Originally submitted by T:
Fitting an Ammeter:
Installation of an Ammeter requires relocating the Alternator Feed from the Battery Positive to the Positive side of the Ammeter.
The Primary Feed must also be connected to the Ammeter's Positive side.
The 200 amp starter lead does not connect to the Ammeter. Note that the Starter Current does not flow through the Ammeter. Wired this way the Ammeter only reports the current flowing into or out of the Battery by every Electrical device in the Car except the Starter. The Starter current is not displayed because it is not measured.
Old And New Ammeters:
In the old days an undesirable plan fed all the Ammeter leads up to a dash mounted Ammeter. A more recent Ammeter fits a device called a "Shunt" between the Battery and Alternator/Primary feed connections.
Ammeter Wired Using a Shunt:
The device is called a Shunt (like shunting goods trucks in a railway yard) because most of the current travels through the Shunt and only a tiny amount is directed to a new generation Ammeter that only needs a tiny amount of current to move the Needle.
Electronically based Ammeters measure the Voltage dropped across a single cable thereby using that cable as a Shunt.
Reading an Ammeter:
When the Engine is running:An ammeter is wired in such a way that it tells the driver whether current is flowing into the battery, out of the battery or when no current is flowing either way.
A positive deflection shows that current is flowing into the battery or in other words the battery is being recharged.
A negative deflection shows that current is flowing out of the battery or in other words the battery is being discharged.
A reading of 0 shows that the voltage on both sides of the ammeter is the same or in other words no current is flowing in or out of the battery and the battery is fully charged.
It also means that the alternator is supplying all the power to the car.
Note that the starter current does not flow through the ammeter.
Current flowing into the -ve terminal makes the Meter read -ve.Current flowing into the +ve terminal makes the Meter read +ve.