anyone know about ammeters?

[williamp]

My 1971 Aston (negative earth) has an unusual ammeter behaviour. It normally sits in the centre (ie 0), but sometimes (and often when the auto box is in top) the ammeter flickers from -50 to +50, like a pendulum of a clock. It cycle time is less then 1 second.

Any ideas? The battery is fine, the car always starts no problem.

[Mikey G]

Sounds to me like something that draws a heavy current is switching off and on, or could be a dodgy wire that is linked to the autobox in some way? what sort of revs are you pulling while it does it?

[eliot]

Or perhaps a short circuit.. better take a look under the car..

[rev-erend]

Or perhaps the ammeter is faulty.. check the battery with another ammeter.

[steve_D]

The ammeter should be connected almost directly to the battery with very fat wires and almost everything else on the car is connected to the other side of it. If the wires were loose or faulty then many other things would be failing to work.
A short could give a -50 reading but nothing is going to suddenly give you a 50 Amp charge.
Conlusion.
I would suspect a duff ammeter.

Steve

[Pigeon]

No, but something switching on and off at the right frequency could excite the resonance of the needle and make it swing wildly over the mark.

My Morris Minor did this occasionally. I think it was the voltage regulation control loop going into oscillation; it seemed to be provoked by big load transients. It was never a problem, so I never tried to do anything about it.