Voltage is always measured parallel to the source, on cars the source is normally the “Battery.” When you measure for available voltage you will typically connect your meters black lead (As Larry stated) to a ground connection. The red lead can be connected to any number of points to determine if you have power at the point being tested; however it will not tell you if there is sufficient current to operate the device (more on this later)

Voltage Tests = Parallel to Positive and Negative

Amperage is always measured in series to a device, or the load. The meter leads can be connected either way in the circuit; however polarity can be an important part to this test. The red meter lead will normally connect to the source side (voltage supply) going to the device while the black lead connects to the input leg of the device. The meter becomes the supply circuit and it displays how much current is needed to operate the device.

Amperage Tests = In series as Positive or Negative

Voltage drop testing is performed parallel to the circuit being tested. If you are measuring the supply side (positive) of a device you will attach the meters red lead to a “Battery +” connection and the black lead to the supply side of the device. By connecting the meter in this fashion the meter essentially becomes another wire providing power to the device. Since it is parallel to the devices wire there should be little to no voltage displayed on the meter. If you have a voltage reading >.500 (1/2 a volt) then there is problem with the supply circuit. This test confirms, or condemns the circuit’s ability to carry sufficient current from the battery in order to operate the device correctly. I consider this to be the “Dynamic Resistance” test because this test is performed while the device is in operation.

Voltage Drop Tests = Parallel to the Positive or Negative circuit


This is just a couple of quick primers for you to digest, if you would like some printed material to reference radio shack has a great introduction book for the basics of electricity.


Ohms law

E=I x R or E/(IxR)

(E)Voltage = (I)Amperage x (R) Resistance

12.6 (V) = 3 (A) x 4.2 (Ohms)

When Resistance is high Current is low

OR

When Current is High Resistance is low

Think of resistance and amperage as two children on a see saw, if one of the children is heavier than the other, one child will be down while the other child is up. Now substitute the children with I and R on this imaginary see saw and it becomes very easy to remember the relationship between current (Amperage) and resistance.