Treating your digital voltmeter like a subtraction calculator may simplify routine voltage drop tests. Voltage drop testing is – and always has been – a vital part of successful electrical diagnosis. That’s because excessive voltage drop is a major cause of electrical failures. Loose, dirty, corroded or broken connections create excessive voltage drop. Other causes are frayed or broken wires or worn electrical contacts.
I have practiced and taught this technique for years. I noticed that some technicians struggle with voltage drop testing simply because they aren’t comfortable doing basic subtraction. I am not a mathematics whiz. However, I learned how to use a digital voltmeter as a subtraction calculator during voltage drop tests.
A digital voltmeter normally acts like a subtraction calculator. This means the meter subtracts the voltage at one of its test leads from the voltage at the other lead. Then it displays the difference between these voltages on its screen. You can observe this just by connecting your digital voltmeter to a common car battery.
Suppose the battery’s voltage is 12.60 volts. If so, then its positive post measures 12.60 volts. When you connect a voltmeter test lead to this positive post, it also measures 12.60 volts. The battery’s negative post always measures zero volts. So when you connect a voltmeter lead to the negative post, it also measures zero volts.
Okay, this digital voltmeter senses 12.60 volts on one test lead and zero volts on its other lead. Therefore, it subtracts one from the other and displays a result of 12.60 volts. (12.60 volts – 0 volts = 12.60 volts).
Supply Side Voltage Drop Test
Let’s apply this method to a basic voltage drop test on a simple bulb circuit. The accompanying illustrations show a series circuit containing a battery, a switch and a bulb. Closing the switch turns on the bulb.
Suppose a customer tells you that his car’s electrical system has been performing fine. However, this particular bulb is dim. The bulb may be failing but experience suggests that a bad connection or damaged wire is a more-likely cause. Here, a popular procedure is to turn on the circuit by closing the switch. Then measure the voltage drop across the supply (positive) side of the circuit. Next, check the voltage drop across the return (ground) side of the circuit. A threshold or guideline is that voltage drop across each side of the circuit should not exceed approximately 0.50 volt.
If the voltage drop is greater than 0.50 volt, pinpoint the cause and repair it. Then retest the circuit. Suppose the voltage drop across each side of the circuit measures less than 0.50 volt. If so, the wiring and connections are okay; suspect a failing bulb.
Now refer to figure one. For the purposes of this brief article, we’ll just focus on the supply side of this bulb circuit, which goes from terminal 1 to terminal 4. Usually, a tech grounds the voltmeter by connecting its negative lead to the negative battery post. Next, he turns on the bulb circuit and touches the positive voltmeter lead to terminal 1 (positive battery terminal). Here, the meter reads a normal value of 12.00 volts.
Next, refer to figure two. Here, the tech has moved the positive voltmeter lead to terminal 4, which is the positive bulb terminal. Terminal 4 also is the end of the circuit’s supply side. The voltage change from terminal 1 to terminal 4 should be approximately 0.50 volt or less. But in this example, supply voltage to the bulb is 10.17 volts. So, 12.00 volts at terminal 1 minus 10.17 volts at terminal 4 yield a voltage drop of 1.83 volts across the supply side of the bulb circuit.
Hopefully, the tech checking this circuit recognizes two facts. First, the 1.83-volt drop is substantially greater than the threshold of 0.50 volts. Second, these results mean that a problem somewhere between terminal 1 and terminal 4 is causing this excessive voltage drop. Next, the tech should check voltage at each section of the supply side, working from terminal 4 back toward terminal 1.
For example, suppose voltage is still an unacceptable 10.17 volts at terminal 3. But voltage at terminal 2 is an acceptable 11.80 volts. If so, then the source of the excessive voltage drop has to be between terminals 2 and 3. The contacts inside that switch are badly worn – replace the switch.
Okay, now look at figure three. Of course, the supply side of the circuit goes from terminal 1 to terminal 4. Here, I have “bridged” the entire supply side by connecting the voltmeter to terminals 1 and 4. Suppose I turn on the bulb circuit and the voltage at terminal 1 is 12.00 volts. If so, then the voltmeter’s sensing 12.00 volts on the test lead connected to terminal 1. Next, suppose that the voltage at terminal 4 is 10.17 volts. If so, then the voltmeter’s sensing 10.17 volts on the test lead connected to terminal 4.
Remember, a digital voltmeter subtracts the voltage at one test lead from the voltage at the other lead. Here, the meter’s calculation is 12.00 minus 10.17, which equals 1.83. Therefore, the meter displays 1.83 volts. First, this represents the voltage drop across the circuit’s supply side. Second, 1.83 volts is substantially greater than the threshold of 0.50 volts. Third, this test confirms that a problem between terminals 1 and 4 is causing a serious voltage drop.
To pinpoint the source of the voltage drop, keep the test lead connected to terminal 1. Then remove the test lead from terminal 4. Use it to check voltage along each section of the supply circuit. Now, suppose you touch this test lead to terminal 3 and the meter still shows 1.83 volts. But the meter reads .20 volts when you touch the test lead to terminal 2. Obviously, this shows that the lion’s share of voltage drop occurs between terminals 2 and 3 – that’s a severe voltage drop across the switch. This means it’s time to replace a worn switch.
Before I wrap up here, note that a tech sometimes connects the test leads backwards during a voltage measurement. First, this won’t harm a digital voltmeter. Second, a digital meter’s voltage measurement is still accurate if its leads are reversed. But the meter displays a minus sign (-) to remind you that the leads are reversed.
In conclusion, the “bridging” approach shown in figure three may be a small step toward simpler electrical diagnosis. Many techs have told me that the method simplified voltage drop testing for them. It could be helpful to you, too.