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How to Do a Voltage Drop Test on Your Charging System

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Excessive voltage drop may cause charging system problems.

Excessive voltage drop may cause charging system problems.

Symptoms of excessive charging system voltage drop may include:

  • Battery undercharging
  • Battery overcharging
  • Dead battery
  • Slow-crank condition
  • No-crank condition
  • Charging indicator light on
  • Vehicle electrical issues

Excessive voltage drop (excessive resistance) in the charging system circuit significantly reduces current flow. This prevents the adequate amount of current from reaching the alternator, the battery, or both, affecting system operation and other electrical systems in the vehicle.

For example, you may notice your battery dies frequently, or that your engine cranks slowly or doesn’t crank at all. The warning indicator on your dashboard may show a problem with the charging system.

If you suspect there’s something wrong with the charging system, this test can help you diagnose the problem. A voltage drop test will help you locate excessive resistance in the circuit, that otherwise would be difficult to detect.

INDEX

I. Why Do a Charging System Voltage Drop Test?

II. Charging System Voltage Drop Test

1. Preparing for the Test

2. Ground Circuit Voltage Drop Test

3. Power Circuit Voltage Drop Test

III. Common Voltage Drop Values

IV. Voltage Drop as a Diagnostic Tool

Check the charging system voltage drop with your digital multimeter.

Check the charging system voltage drop with your digital multimeter.

I. Why Do a Charging System Voltage Drop Test?

For the most part, alternators are pretty reliable. In fact, most automotive charging system malfunctions are caused by faulty wires, connectors or some other component in the system:

  • Loose wires
  • Frayed wires
  • Corroded connections
  • Bad grounds
  • Faulty battery
  • Blown fusible link or fuse

When a charging system fault appears, however, many car owners immediately assume a problem with the alternator, and replace it, hoping this will fix the problem.

Instead, you can do a voltage drop test first. It takes only a few minutes and will help you locate problems in the circuit’s wiring that may not be visible at plain sight.

The test helps you locate loose, corroded or damaged connections or wires.

You’ll be checking for potential circuit problems before replacing an alternator or battery that may be in good operating condition. Voltage drop testing will help you speed up charging system diagnostic.

Be Careful When Testing the Charging System

Some vehicle models equipped with a heated windshield may cause the alternator to output as much as 110 AC voltage. If possible, don’t operate this system while running the following tests.

Check the battery terminals, wires and connectors between the battery and alternator for loose or corroded connections.

Check the battery terminals, wires and connectors between the battery and alternator for loose or corroded connections.

II. Charging System Voltage Drop Test

The next series of tests help you find excessive resistance in the charging system circuit.

For these tests, you'll need a digital multimeter and, possibly, the vehicle repair manual for your particular vehicle make and model. The manual will help you locate and identify wires and components, and look up test results’ specifications, if necessary.

If you don't have the manual for your car, you can find a relatively inexpensive copy through Amazon. Haynes manuals come with step-by-step procedures for many troubleshooting, maintenance and repair procedures you can do at home. So you'll recoup your small investment right away.

1. Preparing For the Test

During the next two tests, maintain engine conditions as described in the following steps:

  • Apply the parking brake.
  • Set your transmission to Park (automatic) or neutral (manual).
  • Start the engine and let it idle.
  • Have an assistant increase and maintain engine speed at 2000 rpm.
  • Turn on the headlights on high-beam.
  • Turn on the wipers.
  • Turn on the AC.
  • Turn on other electrical accessories.

This will allow high current to flow through the circuit while running the tests.

charging-system-voltage-drop-test

2. Ground Circuit Voltage Drop Test

Excessive voltage drop on the ground side of the charging system may lead to an overcharging condition and other electrical system issues. So let’s start by checking the ground path for the charging circuit first.

  1. Set your voltmeter to 2 volts on the DC Volts scale.
  2. Connect your voltmeter black lead to the alternator housing.
  3. Connect the red meter lead to the battery negative (-) post.
  4. Double check that your voltmeter leads are away of moving engine components.
  5. Check your voltmeter readout.

Your drop voltage should be between 0.3 volts and 0.5 volts:

  • If your reading is below 0.3 volts, have the alternator checked to make sure it is charging. Consult your repair manual for the correct specifications for your application.
  • If your reading is over 0.5 volts, check each connection in the ground circuit (tighten alternator mounting bolts, mounting bracket free of corrosion, good engine grounds).
  • Follow the steps in the next section Checking engine grounds.

