My '64 in stock form, had the heavy gauge wire directly from the alternator feeding a shunt inside the actual gauge. That in turn fed the fuse panel. I believe that the ignition circuit was unfused off the battery so an interruption in the ammeter circuit wouldn't keep the car from running. The indicator on the gauge is reading the magnetic field developed.

Being a later model year yours is probably a remote shunt, mounted somewhere safer and the gauge is actually reading + or - volts, displayed as amperage.

Manufacturers eventually gave up on ammeters altogether and gave us volt meters instead.

Richard's post is very good and informative. It's important to note that in his meter rebuild, 'coil wire' must be used because it is varnished (insulated). Regular copper wire would short straight across the windings and produce no magnetism. We would normally say his meter is internally shunted but it really uses magnetism to directly move his meter.

When current flows through a wire it creates magnetism around that wire. We can multiply that magnetism by winding the wire in a coil of turns. Loudspeakers also use this technology to move a cone.

All ammeters are really volt meters. There is no way heavy current can possibly power a small meter movement. Besides, why run 100amps into the passenger compartment, then back out into the engine bay? When generators produced a measly 30amps, #10 wire was possible but today's 100amp systems require at least #4awg which is too heavy.

So, how do we read amps from voltage? We use a 'shunt resistor' with low resistance, usually mounted on the fender apron or close to the starter solenoid. The meter must match the shunt resistor in a specific ratio.

Here is a typical shunt resistor designed to measure 100 amps:


Major current flows through both ends (large connection holes) but notice those two small screws? They connect to both meter wires, so a ratio of power feeds the meter. This shunt resistor is designed to pair with a 50-millivolt (0.05v) meter. Yes, whether the meter is connected or not makes no difference to the shunt resistor. Major current still flows through.

Ford did exactly the same method, using a shunt resistor to save money.

Another method using a Current Transformer can be used. A C.T. is simply turns of wire around the battery cable, directly feeding an ammeter. Again, the number of turns will determine power to the meter. This is how we accurately measure extremely large current, like starter motor draw using no resistance.

Modern cars use a simple volt meter, connected anywhere including the cigarette lighter. The volt meter reports the battery's charge. If your charging system isn't working, battery voltage will be low. If the battery has shorted cells, the charging system will still feed 14volts but the battery will only hold two volts per working cell. Can a car start on 10volts? Yep, some folks never know that their new battery came with a dead cell.