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Automotive Voltage Drop Testing

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Use a multimeter to test for voltage drops.

Use a multimeter to test for voltage drops.

What Does a Voltage Drop Test Do?

Voltage drop (or voltage loss) testing is a great diagnostic skill you can use to deal with many automotive electrical problems.

Over time, electrical devices wear out, and electrical contacts, terminals, and wires may become corroded, loose, or broken, introducing unwanted electrical resistance that prevents the circuit from working properly. Or at all.

A visual inspection of a problematic circuit rarely gives you much troubleshooting information. Even more, a standard voltage or resistance check may not reveal anything wrong, making you waste time and—frequently—money. That's why a voltage drop test should be one of your primary tools in troubleshooting electrical problems. If done correctly, this test can tell you whether a circuit is dropping more voltage than it should.

This guide tells you what a voltage drop test can reveal and how it is used, and it gives you a practical use for the test. Also, you'll find how to turn complex circuits into simple ones for testing purposes and a handful of useful tips. So in a few minutes, you'll be ready to apply this new skill using a digital multimeter (DMM).

Let's see what voltage drop means in practical terms so that you can have an idea of what's going on in those electrical circuits in your car.


I. What a Voltage Drop Can Reveal

II. How Voltage Drop Test Is Used

WARNING: Use the Appropriate Digital Multimeter

III. Applying Voltage Drop Tests to Common Automotive Problems

A) Testing a Starter Motor Circuit

B) Testing an Alternator Circuit

Symptoms: When Do You Need a Voltage Drop Test?

IV. Voltage Drop Testing Tips

V. Testing Complex Automotive Electrical Circuits

I. What a Voltage Drop Can Reveal

A voltage drop test basically means checking for the presence of unusually high (or unwanted) electrical resistance in wires, connectors, fasteners, components, and terminals. Electrical resistance is not bad. In fact, you need resistance in some form for a circuit to work properly.

The purpose of a circuit may be to operate a light bulb, a sensor, an actuator, a fan, or any other appropriate automotive electrical device. Each one of those devices represents resistance in the circuit that is consuming (dropping) voltage to use electrical current to do its job. This is good resistance.

However, corroded, loose, burned, or broken connectors or wires also introduce some resistance to a circuit. But this is unwanted resistance (voltage drop) because it prevents some or all of the electrical current from reaching its intended destination, causing the loads in the circuit to operate poorly or not at all. That unwanted resistance is what voltage drop testing is after.

Figure 1: Simple automotive circuit.

Figure 1: Simple automotive circuit.

II. How the Test Is Used

We'll use the simple diagram pictured above to illustrate the points in a circuit you are concerned with during a voltage drop test.

The purpose of the circuit in Figure 1-A is to illuminate a lamp (it could be any other load). The circuit has a fuse for protection and a switch to turn the lamp on and off.

Let's suppose that the lamp in our circuit illuminates dimly. In other words, full current is not reaching the lamp. Thus, besides the lamp, some other unwanted resistance in the circuit is dropping voltage and 'stealing' current. So we need to locate it.

1. First, we need to check system voltage. Turn on the headlights for one minute. Then turn them off and connect your DMM voltmeter across the battery. At 12.6V or more, your battery is fully charged; at 12.4V, the battery is at 75% charge; at 12.3V, the battery is at 30% charge. If you get a lower reading than 12.4V, charge the battery before continuing with the tests.

2. Now, we start the engine and let it idle. Then, we turn on the switch to activate the lamp's circuit.

3. Next, we check for voltage drop in the power side of the circuit. So we connect the voltmeter across points (a) and point (b) (positive battery post to lamp connector). The meter reads about 0.10 of voltage drop in the power side of the circuit. This is an expected reading. So we assume that this side of the circuit is working as it should.

4. Now we move to the ground side of the circuit. We touch our voltmeter leads to the connectors across points (f) and (c) (negative battery post to lamp connector). The meter reads 0.80 of voltage drop. Anything above 0.2V or 0.3V (because we take into account the voltage drop from the switch, otherwise 0.1V) means there's some unwanted resistance in the circuit.

OK. So our voltage tests reveal we have some unwanted resistance (voltage loss) somewhere in the ground side of the circuit. Now we need to pinpoint where in the ground side of the circuit we are dropping voltage.

Note: One thing to keep in mind here is that the ground side of any automotive electrical circuit should not receive any voltage at all (or close to 0 because of wire and terminal resistances). The loads in a circuit should consume total battery voltage to work properly. For example, if our sample circuit was working properly, measuring with the voltmeter from point (a) across to point (c) should give us a reading above 13 VDC of voltage drop with the engine at idle, because the lamp (the only load in our circuit) would be consuming (dropping) total battery voltage.

5. To locate the unwanted resistance, we do a voltage drop test at each connection (as shown in Figure 1-C, under "connection voltage drop test" on the diagram above) on the ground side of the circuit (including the switch, load and battery connections). We also test each wire between connectors, as shown in Figure 1-B, under "wire voltage drop check" (when checking wires, wiggle the wire while your voltmeter is connected at each end to check for internal opens, and visually inspect the insulation along the wire for signs of burns or other damage).

