Quote Originally Posted by Frank S View Post
The best way to keep someone from stealing your vehicle is to have a normally closed solenoid valve installed at the tank that shuts off the fuel flow and vents the downstream line to air if the correct procedure is not followed, the solenoid can be controlled by a relay that requires another vehicle system to be switched on before starting use a system that normally would not be thought of. If you use a DPDT relay it can be made into a latching relay which will remain on after the vehicle is started until the ign. is switched off once again the secret system could be just about any electrical system and can be used only as momentary contact or left on.
Works on all vehicles even older carbureted with or without fuel pumps in the fuel tank. Even if the vehicle starts and runs it will only do so momentarily. Works especially well for vehicles owned by persons who might be inebriated. Will now work on vehicles you want to prestart before getting in.
A fuel shutoff with hidden switch is a good example of security through obscurity, which is pretty effective against the usual opportunists.
I did something similar to the latching relay circuit you described to a golf cart, except in this case it controlled the ignition. They are notoriously easy to steal because each manufacturer generally uses the same key for all of their models. The owner had a few carts stolen because the keys are readily available. He never went anywhere without a can of soda, so I put a pair of contacts in the bottom of one of the cup holders. It sometimes needed a little twist to make contact, but wouldn't start unless there was a can of something in the holder to complete the circuit. It worked a lot better than expected.