Consult your repair manual for the correct specification for your model, if necessary. However, you can expect 0.1 volts or less of voltage drop per connection. If you don’t have your manual, refer to the numbers in the section below Common Voltage Drop Values.

If the ground path didn’t past the voltage drop test, continue with the following voltage drop ground tests; otherwise go to the next section, "Power Circuit Voltage Drop Test."

Checking Engine Grounds:

During this test, notice at which point voltage drop goes down to an acceptable level. Your trouble point will be the previous one to this point.

  1. Move the voltmeter red lead to the alternator mounting bracket. Keep the meter black lead on the battery negative post, and take a note of your reading.
  2. Then move the red lead to the engine block.
  3. Then move the red lead to the firewall.
  4. Move the meter red terminal to the battery ground terminal that connects the negative post to the vehicle’s body, if your battery has one. Take a note of your reading.
  5. Move the meter red terminal to the battery’s black post terminal and take a note of your reading.

If necessary, make sure the alternator is properly mounted and there’s no oil or grease between the alternator and mounting bracket and engine; clean a terminal or mounting point of corrosion; properly tight loose connections; repair or replace frayed wires.

charging-system-voltage-drop-test

Subsection 3. Power Circuit Voltage Drop Test

Excessive voltage drop on the power side of the charging circuit may lead to an undercharged condition and other engine electrical system issues. So now, let’s check the power side of the charging circuit.

  1. Set your voltmeter to 2 volts on the DC Volts scale.
  2. Connect the voltmeter red lead to the output battery post (B+) on the back of the alternator.
  3. Connect the voltmeter black lead to the positive (+) battery post.
  4. Make sure your voltmeter leads are away of moving engine components.
  5. Then check your voltmeter readout.

Your drop voltage should be between 0.3 volts and 0.5 volts:

  • If your reading is below 0.3 volts, have the alternator checked to make sure it is charging. Consult your repair manual for this specifications, if necessary.
  • If your reading is over 0.5 volts, check each connection in the power side of the circuit. Follow the next steps.

Usually you should expect 0.1 volts or less of voltage drop per connection. Consult your vehicle repair manual for the correct specifications for your particular model, if necessary. If you don’t have your manual, take a look at the numbers in the section Common Voltage Drop Values below.

If voltage drop is too high:

  1. Move your meter red lead to the terminal on the B+ post.
  2. Then check the next connection in the circuit (perhaps a fusible link) until you reach the battery terminal that connects to the positive (+) battery post.
  3. Notice at which point your voltage drops to an acceptable level.
  4. Your trouble point will be the previous one to this point.
Compare your readings to your manufacturer specifications, if necessary.

Compare your readings to your manufacturer specifications, if necessary.

III. Common Voltage Drop Values

Although a perfectly working wire, cable or connection can run its loads trouble-free, some resistance exists in the wire or cable. For the most part, these common voltage drop values are expected:

DeviceVoltage Drop

Connection

0.0

Switch

0.3

Wire or cable

0.2

Ground

0.1

Take these values as a reference point. Whenever possible, check the voltage drop values expected for your particular application as listed in your vehicle repair manual.

Have your car battery and alternator checked if your charging system circuit seems to be OK.

Have your car battery and alternator checked if your charging system circuit seems to be OK.

IV. Voltage Drop as a Diagnostic Tool

Excessive charging-system circuit voltage drop is often overlooked. However, it’s often the cause of low charging voltages or poor electrical systems performance. This is often due to corroded or poor connections at the battery or alternator.

You can waste your time, and money, replacing relays, motors, light bulbs and other components unnecessarily trying to fix a circuit issue.

If your voltage drop tests didn’t reveal any excessive resistance in the charging circuit, but you suspect a problem in the system, check the alternator and battery. If necessary, most auto parts stores will check your alternator and battery for free.

Next time you run into a problem with the charging system, starting system, or some other vehicle electrical system or component, start by checking that circuit’s voltage drop first. This strategy can save you time and money. And don’t be surprised to find the problem hiding under a loose, corroded or damaged connector, terminal or wire.

The following video, gives you a visual description of the charging system voltage drop test.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Dan Ferrell

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