I have a one wire alternator on my car and an aftermarket ampmeter. When I turn the ignition on it shows a slight discharge. After starting the engine it still shows the same exact discharge(reading)no matter how high the rpm. When air or other accessaries are turned on it shows a larger discharge, no matter the rpm. The alternator is charging and the battery is never dead. What is wrong with ampmeter reading or is this common with one-wire alternators?