How to Test an Alternator Diode

The alternator is that part of a car that produces current for the electrical functions of the car.

It converts the engine’s mechanical energy into electrical energy, and channels part of this energy to recharge your car battery.

If you experience a change in how the electrical systems of your car function, the alternator could have a problem.

It’s important to know what the early signs of failure are as well as a few easy tests you can do. This will save you from having a costly breakdown.

Below are a few steps to test the alternator:

Testing with a voltmeter

Here you use a device called a voltmeter.

Step 1: measure your battery’s base voltage.

Ensure your battery has full charge before you do this.

Set your voltmeter to 20V(DC mode). For accurate readings make sure the voltmeter’s batteries are brand new.
Familiarize yourself with the color code of the cables- red is positive, black is negative.
Connect the probe tips to the battery’s terminals-negative to negative, positive to positive.
Take note of the change in the voltmeter display upon contact(12.6 volts for a fully charged battery). If the battery’s base voltage is between 6.5V and 10.8V, recharge your battery or replace it.

Step 2: Measure the charging system’s No-load voltage

Have someone start the engine and run at idle for a while. Ask him/her to maintain the engine’s speed at 1500 RPM and switch off all other accessories.
Connect the probe tips as before.
Take note of this new reading. The expected change is an increase of 0.5V to 2.0V(13.1V to 14.8V, if your base was 12.6V). An increase above 2.0V signifies an overcharging issue. If there is no change in the voltmeter’s reading or lower reading than the base voltage, your charging system is malfunctioning.
Recheck your connectors and system wires to ensure they are in good condition. If the reading doesn’t change after checking, your alternator has a problem.

Step 3: measure the charging system’s Load Voltage

This is done when the charging system passes the No-load voltage test.

Restart the engine, and maintain a speed of 2000 RPM and switch on all accessories.
Reconnect the voltmeter.
Take note of the reading. The difference between this reading and the base voltage is the charging voltage of your system. The ideal charging voltage should be at least 0.5V higher than the base voltage.(13.1V to 15.0V). A higher or lower value than this signifies a faulty alternator.
If your charging system failed the No-Load or Load test, you need to check your alternator’s voltage regulator. There are different designs for charging systems. Before you test the alternator voltage regulator make sure you consult your vehicle’s service manual. The manual will guide you on the type of tests you can do on your own.
If tests prove that the alternator voltage regulator is bad, find a replacement online or in a local auto parts shop.
An alternator voltage regulator that is faulty can cause charging problems.
Internal voltage regulators found in late-model cars can also fail from high levels of heat.

Step 4: test the alternator diode

Reset the voltmeter to the lowest AC voltage scale setting possible.
Restart the engine, run it for a few minutes at 1200 RPM
Connect the voltmeter probes as before
The display should read 0.00. A fraction(eg. 0.001) in the display points to a bad alternator diode. You’ll need to replace the whole alternator because the diode cannot be replaced alone.

Testing without a voltmeter

Although using a voltmeter gives an accurate report on your car’s alternator problems, it is not the only available way to test.

Here’s how to test without a voltmeter:

Assess the Alternator belt drive condition

The energy from the engine reaches the alternator through the drive belt. A loose or worn-out belt leads to charging problems.

Listen for an unusual sound from your car’s engine.

As your engine runs, listen for a loud clicking sound. It indicates alternator problems.

Do a headlight test

For accurate results on the headlight brightness do this in a darkened environment.

Switch on the headlights
Start the engine, the lights’ brightness may fade but go back to the original brightness after a while. This means the alternator is charging your system properly.
If the headlights brightness level does not go back to the original level, you might be having alternator problems.

Check the bearings of the alternator

Use a rubber hose to do this. It works like a stethoscope.

Place one end of the hose on the metal alternator case and the other to your ear. If there is loud grinding or squeaking as you listen there could be a bearing failure that begins in the alternator and you’ll need to replace it.

Testing with a multimeter

For this test follow the steps in testing with a voltmeter. The only difference is that with a multimeter you will have to let the car sit overnight or for an hour before testing the battery.

Testing the battery when the car is running will affect the multimeter’s ‘true’ reading.

The alternator test readings should be similar to those in testing with a voltmeter.

Set the multimeter’s voltage to 20DC. The display should read ‘0.00’
Connect the multimeter probes with the respective battery terminals. A 12.6V means the battery is fully charged, 12.2V means the battery is at 50% charge, lower than 12V means the battery is dead and should be replaced.
If your battery is in good condition, test the alternator. A good alternator produces 13.1V to 14.6V when the engine is running but the car accessories are switched off.
A reading higher or lower than this value means your car is either overcharged or undercharged.

Testing with a screwdriver

Bring the tip of your screwdriver close to the nut on the alternator’s pulley.

