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How to Test the Alternator Diode[FAQ]?

Author: Apogeeweb
Date: 12 Mar 2022
how to test alternator diode



Ⅰ How To Test An Alternator Diode

1.1 Testing With A Voltmeter

1.2 Testing Without A Voltmeter

Ⅱ Alternator Diode Replacement


1. How do you Know if your Alternator has a Bad Alternator Diode?

2. What does a Alternator Diode Do?

3. What is a Bad Alternator Diode?

4. How Much does it Cost to Replace an Alternator?

5. What are Symptoms of Bad Alternator Diodes?

6. Why do you Need in an Alternator Diodes?

7. What Does an Alternator Diode Rectifier do?

8. How do the Alternator Diodes in an Alternator Work Video?

9. Can a Bad Alternator Diode on an Alternator be Replaced?

10. Can a Bad Alternator Diode Cause a Car Battery to Run Down?

11. What Causes an Alternator to Overheat in a car?

12. How are the Alternator Diodes Used?

13. How to Check Alternator Diode?

14. How Long can a Car Run Without Alternator?

15. How many Diodes Does an Alternator Have?


The alternator diodes are a component of the rectifier assembly, which transforms the alternator's alternating current output to direct current. The charging output of the alternator passes through six diodes in the rectifier assembly before reaching the battery and electrical system. As a result, the larger the charging load, the hotter they become. If you notice a change in the operation of your vehicle's electrical systems, the alternator may be malfunctioning. It's critical to understand the early warning signals of failure, as well as a few simple tests you can perform. You will avoid a costly breakdown as a result of this.

The following are a few steps for testing the alternator diode.

Ⅰ How To Test An Alternator Diode

In this video, we will demonstrate how to test an alternator diode with a multimeter to determine whether the alternator diode pack is good or defective. Flickering dash lights and headlights can be caused by a bad alternator diode, however it is not always the case. While the alternator diode pack fails, the alternator will kill your car battery even when the engine is turned off. This is due to the fact that the alternator will get current from the battery.


1.1 Testing With A Voltmeter

A voltmeter is used in this situation.

Step 1: Measure Your Battery’s Base Voltage.

Before you begin, make sure your battery is fully charged.

  • Set your voltmeter to 20 volts (DC mode). Make sure the voltmeter's batteries are brand new for reliable readings.
  • Familiarize yourself with the cable color coding—red is positive, black is negative.
  • Connect the probe tips to the terminals of the battery—negative to negative, positive to positive.
  • Take note of any changes in the voltmeter display that occur as a result of contact (12.6 volts for a fully charged battery). If the battery's base voltage is between 6.5V and 10.8V, it should be recharged or replaced.

Step 2: Measure The Charging System’s No-Load Voltage

  • Start the engine and let it idle for a few minutes. Request that he/she keep the engine speed at 1500 RPM and turn off all other accessories.
  • Connect the probe tips in the same manner as before.
  • Make a mental note of this new reading. The projected change is a 0.5V to 2.0V rise (13.1V to 14.8V, if your base was 12.6V). An increase above 2.0V indicates an overcharging problem. If there is no change in the voltmeter reading or if the reading is lower than the base voltage, your charging system is faulty.
  • Check the connectors and system cables to ensure they are in good working order. If the reading does not change after inspection, your alternatoris not working properly.

Step 3: Measure The Charging System’s Load Voltage

When the charging system passes the No-load voltage test, this is done.

  • Restart the engine and maintain a speed of 2000 RPM while turning on all of the accessories.
  • Reattach the voltmeter.
  • Take heed of what you're reading. The charging voltage of your system is the difference between this reading and the base voltage. The ideal charging voltage is at least 0.5V more than the base voltage. (13.1 to 15.0 volts). A value greater or less than this indicates a defective alternator.
  • This is done when the charging system passes the No-load voltage test.
  • If your charging system failed the No-Load or Load test, you should inspect the voltage regulator on your alternator. Charging systems come in a variety of designs. Consult your vehicle's service manual before testing the alternator voltage regulator. The guidebook will instruct you on the kind of tests that you can perform on your own.
  • If testing shows that the alternator voltage regulator is faulty, look for a replacement online or at a local auto parts store.
  • A defective alternatorvoltage regulator might create charging issues.
  • Internal voltage regulators used in late-model autos can also fail due to excessive temperatures.

