When testing wires for an application that requires x voltage or x ohms, do you do this with the car running and battery fully charged or can you do it with the ignition switched to auxillary and a fully charged battery?

 

Voltage drops are the easies way to tell your wires condition. You should be able to get away with testing certain circuits Key on, engine off. Regardless, a fully charged battery is a good idea.

 

When you do a voltage test, you always measure the battery voltage first, as it will vary slightly, most diagrams state that you must have battery voltage at a certain point, not just 12.6V but whatever the battery is putting out.

Resistance checks are always done with the circuit disconnected without power.