What an ammeter does depends on a few things:

The charge on the battery

The amount of load, like lights, heater, A/C

The RPM of the engine.

When you start a car, say in the spring, when you don't need heat or A/C, and get out on the blvd and cruise for a few miles, it should not take long for the meter to "zero" when cruising at a steady speed. At speed, say you turn on the A/C. The meter might jump a little, but it should go right back to center.

Now, here you are at the "red light." The engine is idling slowly, maybe you've got both the lights and the A/C running, so there's lots of load. The alternator cannot keep up, so the meter goes negative some amount.

If you rev the engine a little, or go through the light, the meter should charge again, for a little while, until it gets the battery "caught up."

Some good indicators of problems IN THE LONG RUN are things like wild fluctuations, dim or really bright lights, odor from the battery, or worse, damage to the cables and paint around the battery.

Of course if you let the dreaded smoke out, that's still another story.

The other problem with ammeters, is, it turns out all these late '60's and early '70's cars had a poor design, namely the wire size was marginal, the firewall connector is a problem, and the ammeter connections can get hot, loose, and fail.

Also, when you start rewireing them, you can end up with loads--like accessory lamps, or even things being re-wired--you can end up with loads on the "wrong side" of the ammeter, electrically. This will cause the ammeter to give you erroneous readings.


My problem is, I really like ammeters, but more and more, as I read the posts on these forums, I realize what a pain they've become.