Ammeter NEVER shows discharge

1970 Dart. Long ago after seeing all the horror stories I installed an aftermarket ammeter below the dash, hooked the wires from the ammeter in the cluster to it and everything has been fine for 20 years. I had the system overcharge, as shown on ammeter and voltage gauges (regulator went haywire), and it melted connections in the bulkhead connector. I installed new terminals in a new bulkhead connector, and replaced any damaged wire (very little). Now the ammeter works fine and the electrical system seems to work fine, but with one small issue: when you start the car the ammeter shows charge and slowly comes down, but the ammeter NEVER shows a discharge. You can turn on the ignition, without starting the engine, pull on the high beams, turn on every accessory and no discharge indication. I thought I toasted the aftermarket ammeter, but tried two others and get the same result.

Suggestions???