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How to Test Your Alternator's Voltage Regulator

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

Alternator, voltage regulator and other components.

Alternator, voltage regulator and other components.

Symptoms of a bad voltage regulator may include:

  • High voltage output
  • Low voltage output, sometimes
  • No voltage output
  • Lights dim or flicker
  • Faulty high-beam headlamp bulbs
  • Engine working erratically (weak or flickering ignition system)
  • Adding water to the battery frequently
  • Growing corrosion around battery terminals and top
  • Dead battery
  • Battery or check engine light indicator lit on dashboard

Some of these symptoms may come from loose or corroded charging system connections.

Index

I. What Does a Voltage Regulator Do?

II. Voltage Regulator Test

1. Checking Wires Using Voltage Drop

2. Voltage Regulator Bypass Test

3. Voltage Regulator Adjustment

4. Testing a Contact-point Voltage Regulator

III. Voltage Regulator Replacement

Check the battery and charging circuit to make sure they are not interfering with charging system operation.

Check the battery and charging circuit to make sure they are not interfering with charging system operation.

I. What Does a Voltage Regulator Do?

A voltage regulator controls the alternator voltage output to maintain a preset charging voltage for the battery. It also controls electrical power to the vehicle's different electrical systems.

Without a voltage regulator, an alternator may put out up to 250 volts. This is enough to destroy the car's battery and electrical system.

The voltage regulator is usually found inside or on the back of the alternator case. Increasingly, though, late-model vehicle have the engine control module (ECM) regulating alternator voltage output through a special circuit.

Older models used an electromechanical, external voltage regulator, mounted somewhere in the engine compartment.

On a computer controlled charging system, the electronic or powertrain control module can monitor system operation, cut off charging output if voltage levels are too high, and trigger diagnostic trouble codes. This is part of a fail-safe-circuit in the computer, and can greatly help you diagnose system problems and describe potential faults.

Use a digital multimeter to check voltage regulator operation and charging circuit voltage drop.

Use a digital multimeter to check voltage regulator operation and charging circuit voltage drop.

II. Voltage Regulator Test

This test is a simple procedure to check alternator voltage regulator output. You need a digital multimeter for this test.

  1. Set the parking brake and shift the transmission to Neutral (manual) or Park (automatic).
  2. Set your multimeter to DC Voltage and select the 20 Volts in the scale.
  3. Connect the meter's red lead to the battery's positive (+) post and the meter's black lead to the battery's negative (-) post.
  4. Notice the open-circuit voltage of the battery. Your battery should be at about 12.6 volts, 12.4 volts minimum; otherwise, charge the battery and continue with this test.
  5. Now, ask an assistant to start the engine and run it at 1500 rpm.
  6. Take a note of your voltmeter reading.

A good output voltage should be about 2 volts higher than your battery open-circuit voltage. Consult your vehicle repair manual, if necessary, to check the correct specifications for your particular model.

  • If you noticed an output voltage reading below 13 volts right after starting the engine, there could be a charging system problem.
  • If the output voltage reading is 16 volts or higher, there's an overcharging problem. This usually indicates a bad voltage regulator.
  • If voltage seems to fluctuate during your test, switch your voltmeter to the AC voltage scale and take another output voltage reading with the engine still running.
    • This time, connect your meter's red lead to the B+ terminal on the back of the alternator, and the meter's black lead to battery negative (-).
    • Usually, the presence of 0.25 AC volts means a leaking diode that requires replacing the alternator. But some manufacturers recommend replacing the alternator if 0.50 AC volts is detected.
    • However, if you have noticed engine performance issues, this might be the problem. Consult your vehicle repair manual for acceptable diode leak rate, if necessary.

If your output voltage is within specifications, continue with this test:

  1. With the engine running, increase engine speed to 2000 rpm.
  2. Turn on the headlights, AC, defogger, and other high current accessories you may have.
  3. Take a note of your voltmeter reading.

The voltage output reading should be about 0.5 volts higher than your battery's open circuit voltage.

Most voltage regulators are calibrated to output between 13.5 and 15.5 charging volts on a fully charged battery at normal temperature with no accessories or lights on. Consult the specifications in your vehicle repair manual for your particular application.

Keep in mind that a worn or loose drive belt and other vehicle operating conditions like high temperatures can affect how the voltage regulator operates.

When your test shows a steady or intermittent high or low voltage output, the voltage regulator is possibly bad. Most voltage regulators fail by allowing a high voltage output, though. However, before going any further, check that all the connections to the alternator and battery are good and clean as described in the next section.

1. Checking Wires Using Voltage Drop

A quick way to examine the wires and connections in the charging system is to check for voltage drops.

