle, the fuel solenoid is cut off.

The solenoid draws 2 amps of 12VDC.

I was thinking I could power the solenoid with a transistor and feed the gate with power from two sources; 1)A SPST NC microswitch to sense idle power position, and 2) Something that switches off above idle speed.

I was thinking that the alternator might be a good way to sense the speed, either by tapping one of the windings before the rectifier, or looking at the fan as a chopper for a photo-diode, or looking at a reluctance variation as the fan blades pass with a Hall effect sensor or even an aerodynamic actuated switch using the wind generated by the alternator fan like a lawnmower governor.

I have a manual switch I can do this with now, so I am sure if I could automate this function, I could save a pretty good amount of fuel. The manual switch is pretty reliable with an on resistance about .2 Ohms. An earlier switch that ran anywhere from .5 to 1.2 Ohms when on was horribly unreliable about getting the solenoid to lift and start the popcorn fryer again. I am hoping a transistor can be very reliable

The same logic might be employed to cut out the field current of the alternator, so it only charges when I am coasting down.

Summit Racing has an adjustable speed switch that provides a ground connection on one wire below the set speed and a different wire above the set speed. But it requires a speed signal at 12 VDC, and I am not real sure what to do with a ground. Would I isolate the transistor case from ground and ground it through their wire? Or use the ground to run a relay to drive the gate? Also, they want like $50, and that seems pretty high to me.

Thanks for your help if you can help me out.