How to bench test an alternator

How To Bench Test an Alternator

Bench testing an alternator is a crucial process to assess its functionality and diagnose any potential issues. By conducting a bench test, you can evaluate the alternator’s output voltage. This will ensure its proper operation. So, how to bench test an alternator?

You can easily bench test the alternator with proper guidance.

I deeply researched how to bench test an alternator. After trying various methods, I was able to find a successful method to do it myself. In this article, I will show you exactly how I bench tested the alternator.

What Is Bench Testing an Alternator?

Multimeter with bag

Bench testing an alternator is a process to verify whether or not the alternator’s output is within specifications.

What Happens If the Alternator Is Bad?

If your alternator is malfunctioning, it will produce a lower or higher voltage than the default or no current at all.

What Are the Symptoms of an Undercharging Alternator?

You can experience dim headlights, slow or malfunctioning accessories, trouble starting the car, frequent stalling, and a dead battery if your alternator produces low power.

What Are the Symptoms of an Overcharging Alternator?

The check engine light, blown fuses and lights, a red battery light on the dashboard, battery damage, and overly bright lights are all common signs of an overcharging alternator. The electrical system of your car is safeguarded from an overcharging alternator by the voltage regulator.

How To Remove the Alternator from the Vehicle?

For the bench test, I pulled the alternator from the vehicle.

Step 1: Take Safety Precautions

First, I set the parking brake. Then I waited some time to allow the engine to cool, as I just came home in the car. I wore eye protectors and protective gloves for additional protection.

Step 2: Access the Alternator

I identified the alternator in the engine compartment. Next, I removed the heat shield to get better access to the alternator.

Step 3: Disconnect the Electric Cables

I disconnected the electric cables coming to the alternator.

Note: Remember their positions. If not, you will fall into big trouble when reconnecting them.

Step 4: Remove the Belt

Then I loosened the drive belt tension. I removed the serpentine belt from the alternator pulley.

Serpentine belt of a car, tensioner pulley is loosened

Step 5: Pull Out the Alternator

Next, I removed the mounting bolts that secure the alternator to the bracket and pulled the alternator away from the engine compartment.

How To Bench Test an Alternator

Test ParameterExpected ValueInterpretation
Output Voltage13.5 to 14.5 voltsWithin acceptable range (indicating proper output)
Diode RectifierContinuityDiodes functioning correctly (good rectification)
No ContinuityDiodes failed (poor rectification)
Regulator FunctionalityIn the range of 13.5 to 14.5 voltsRegulator regulating voltage effectively
Fluctuating in a higher rangeRegulator malfunctioning (voltage regulation issue)

So, how to bench test an alternator? This is how I bench-tested my alternator.

Tools Needed:

  • Factory plug with a four-pin connector
  • Voltage meter
  • Low five-watt light bulb
  • Jumper wires
  • Drill with a socket
  • Optional switch (for controlling charging)

Step 1: Identify the Primary Wire

First, I identified the brown wire in the four-pin plug. It is the primary wire that needed to be focused on during the test.

Step 2: Measure the Battery Voltage

Then I connected the volt meter to the battery. The battery voltage was 12.4.

Note: It should be around 12.5. If your battery power is low, you should replace it with a fully charged battery to ensure power is sufficient for the test.

Step 3: Connect the Wires

Next, I took out the jumper wire. I connected one end to the brown wire of the plug and the other end to one side of the bulb.

I connected another jumper wire to the other side of the bulb and the positive ignition source.

Important: Use a 50-ohm resistor in series with the jumpers to prevent damaging the voltage regulator.

Step 4: Connect the Drill

I connect the drill to the positive post of the alternator.

Step 5: Observe the Voltage

I started the drill and began spinning the pulley. Then I observed the volt meter, and there was no charge indicated. Initially, with the switch off, there should be no charging indicated. The light was illuminating.

Step 6: Turn the Switch On

Then I flip the switch to the on position while spinning the pulley. The bulb gradually dimmed and turned off completely. This indicates that the alternator is doing the charging work. Moreover, the drill also slowed down indicating that the alternator was generating current. That’s it. My alternator was good.

Note: If the alternator doesn’t charge, spin the pulley near 1500 RPM. This achieves the necessary speed for charging.

If you are not clear about any step I mentioned, watch this video.

What Should I Do If Your Alternator Shows No Voltage?

Here are the things you should check if the alternator is faulty.

Voltage Regulator

First, test the voltage regulator. The voltage regulator’s job is to regulate the electrical output of the alternator.

Rectifier Diodes

Diode failure leads to low or no voltage output from the alternator. You can use a multimeter to test the rectifier diodes of the alternator. If the rectifier diodes are failing, you should replace them.

Bushes And Slip Rings

You know that brushes make electrical contact with the slip rings of the alternator. Make sure the brushes have proper contact with the rings. If bushes are worn out or damaged, you have to replace them. You should also clean the rings to remove any debris from them. This will ensure proper electrical conductivity.


Can you clean an alternator?

Yes, you can clean the alternator.

  1. Get an off-the-shelf degreaser and dilute it with water to a 50/50 mixture.
  2. Apply a cloth or gently spray it onto the dirty areas of the alternator.
  3. Let the cleaner soak on the alternator for about 5 minutes.
  4. Take a soft brush with short bristles (around 2-3 inches long) and use it to clean heavily soiled areas and the terminal connection points of the alternator.
  5. Gently scrub the dirt and grime. Make sure not to apply excessive force that could damage the alternator.
  6. After cleaning, wipe away any residue with a clean cloth.
  7. Allow the alternator to air dry completely before reconnecting any electrical connections or installing it back into the vehicle.
  8. You can also paint the alternator for protect it from rust.

Do jet skis have alternators?

In opposition to cars, which use alternators, jet skis have alternative charging systems with a stationary stator and a rotating flywheel all around them. Stators also generate electricity, but they can only maintain the battery’s charge.

How many alternators does a 6.7 Powerstroke have?

F250/F350 6.7L Diesel trucks have 370 Amp XP High Output single or dual alternator configurations. The single or lower dual alternator mounts are compatible with this item.

How long will a whining alternator last?

The whining noise of the alternator is due to a bad alternator pulley or a worn-out bearing. It can travel up to a few thousand kilometers before breaking down if it is in the early stages.


Now you know how to bench test an alternator. An alternator bench test is a simple task that can be done by yourself with proper guidance. Have a nice day!

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