How to test your alternator

Tools Needed for Testing

Outside of basic tools, there are a few things you’ll need for testing your alternator.

Using a Multimeter

A multimeter is an essential tool for testing an alternator. It’s a versatile device that can measure voltage, current, and resistance. When testing an alternator, the focus is primarily on measuring voltage output to ensure it’s within the recommended range. A basic digital multimeter, easily available at most hardware stores, will suffice for this task.

Safety Equipment

Before beginning the testing process, it’s important to have some basic safety equipment on hand. Rubber gloves and safety goggles can protect you from accidental shocks or splashes of battery acid. Additionally, make sure your vehicle is parked on a level surface, the engine is off, and the keys are removed from the ignition before starting any tests.

How to Test the Battery

Before jumping to the alternator, it’s wise to check the battery’s health with a multimeter. A weak or dead battery can mimic alternator problems, so it’s crucial to rule out the battery as the source of the issue first.

1. Set the Multimeter: Start by setting your multimeter to the DC voltage setting. This setting typically has a “V” with a straight line next to it.
2. Connect the Multimeter: Place the multimeter’s red probe on the positive terminal of the battery and the black probe on the negative terminal.
3. Read the Voltage: A healthy, fully charged battery should read between 12.6 and 12.8 volts when the engine is off. If the reading is below this range, the battery may need charging or replacement before proceeding to test the alternator.

Testing the Alternator with the Engine Off

One basic test to begin with is the key-on, engine-off (KOEO) test. This involves turning the key in the ignition to the "on" position without starting the engine. Look for any warning lights on the dashboard, particularly the battery or alternator light. If these lights don’t illuminate, there might be an issue with the alternator or the associated wiring.

Testing the Alternator with the Engine Running

Once you have tested the battery, start the engine. Leave it idling and turn off all electrical components like the radio, air conditioning, and headlights.

As before, set your multimeter to the DC voltage setting.

With the engine running, connect the multimeter’s positive probe (usually red) to the battery’s positive terminal and the positive probe (usually black) to the negative terminal. The voltage reading should be between 13.8 and 14.8 volts. This range indicates that the alternator is charging the battery properly.

If the voltage is below 13.8 volts, the alternator may be undercharging the battery. A reading above 14.8 volts suggests the alternator is overcharging, which can also lead to battery damage.

The Load Test

To further test the alternator’s ability to handle electrical loads, turn on various components like the headlights, air conditioning, and radio. As each component is turned on, watch the multimeter’s reading. The voltage should remain within the 13.8 to 14.8 volts range. If the voltage drops significantly when a load is applied, the alternator may be failing.

Testing the Alternator’s Diode

The alternator contains diodes that convert the AC electricity generated by the alternator into the DC electricity used by the vehicle. If one or more diodes fail, the alternator might produce insufficient power, or the battery may discharge when the vehicle is off.

How to Test the Diodes

1. Switch the Multimeter to AC Voltage: After performing the load test, switch your multimeter to the AC voltage setting.
2. Measure the Voltage Again: With the engine running and the multimeter probes still connected to the battery terminals, check the AC voltage reading. A reading above 0.5 volts suggests that one or more diodes are failing, allowing AC voltage to pass through, which can damage the vehicle’s electrical components.

Using an Alternator Test Bench

For a more comprehensive test, you can use an alternator test bench. This device can simulate various conditions and provide a detailed analysis of the alternator’s performance. While this method requires removing the alternator from the vehicle, it can be more accurate than in-vehicle testing.

On-Board Diagnostic (OBD) Tools

Modern vehicles equipped with On-Board Diagnostics (OBD) systems can provide real-time data about the alternator’s performance. By connecting an OBD scanner to the vehicle’s diagnostic port, you can read fault codes related to the alternator and monitor voltage levels directly from the vehicle’s computer system. This method is especially useful for those who prefer not to use a multimeter or want more detailed information.

Diagnosing Common Alternator Problems

Being able to identify smaller problems with your alternator before they develop into more serious issues can save you a lot of time and money in the future.

Recognising Voltage Regulator Issues

The voltage regulator, often integrated into the alternator, controls the amount of voltage sent to the battery. If the regulator fails, it can lead to overcharging or undercharging. Symptoms of a faulty regulator include fluctuating dashboard lights, a burning smell, or an excessively high or low voltage reading during testing.

Identifying Bearing Problems

Alternators contain bearings that allow the rotor to spin smoothly. Over time, these bearings can wear out, leading to a whining or grinding noise when the engine is running. If such noises are heard during testing, it may indicate that the alternator’s bearings are failing and need replacement.

Checking for Loose or Damaged Belts

The alternator is driven by a belt connected to the engine. A loose or damaged belt can cause the alternator to underperform. During testing, inspect the belt for signs of wear, such as cracks or fraying, and ensure it has the proper tension. A slipping belt can lead to a lower voltage output, mimicking alternator failure.

Maintenance Tips for a Healthy Alternator

There are a number of ways you can help to increase the longevity of your alternator and keep your vehicle on the road for longer.

Regular Inspections

Incorporating regular alternator inspections into your vehicle’s maintenance routine can prevent sudden failures. During routine services, ask your mechanic to check the alternator, voltage regulator, and related alternator parts. Early detection of issues can save time and money on repairs.

Keeping Connections Clean

Corroded or loose connections can lead to poor alternator performance. Periodically inspect the battery terminals and alternator connections for corrosion or looseness. Clean any corrosion with a wire brush and ensure all connections are tight and secure.

Avoiding Electrical Overloads

Modern vehicles come equipped with numerous electrical devices, but overloading the system with aftermarket accessories can strain the alternator. Being mindful of the electrical load can help extend the alternator’s lifespan. If you plan to install additional electronics, consider upgrading the alternator to handle the increased demand.

Alternators at Repco

Testing an alternator is an essential aspect of vehicle maintenance that can prevent unexpected breakdowns and costly repairs. Regular maintenance, combined with a solid understanding of how to test and interpret results, will ensure that the alternator remains in good working condition for the long haul, which will save you precious time and money.

If you need to replace your alternator, Repco offers a large selection of replacement alternators as well as alternator parts and tools for the job. Use our Rego Search to get the right fitment for your vehicle or head in-store to get direct help from our staff.