6. Let's say that we got a voltage drop of 0.6 at connection (e) of the switch, instead of the expected 0.0V. This means that part of the switch connection has some corrosion, the connection is loose, or some of the wire strands are broken and only a few are still attached to the connector, introducing unwanted resistance.

So we found the part of the circuit that was causing our lamp (or load) to work poorly. We fixed the problem, and now our lamp is working properly.

Figure 2: Starter circuit diagram.

Figure 2: Starter circuit diagram.

III. Applying Voltage Drop Tests to Common Automotive Problems

Okay, it's time to apply a voltage drop test to a common automotive electrical problem. We'll do a voltage drop on a starter motor circuit because the motor draws high amounts of current (above 100 amps) to operate, and a little unwanted resistance (like a corroded or loose terminal or connection) can easily upset circuit operation.

Let's suppose that you hopped into your car this morning, turned the ignition key to fire up the engine, and instead of the usual rumbling sound of the engine coming to life, all you heard was a chattering sound. Usually, this is a symptom of insufficient current reaching the starter motor.

The best part of a voltage drop test is that you don't have to disconnect anything (although you still may have a hard time trying to reach every single connection in a circuit you want to test).

A) Testing a Starter Motor Circuit

As mentioned above, with a voltage drop test you are looking for any connection or component with abnormal high resistance.

Testing the Power Side

Continuing with the starter circuit test (refer to Figure 2 above):

1. First, disable the ignition system or fuel pump to prevent the engine from starting. To disable the ignition system, if you have a distributor, unplug the thick wire connected to the center of the distributor cap and connect the wire to a metal component on the engine using a jumper wire. Otherwise you can remove the fuel pump fuse.

2. Now, check the battery. Set your digital voltmeter to 20V on the DC Volts scale. Connect the meter red lead to positive battery post and the black lead to negative battery post. Crank the engine. Your meter should read 9.5V or higher, otherwise, charge the battery and test again.

3. Now take a voltage drop reading on the power side of the starter circuit. Touch your red meter probe to the positive battery post (not the wire), and the black meter probe to the battery terminal stud (not the wire) on the starter motor. While an assistant cranks the engine for five seconds, keep the leads connected, see your voltmeter reading, and then stop cranking the engine.

NOTE: Let the starter motor rest between cranks for about 30 seconds to prevent wearing out or burning the motor.

4. A voltage drop reading on the power side above 0.5 Volts means you have some unwanted resistance on the power side of the circuit. Check every connection, and cable, between the positive battery post and the starter motor battery terminal post as shown in Figure 1-B and Figure 1-C, while cranking the engine.

Testing the Ground Side

1. To check for voltage drop on the ground side of the starter circuit, connect the black meter lead to the negative battery post and the red meter lead to the starter motor case while the engine cranks.

2. A voltage drop reading above 0.3 Volts means you have some unwanted resistance in the ground side of the circuit. Check that the starter motor is properly mounted, mounting bolts tight, and there's no grease or dirt between the starter motor housing and the engine. Your motor housing is the ground connection and should have proper contact with the engine block during installation. A voltage drop reading between the starter motor case and the engine should be 0.1V or less. Also check the connections at the negative battery cable as shown in Figure 1-B and Figure 1-C while cranking the engine, the battery negative post to body wire connection; and, if necessary, engine ground connections to body.

If both your power and ground voltage drop tests are within specifications, you probably have a bad starter motor. To double-check your findings:

1. Connect your positive meter probe to the starter motor battery post and the black meter probe to the starter motor case.

2. Crank the engine and check your meter reading.

3. If your reading is exactly the same as your battery voltage test, 9.5V or above, then your starter motor is no good.

Voltage drop tests results above 0.1V on a connection, 0.2V on a wire, or 0.3V for most switches (remote relay or solenoid in this case), means too high resistance present. Check for corroded, dirty or loose connections; broken wire strands at the connection or between wire ends; or burned contacts within switches, solenoids or relays.

"Cranking-no start" and "no-cranking no start" conditions are frequent automotive starting system problems, but remember that voltage drop tests are important in any circuit, especially where sensors are involved because even a small amount of unwanted voltage drop can send the wrong signal to the car computer and translate into engine performance problems.

B) Testing an Alternator Circuit

Another common voltage drop test is the one you can do on the charging circuit. This test comes in handy when you seem to have problems with the battery or alternator.

*To check the power side of the alternator circuit:

1. Connect your red voltmeter lead to the alternator output (battery connector, B+) on the back of the alternator

2. Connect the black voltmeter lead to the positive battery post.

3. Start the engine and turn on the headlights, blower motor, radio and other accessories (except the back window defroster).

4. Increase engine speed to 2000 rpm.

5. Voltage drop should be 0.5V or less; otherwise, check for voltage drop on every connection and wire on that part of the circuit, just as you did on the starter circuit voltage drop test.

