I´m trying to moddify a fuel tank in c#, help with the script?

I´m trying to add an input controller on a fuel tank(clone) but it is not enough to prevent the fuel going to the engines.

if input=0 then fuel = 0, else, fuel = remaining value of fuel.

If fuel = 0, no engine sound is played, above 0 it starts playing :) So the input also would work like a switch to acess the fuel tank.