Originally Posted by
scirockett
A good option would be to wire the HHO gen off the fuel pump relay. This relay is only turned on when the engine is running. In newer cars, it's computer controlled (ground from ECU to fuel pump relay) and the ECU requires action from the hall sendor. On older cars, same deal, except no ECU is involved, and the FP relay requires pulsed ground (hey, go figure) from the coil (just like a tach).
This is a safety feature so the fuel pump is running ONLY when the engine is running. If you get in an accident, your ignition switch may still be on but with the engine not turning, your fuel pump is off.
Now you might think it would be difficult to wire into the fuel pump relay, but typically other power is controlled by this relay than just the fuel pump. most VW's (digi1/2, motronic, and even CIS/E) use the fuel pump relay to control power to the O2 heater, and power to the fuel rail. Either of which do now have power unless the motor is turning, and would be ideal to relay a HHO gen off because they're already in the engine bay.
no brainer way to prevent this kind of problem.