Don’t let the term scare you. The biggest challenge is knowing what is "normal" so you recognize a defect when you see it. Voltage drop may be thought of as a reduction of voltage because of resistance in the circuit. I’ll try to give a quikie lesson here.
Refer to the hasty diagram. Notice we have the battery, two cables, a push button start switch and the starter, all in a string (in series). As far as this example goes, the resistance between the battery and starter is the sum TOTAL of all the individual resistances in the string. Everything has some resistance, but in this case the resistance of all the parts would be so low we can assume it to be zero. If it is zero total, we can expect the voltage coming out to be the same as voltage going in.
So, what do we know here? We know the starter is good because it works with a jumper cable. For the same reason we know the ground circuit is reasonably good. So, let’s check the supply circuit.
With a voltmeter connected between test points 1 and 8, and the start button pushed, we expect to see 12V at the starter. But the test showed little or no voltage at the starter. That means there is a high resistance in the string. We can test the voltage drop across any two test points, as convenience allows. Let’s put the voltmeter leads between test points 4 and 5 (the switch). We expect no voltage drop (0 Volts) when the switch is closed because we expect no resistance through the switch. But Aha! We get 12 Volts drop because there is a very high resistance through the switch. That means either the switch is not closed, or it is burned up. In this case, it was burned up.
You can do the same thing anywhere along the line. For instance you can check the drop on everything between test point 1 and 7 by connecting the voltmeter between those two points. As before,we expect 0 Volts because we expect zero resistance. A real handy test would be between test points 1 and 2 (the cable connection to the battery post). We expect 0 Volts drop because we expect zero resistance. What if we get 8 Volts drop?? Dirty, corroded, or loose cable connecton causing high resistance. That’s 8 Volts wasted through the resistance and only 4 left to run the starter (12 – 8 = 4)
Clear as mud???