*To check the ground side of the alternator circuit:

1. Connect your black voltmeter lead to the alternator case.

2. Connect your red voltmeter lead to the negative battery post.

3. With the engine running at 2000 rpm and accessories turned on (except back window defroster), you should get a voltage drop of 0.2V or lower; otherwise, check for voltage drop on every connection and wire on that part of the circuit, just as you did on the starter circuit voltage drop test.

IV. Testing Tips

When doing a voltage drop test, follow these important tips:

  • Make sure your battery is charged and operating properly.
  • Energize (activate) the circuit you want to test so that you have current flowing through it.
  • Begin your diagnostic on the ground side of the circuit where unwanted resistance shows up more frequently.
  • In general, you can expect voltage drops of 0.0V across connections, 0.3V or less at most electrical switches, 0.2V or less from end-to-end of a wire (consult your vehicle repair manual, if necessary).
  • Unless you are dealing with the starting, charging or any other high amperage circuit, a good rule of thumb is to take 0.2-0.3 voltage drop as a standard loss for the power or ground side of a circuit. However, some manufacturers suggest 0.1 voltage drop or less, especially for circuits with computer-controlled components (emission-related sensors, actuators), as a desirable limit. Higher voltage drops may cause components to behave erratically or become inoperative.
  • Make sure the circuit you need to diagnose has the correct wire size for that circuit. Using the incorrect wire gauge may cause a significant voltage drop.
Series and parallel circuit.

Series and parallel circuit.

V. Testing Complex Automotive Electrical Circuits

The example circuit shown in Figure 1 is a simple series circuit. Mostly, you'll be dealing with series-parallel circuits. But don't get scared, you can turn complicated circuits into simple circuits when testing for voltage drop.

First, let's see briefly what series and parallel circuits are.

A) Series Circuits

Figure 3-A in the schematic above is what is called a series circuit. Simply put, all the devices in the circuit share a single electrical line—or branch—to operate. The series circuit has its own rules. The two most important ones are:

  • Each component (load) will create a certain amount of voltage drop, depending on the amount of resistance introduced by that component into the circuit.
  • Electrical current value (amperage) will be the same across the circuit.

B) Parallel Circuits

Figure 3-B in the schematic above is what is called a parallel circuit. This simply means that the circuit is divided into two or more branches. Each branch may have one or more loads (components) connected to it. And just like the series circuit, it has its own rules. The two most important ones:

  • Voltage drop across the loads remains the same.
  • Electrical current value (amperage) in each branch varies according to the resistance in each branch.

Whether you are dealing with a series or parallel circuit, the loads in the circuit will consume the total voltage in that circuit, just like we saw with the simple circuit in Figure 1.

As we mentioned above, most automotive circuits you'll be dealing with will consist of circuits that combine series and parallel ones. Regardless of the circuit you are dealing with, though, you can make your voltage drop tests simple by breaking your circuit into small series circuits. For example:

  • First, whenever possible, start your tests at the ground side of the circuit you need to troubleshoot, and then the power side.
  • Test for incoming voltage and reference voltage as appropriate (consult your vehicle repair manual for information on this).
  • Test the suspect component if necessary (consult your vehicle repair manual).
  • Make a voltage drop test of the connectors that link the suspect component to the circuit.
  • Next, voltage-drop test the wire attached to the troubled component.
  • Then, voltage-drop test the connector at the other end of the wire that connects to the troubled component.
  • Keep working your way through the branch as access to connectors and components allow.
  • Skip to other components and sections in the circuit as access allow.

Use the wiring diagram of the circuit you are troubleshooting to locate components and wires that belong to the circuit you are diagnosing (you can find the diagrams for the most important circuits in your car in the repair manual for your particular vehicle make and model. Buy an inexpensive aftermarket vehicle repair manual online or at your local auto parts store).

A Useful Diagnostic Skill

Voltage drop testing is an excellent electrical diagnostic skill when dealing with unwanted high resistances. Use it to test circuits where you see problems with light bulbs, horns, starter motor, fuel pump, sensors, actuators, relays, other switches, wires and connections, or any component or circuit indicated by Diagnostic Trouble Codes (DTCs) stored in your computer memory. Without the need to remove components or disconnect wires, this simple electrical test helps eliminate guesswork and unnecessary replacement of good parts.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How do I connect a voltmeter to test for voltage drop?

Answer: When checking the same side of the circuit (either power or ground), the red lead goes to the most positive side of the circuit and the ground to the opposite. If you are checking the ground side, for example, the black meter lead will be toward the ground (negative) side of the battery and the red lead to the other side. Reverse the leads when checking the power side.

Question: What happens if during a volt drop test the negative on the battery is touched to B+ on the alternator?

Answer: if you are using a multimeter, this would be like reading open-circuit battery voltage. One lead connects to negative on the battery and the other lead connects to the other end of the wire connected to the alternator, coming from the battery.

Question: What is the total resistance for two 2-ohm resistors connected in parallel?

Answer: You should get a lower value than the resistor with the lowest value in the circuit.