ammeter jumping

[dshebuski]

I have an alternator on my 31 coupe. The ammeter needle jumps from -20 to+20 continously as i drive at any speed. I have checked for a loose wire and also have the proper belt tension. Any suggestions???DWS

 

[Ted]

Does it have a good ground connection?

 

[Barry B./ Ma.]

If all the connections are tight and the grounds are good I would suspect the alternator. I've had new alternators before that had bad regulators but usually the just overcharge.Could be a bad regulator circuit in the alternator but with it pulling down 20 amps I would first suspect a short somewhere. Disconnect the lighting circuit wire from the alternator first and try it again to verify that it's not in that circuit then check the connections on your ammeter.

 

[Tom Wesenberg]

In addition to what Barry mentioned also check the terminal box for a bad connection. With the car off, turn on the headlights. Set your multimeter to measure the lowest DC volts and connect the 2 meter leads to the 2 terminal box wing nuts. You should measure no volts. If the meter measure some voltage or if either stud gets warm it indicates a bad connection at the studs or the amp guage.

As Barry mentioned, I've also seen alternators with bad voltage regulators.