Step 4: Test The Alternator Diode

  • Reset the voltmeter to the lowest allowable AC voltage scale setting.
  • Restart the engine and run it at 1200 RPM for a few minutes.
  • Connect the voltmeter probes in the same manner as before.
  • The display should be set to 0.00. A fraction (for example, 0.001) in the display indicates a faulty alternator diode. Because the diode cannot be replaced on its own, you must replace the entire alternator.

1.2 Testing Without A Voltmeter

Although a voltmeter provides an accurate report on alternator faults in your vehicle, it is not the only option to test.

Here’s how to test without a voltmeter:

Assess The Alternator Belt Drive Condition

The drive belt transports energy from the engine to the alternator. Charging issues are caused by a loose or worn-out belt.

Listen For An Unusual Sound From Your Car’s Engine.

Keep an ear out for a loud clicking sound when your engine runs. It denotes alternator issues.

Do A Headlight Test

Do this in a darker setting for accurate results on headlight brightness.

  • Turn on the headlights.
  • When you start the engine, the brightness of the lights may fade, but it will return to normal after a time. This indicates that the alternator is properly charging your system.
  • If the brightness of the headlights does not return to its former level, you may have alternator troubles.

Check The Bearings Of The Alternator

To do this, use a rubber hose. It functions similarly to a stethoscope.

Place one end of the hose on the metal alternator case and the other against your ear. If you hear loud grinding or squeaking while listening, you may have a bearing failure that starts in the alternator and will need to be replaced.

Testing With A Multimeter

Follow the instructions in testing with a voltmeter for this test. The only difference is that you must let the car sit overnight or for an hour before checking the battery with a voltmeter.

When testing the battery while the car is operating, the 'real' reading of the multimeter will be affected.

The alternator test results should be comparable to those obtained when testing with a voltmeter.

  • Set the multimeter's voltage to 20 volts of direct current. '0.00' should be displayed on the display.
  • Connect the multimeter probes to the appropriate battery terminals. A voltage of 12.6V indicates that the battery is fully charged, a voltage of 12.2V indicates that the battery is halfway charged, and a voltage of less than 12V indicates that the battery is dead and should be replaced.
  • Test the alternator if your battery is in good condition. When the engine is running but the car accessories are turned off, a decent alternator produces 13.1V to 14.6V.
  • A reading that is higher or lower than this figure indicates that your vehicle is either overcharged or undercharged.

Testing With A Screwdriver

Bring the tip of your screwdriver up against the nut on the alternator pulley.

The presence of high magnetism implies that the alternator is in good working order. The absence of magnetism suggests that there is an issue with your alternator.

Ⅱ Alternator Diode Replacement

Replacement Denso Dynamo brushes and alternator diodes Remove the B+ post's 10mm nut, three 8mm nuts, and the 8mm cap screw. To remove the fracture, use a little screwdriver to carefully pry it open. Brushes and diodes are now operational.


1. How do you Know if your Alternator has a Bad Alternator Diode?

Normally, a broken alternator diode causes the instrument cluster's headlights or turn signals to blink or darken, and the battery might drain overnight or within minutes. * To look for a damaged alternator diode, set the voltmeter to the lower end of the AC (AC) scale.

2. What does a Alternator Diode Do?

In a brushless AC alternator, the alternator diodes convert the alternating current (AC) current induced in the exciter rotor by the exciter stator to direct current (DC). This occurs because diodes only allow current to flow in one way; so, because AC is current that runs in both directions, a diode will halt one of the flows from the AC to convert it to DC.

