Lesson 3/Learning Event 1
Make sure that all switches are turned off including the master (battery) switch.
You should always try to determine the amount of amperes or current that you are going
to measure. This information can be found in the vehicle technical manual. If you are
not able to find what amperage the circuit does carry, start with the highest range on the
ammeter and work down. Through this method you will not damage the ammeter by
using a range too small to handle the amperage. In fact, it is always best to start testing
on a scale higher than you think is necessary for a particular circuit.
Let us follow the hookup of an ammeter in a circuit with a fully charged 12-volt battery, a load,
and a switch to connect or disconnect the load to or from the battery. The load is a large lamp
similar to the headlamps on a vehicle. The information included with this circuit indicates there
should be about 10 amperes flowing when the switch is closed. The ammeter has a range from 0
to 10 amperes; however, we will use the 0- to 50-amp range just in case the flow happens to be
more than 10 amperes, which could harm the meter in the 10-ampere range. We connect the red
(positive) test lead to the common terminal of the meter because it is marked positive (+). The
black (negative) lead is then connected to the 50-ampere terminal on the meter. We know the
ammeter must be connected in series with the circuit, so we have connected the red lead to the
positive side of the circuit and the black lead to the negative side of the circuit.
In a generator output rate test, the current flow will be from the generator back to the battery. To
read upscale, the leads must be connected to the circuit in reverse of a load test. That would be
red (positive) lead to negative, and black (negative) lead to positive. Any load on the circuit would
then be indicated to the left of zero or downscale. The charging rate from the generator would be
Once you are connected correctly for a load test, complete the circuit by closing the switch.
Current will then flow from the battery, through the load, through the ammeter to the switch, and
back to the battery. As the current passes through the ammeter, the amount of current flow will be
indicated on the upscale side of the ammeter.
The amperage capacity of an ammeter depends on its construction. The ammeter used for the load
test above can accept no more than 100 amperes through the meter. With an external shunt,
greater amounts of current can be measured. You will learn how a shunt is used later.