Recently, I began having trouble starting my '73 Dart Sport 340 with factory electronic ignition. The engine would crank okay, and the carb was getting fuel, but it just wouldn't fire up. My mechanic friend checked the electrical system with a meter and found the voltage was low. We replaced the ballast resistor (original, 35+ year old unit) on the driver's side firewall. The car started okay for a week or two, and then repeated the problem.

Next, we put a charger on the battery (about 4 years old), and the engine fired right up. That prompted me to replace the battery last weekend.

The car will now start, but the alternator gauge is acting strangely. When you first fire up the engine, the needle is way over on the positive side. After the engine runs for a few minutes, the needle comes back towards the center a little -- but is not in the middle range. Plus, the needle jumps around quite a bit. This never happened in the past -- the alternator gauge always remained stable at dead center.

Any ideas on my problem?? This is starting to drive me nuts. Thanks for any advice you can offer.

I suggest testing your charging system. Sounded like your initial problem was with the battery either not taking a charge or the charging system not delivering enough to keep the battery charged with enough voltage to fire the ignition. Test you charging system with a reliable volt meter and and amp meter and find out what the system is really doing. The dash gauge if oem is a indicator at best.
In short your ignition problem is lack of charge to the battery. The Chrylser ignition need I believe 12-12.5 volts to fire properly and with your old battery it wasn't getting it.

I imagine you are referring to the ammeter? Did you add any new equipment that would be pulling straight from the battery?

Its possible your alternator is going bad. If you can't put enough charge into the battery the ammeter will show you that you are heading down that path.

check the brushes/clean the bulkhead terminals (small metal bristle brush for the male ones, a NAPA male terminal 725147 for the female ones/clean all terminals/connections in the field circuit, positive and neg batt large cables including eng to firewall to neg post and reg ground & check alt case to neg post resistance