Hello C.,
When used in this manner, with an input signal that is switched between 12V and "floating", the GEA 24 discrete input Active High configuration may also be thought of as "Active When Not Grounded". That is to say, it's very common in the electronics world for "a voltage" and "wire not connected" to be treated the same, at least when talking about a high-impedance digital input.
One approach would be to use a pull-down resistor of around 10k ohms (the value is not critical) between the GEA 24 discrete input and ground. This will ensure the discrete input pin is really sitting at a low voltage when the fuel pump switch is off.
Another method would be to use a DPST switch for your fuel pump, with one pole being used to switch power to the fuel pump, and the other pole being used to ground a discrete input pin when the fuel pump is on. That's how I did it in my airplane. This method has the advantage of keeping the fuel pump and annunciator circuits completely separate.
- Matt