This direct current is then passed via the main rotor, causing a current to flow through the main stator.

The magnetic field in the alternator of a generator relies on the flow being DC, hence it is required that the flow be DC.

The AVR houses the diodes in an alternator. There is the same number of forwarding biased and reverse biased diodes, allowing the alternator to make use of both directions of the alternating current. This means that all of the alternating current power can be used to generate electricity rather than just half of it.

3. What is a Bad Alternator Diode?

Generator failures are frequently caused by faulty diodes. The rectifier, which transforms the alternator output to direct current, contains diodes. The charging current from the generator is routed through the rectifier's six diodes before being supplied to the vehicle's battery and electrical system.

4. How Much does it Cost to Replace an Alternator?

It is critical to test the charging system if the headlights are dim. Waits estimate that replacing an alternator with a refurbished alternator costs $ 400 on average, whereas Gunning estimates that a refurbished alternator in a standard house automobile costs $ 300 to $ 500, including materials and labor.

5. What are Symptoms of Bad Alternator Diodes?

7 Symptoms of a Faulty Alternator Battery Warning Sign. The driver's first indication that there is an issue with the charging system is an illuminated battery warning light on the instrument panel. The inadequate performance or sluggish behavior If there is a complete power outage, the generator is most likely defective. Noise. Serpentine belt is torn. The odor of burning. Storage problems. Alternator or battery failure.

6. Why do you Need in an Alternator Diodes?

Diodes are used in the conversion of alternating current (A/C) to direct current (D/C) because they allow electrical current to travel in just one direction. When the generator is turned on, it generates power, which is routed through diodes to the generator's battery connector.

7.  What Does an Alternator Diode Rectifier do?

Alternator diodes are components of a rectifier assembly, which transforms alternating current at generator output to direct current. The charge produced by the generators is routed through six diodes in the rectifier unit before being delivered to the battery and electrical system.

8. How do the Alternator Diodes in an Alternator Work Video?

The current drawn from the battery must have the correct polarity to charge the battery while the alternator is functioning (positive to positive, negative to negative). While charging the battery, the rectifying diodes operate as a check valve to maintain steady polarity.

9. Can a Bad Alternator Diode on an Alternator be Replaced?

This has the potential to drain the battery over time. Can you also replace the defective alternator diode? If one or more diodes in your vehicle's alternator fail, the diode rectifier must usually be replaced. Your vehicle's alternator diode rectifier can be replaced at Grimmer Motors.

10. Can a Bad Alternator Diode Cause a Car Battery to Run Down?

Generator failure is frequently caused by faulty diodes. If only one or two diodes fail, the generator can still provide enough current to power the car, but not enough to handle larger loads or keep the battery completely charged. This has the potential to drain the battery over time.

 11. What Causes an Alternator to Overheat in a car?

When using a generator to fully charge an undercharged battery, the diode can overheat. Excessive current will overheat the diodes and cause them to fail. Attempting to change the battery while the engine is running may cause the diode to be damaged.

12. How are the Alternator Diodes Used?

The generator is powered by a motor that rotates a magnetic rotor continuously. Diodes are intended to rectify current while rotating. The battery and other electrical systems rely on rectifiers and diodes to create the necessary current.

13. How to Check Alternator Diode?

Connect the major terminal of the generator (the generator's positive terminal) to the red wire of the multimeter, and connect the other wire to the generator housing. If a multimeter is used to test resistance, it should read resistance greater than 550 ohms if the alternator diode is in good working order.

14. How Long can a Car Run Without Alternator?

25 miles should be sufficient. In theory, you can drive an older car indefinitely once the engine is started; but, you will be without any electrical conveniences in the vehicle. Newer vehicles are far more dependant on the battery/charging mechanism, and many will not even start if the battery is dead.  

15. How many Diodes Does an Alternator Have?

Six alternator diodes.








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