Join Date: Mar 2002
Location: Austin, TX, USA
In a general sense, an amp meter must have a shunt accross it's leads. A shunt is basically a small value resistor. All resistors develop a voltage drop accross them as current flows. The shut provides a very small voltage drop - just enough to swing the needle. A volt meter on the other hand has internal resistors configured in such a way as to move the needle when 10s of volts are applied. Typically a meter movement takes about 100 millivolts to swing full scale.
Now having said all that, I can't tell you exactly how an ampmeter on a car works. I believe it gets it's signal from the alternator (or generator). There must be some sort of current divider circuit in there that measures the amount of charging current and sends a current signal to the ampmeter. I'd have to see a schematic for an automotive electrical system with an ampmeter in it to give a definate answer.
While the acutal guts of the meter might be the same, the face markings and internal resitor networks are completely different.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI