A functioning ammeter always "works" in that it will measure the current flowing through it. When all of the circuits on the alternator side of the ammeter are off (and the car is off), there is no current flowing for it to measure. (This is a simplified view of things for clarity's sake.)

If the meter is blown, the connection is open. In this case, circuits that hang off of the white/red wire would not work with the car off. This includes the headlights and parking lights to mention a couple of obvious ones.

How to diagnose? the easiest way is to pull the ammeter and replace it with a shunt (piece of wire). If the headlights work with the shunt and not with the ammeter, the ammeter is bad. (They can be repaired, though.)

You can also test the ammeter by itself out of the car. Let me know if you are interested in trying that, and I'll draw up the circuit for you.

SteveJ,

If I can ask, to test the ammeter do you have to take it apart? I have one to test but don't know how to put 45 amps through it to test.

Mike

To test it, you don't need to take it apart; just take it out of the dash. A bad ammeter will act as an open circuit. You can test it with a 12VDC source and a test light.

I suggest using two lantern batteries wired in series for the 12VDC source. Wire the positive to the positive on the ammeter. Wire the negative of the ammeter the test light, and touch the other end of the test light to the negative on the battery string. If the lamp lights, the ammeter is good, just not necessarily accurate.

If you want to test accuracy, use the 12 VDC source with at least a 500W rheostat. The 500W will allow about 41A before you exceed its rating. Unfortunately, 500W rheostats are pretty pricey. The cheapest I could find in a quick search was $165.

Thanks Steve I will give it a shot and post my results

If you can wait until the weekend, I have a spare ammeter, and I can fabricate the circuit and take pictures of it for a reference.