  1. Set your voltmeter to 2 volts.
  2. Start the engine and let it idle.
  3. Measure for voltage across individual wires and connections in the charging system.
  4. If there's voltage over 0.2 volts in any wire or connection, check for corroded, damaged or loose wires.
  5. When fixing wires and connections, aim for a voltage drop of less than 0.1 volts or 0.

Check voltage drop around engine grounds as well, if necessary.

If the charging circuit connections are good, continue with the following tests. You can check whether your voltage regulator is at fault through a regulator bypass test, as described in the following section.

The following video gives you a charging system check overview that you can follow as well to test your charging system, if necessary.

2. Voltage Regulator Bypass Test

On many alternators (except those with computer voltage regulation), you can bypass the voltage regulator to test whether your voltage regulator or some other component (alternator or charging circuit) is at fault.

There could be several ways to bypass the voltage regulator, depending on the charging system configuration for your particular vehicle model.

  • If the rear of your alternator has a 'test tab', you need to short this tab to the alternator frame using a screwdriver while checking voltage output at the battery with the engine running.
  • On other systems, you may need to connect the battery and field terminals using a jumper wire while checking voltage output at the battery with the engine running.

Consult the vehicle repair manual for your particular model to conduct this test, if necessary.

  • When the voltage regulator is bypassed, you should see maximum voltage output.
  • If voltage output is at a normal level, most likely the voltage regulator is faulty.
  • If voltage output remains at the same level as in your initial test, most likely you have a faulty alternator.

The next video shows you how to test an external voltage regulator and how to bypass it.

3. Voltage Regulator Adjustment

Some alternators with an old configuration allow adjusting of the voltage regulator. On these units, you can find a small adjusting screw on the voltage regulator.

  1. Connect your voltmeter across the battery posts.
  2. Set the Parking brake.
  3. Shift the transmission to Neutral (manual) or to Park (automatic).
  4. Start the engine and let it idle. Turn off any accessories, if necessary.
  5. Check battery charging voltage.
  6. Turn the adjusting screw using a small screwdriver to adjust charging voltage to specifications.

Refer to your vehicle repair manual to make sure you have an adjustable voltage regulator, locate the adjusting screw, and set voltage output to specifications.

4. Testing a Contact-point Voltage Regulator

Old type DC generators and early vehicle charging systems used a contact-point type voltage regulator. Basically, it consisted of a coil, a set of points and resistors to control alternator voltage and current output. These regulators were replaced by electronic or solid-state voltage regulators.

Still, there might be some vehicles on the road today fitted with this type of regulator.

Usually, the contact points in the regulator are the ones to cause trouble after many miles of service due to wear or pitting.

To repair a contact-point voltage regulator:

  • File, test and adjust the regulator points, as necessary.
  • If still, voltage output is out of specifications, replace the regulator.

Refer to the repair manual for your particular vehicle make and model.

If necessary, replace the voltage regulator or install a new alternator.

If necessary, replace the voltage regulator or install a new alternator.

III. Voltage Regulator Replacement

To replace it, remove the voltage regulator from the back or inside the alternator. Consult your vehicle repair manual for the procedure for your particular model.

If you don't have the manual yet, you can buy a relatively inexpensive copy online through Amazon. Haynes manuals come with step-by-step procedures for many maintenance, troubleshooting and component replacement projects you can do at home. So you'll recoup your small investment soon.

Some vehicle models use alternators with internal voltage regulation, most likely you'll need to replace the alternator, if voltage regulation has failed.

Also, models with computer-controlled voltage regulation, problems with this circuit means you need to replace the powertrain control module (PCM) has to be replaced.

If you suspect a charging system problem, other than the voltage regulator, you may want to check the charging system.

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.

Questions & Answers

Question: When you wrote, "Without a voltage regulator, an alternator may put out up to 250 volts," is that referring to when a car is on the highway or a racecourse?

Answer: It was referring to how much the alternator may put out if it didn't have a regulator.

Question: Is there a more simple solution for the "over voltage" problem from the alternator regulator? For example, can we not put another filter (voltage regulator) after the alternator to keep the voltage between 12 to 14.5 volts?

Answer: If the original internal regulator failed, usually you can replace the unit with an OEM part. It's calibrated for your specific application. External regulators require different wiring set up to work.

© 2019 Dan Ferrell

Comments

Tomas Jacob Organ on August 30, 2020:

How many watts maximum can an alternator without a regulator generate? And how hard is it to get an alternator to its maximum rpm.

Dan Ferrell (author) on August 18, 2020:

When the regulator fails. Let's suppose it surpasses its maximum current (possibly 4 amps). At 12 volts, increasing rpm to its max you may get around 150, in some models over 200.

TJ Organ on August 17, 2020:

In what circomstance will an altinator qithout a regulator generate 250V

Dan Ferrell (author) on January 06, 2020:

Check the circuit between the alternator and the dashboard light. You may need the repair manual for your particular model to identify the wires and test. Hope this helps.

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