Electrical Testing Basics

The difference between voltage and current is a first-year basic in any automotive technology course, so much so that few of us think about it when using a multimeter to troubleshoot vehicle electrical systems. It’s simple, yes, but thinking about where you insert the meter is important to fast, accurate diagnosis. Take a look at the simplified blower motor circuit to the right (Figure 1):

Will the voltmeters show the same reading? No, they will each measure a different voltage drop across each part of the circuit. The total of all the voltage drops should sum to the drop between battery and ground.

The other important fact to remember about measuring voltage and current with multimeters is that the internal resistance of the meter when testing current is very low, to avoid loading the circuit unnecessarily. When testing voltage, it’s the other way around, as the meter’s internal resistance is very high to avoid creating a parallel current path across the resistance under test. Forget to switch from voltage to current on your multimeter, and the high meter ‘impedance’ will protect both circuit and meter. Forget to switch from current to voltage, however and you’ll introduce what is essentially a dead short across the component you’re testing. If it is a protective device like a fuse or breaker, don’t be surprised if something downstream fires into life for an instant.