High magnetism indicates that the alternator is in good shape. Absence of magnetism indicates a problem with your alternator.

Bad diode symptoms

A diode is a semiconductor device that only conducts current in one direction. It has two terminals-anode and cathode.

Current flows from the anode to the cathode. It can’t flow in the reverse direction. The alternator diode test readings for a bad diode are the same in both directions.

Bad diode symptoms are:

Blinking warning indicator light on your dashboard
Dim or flickering headlights
Dashboard gradually dimming while the car runs
Malfunctioning of the car’s electricals eg, air-conditioning unit
Buzzing noise through the radio’s speakers
A squealing sound from the front while the engine runs
Problems when starting your car or frequent stalling
Frequent dead battery experiences

How do alternator diodes fail

Diodes fail when they are exposed to current/voltage that’s higher than their limits.

The alternator handles a substantial load while generating electricity. Because of this load, the diodes fail from overloading and overheating.
A defective connection between the battery positive terminal and the alternator output may cause diode failure. A defective connection is caused by an open circuit caused by a break in some battery cable stands or by corrosion. This causes the current to find an alternate route that may lead to excessive flow of current, making the diodes overheat and fail.
When an alternator is used to bring a battery that’s undercharged to full charge, diode overheating may occur. Excessive current flow overheats the diodes, causing failure.
An attempt to swap out the battery while the engine runs can cause diode damage. This is because the current seek out an alternate route to flow as soon as you disconnect the battery cables, leading to overheating and failure of the diodes.
A connection between a high amperage set battery charger and a battery that’s in reverse polarity while still connected to the alternator causes diode damage.
Jump starting a battery can also cause diode damage because any excessive power surge burns out the diodes on either vehicle.

To avoid diode damage caused by human error, always seek out the help of a certified mechanic.

Perform regular checks and maintenance to ensure there are no open circuits or corrosion.

Alternator rectifier failure symptoms

Diodes as part of the rectifier assembly convert the AC output from the alternator to DC. Before flowing to the battery and electrical system, this charging output goes through six diodes located in the rectifier assembly.

Alternator rectifier failure symptoms show when one or more diodes fail or when there’s a significant reduction in the alternator’s output which leads to a battery drain. If the battery is the cause of a faulty regulator rectifier you’ll notice signs like problems when starting the car or dimmed headlights. Once you do, use a voltmeter to check the voltage instead of relying solely on these symptoms to make a diagnosis.

Another alternator rectifier failure symptom is battery overcharge. Battery overcharge results from the inability of the rectifier to moderate the battery voltage levels.

Can an alternator still charge with a bad diode?

The alternator’s charging output drops significantly when diodes fail. It may produce sufficient current for all the car’s electrical needs when one or two diodes only have failed but this isn’t enough to keep the battery charged. With time the battery will run down.

Bench testing an alternator

A bench test is the best way to identify problems with your alternator. A bench test checks diodes and internal regulators to gauge the alternator’s ability to produce its rated voltage and current.

Steps to bench test an alternator

Fit the alternator into the bench test machine and connect the drive belt.
Identify each connection at the rear end of the alternator. There is one small connection and two large ones.
Connect the two large connections with the B+ wire. Use one or both.

The smaller connection is for the warning lamp.it passes electricity through the voltage regulator to the rotor and back to the battery.

Connect the green wire at the back of the alternator to the warning lamp terminal, and the light red wire to the B+ terminals.
Use these two modes to test the alternator:

The amperage mode to show you the maximum output of the alternator. This is done when the unit is spinning. Ensure you keep your clothing and hands clear of the rotating parts.
The voltage mode to show you the regulated voltage of the alternator. To charge a battery, the voltage needs to be higher than 12V( the nominal voltage of the system).

How to fix a bad alternator diode

If your car’s alternator diode is bad, the diode rectifier assembly will need replacement. Replacement allows the alternator to keep your car battery fully charged.

Replacement of diodes is a complicated issue because diodes are usually pressed into the diode plate. This press becomes a challenge because any wrong contact with the diode plate results in the death of the diodes.

Also, sourcing for matching diodes for replacement is difficult. Repair becomes labor and time-intensive and also prone to errors.

It is easier and cheaper to replace the entire alternator.

Safety tips when testing an alternator

Your car should be parked on level ground for ease when working
Turn off the engine before you open the hood
Use wheel chocks to secure the car around the rear tire on the driver’s side to prevent movement
Put on safety glasses

Check this too: How Long Does An Alternator Last

Conclusion

If from your tests you do not find any problems, but you still experience persistent battery warning on the dashboard or have a dead battery, you need further testing. This should be done by a certified mechanic.

Though it’s easy to test an alternator, some things may require professional evaluation.