How to Test an Alternator

Summary

In this video, ChrisFix walks viewers through how to test an alternator to ensure it's functioning properly. He covers common symptoms of a bad alternator or battery, such as weak lights and slow cranking. Using a multimeter, he demonstrates how to check the voltage across the battery and alternator, highlighting the ideal voltage range. ChrisFix also explains how to perform a voltage drop test, check connections, and inspect the alternator, wiring, and belt. He offers practical tips to diagnose issues and maintain a healthy alternator, helping viewers avoid expensive repairs.

Takeaways

• 😀 Test your alternator by setting your multimeter to DC volts, above 15V (e.g., 20V).
• 😀 A fully charged battery should read around 12.6V when the car is off.
• 😀 When the engine is running, the alternator should charge the battery between 14.2V and 14.7V.
• 😀 Voltages higher than 14.7V indicate overcharging, while lower than 14.2V suggests weak charging.
• 😀 When turning on accessories (headlights, radio, off-road lights), ensure the voltage doesn’t drop below 13V.
• 😀 After turning off accessories, the voltage should rise slightly, typically around 12.9V.
• 😀 Ensure proper connections at the battery terminals, alternator, and voltage regulator for accurate readings.
• 😀 If the voltage drops below 13V, check the wiring for corrosion, frays, or loose connections.
• 😀 Perform a voltage drop test to check the ground connections (e.g., negative terminal to alternator case) for resistance.
• 😀 A healthy alternator connection should show less than 0.1V drop during testing, with no significant drops at the alternator bracket or engine block.

Q & A

What are some common symptoms of a bad alternator or battery?

Some common symptoms of a bad alternator or battery include the battery light being on, the engine cranking slowly or not at all, weak interior lights or headlights, and rumbling or squeaking noises coming from the alternator indicating a potential bad bearing.

How do you set up the multimeter to test the alternator?

You need to set your multimeter to DC voltage (DCV) and set it to a value above 15, ideally 20 volts, in order to properly test the alternator.

What voltage should the battery show when the car is off?

The battery should show approximately 12.6 volts when the car is off. This is the target voltage for a healthy, fully charged battery.

What voltage range indicates the alternator is working correctly?

The alternator is working correctly if the voltage ranges between 14.2 and 14.7 volts while the engine is running. This means the alternator is charging the battery properly.

What should you do if the alternator is overcharging the battery?

If the voltage is over 14.7 volts, the alternator is overcharging the battery, which could cause damage. You should have the alternator inspected or replaced.

What is the significance of the voltage dropping below 13 volts when accessories are on?

If the voltage drops below 13 volts with accessories on, it indicates that the alternator is not charging the battery sufficiently, which can lead to battery depletion over time.

How do you perform a voltage drop test on the battery's negative side?

To test the voltage drop on the negative side, connect the black lead to the battery's negative terminal and the red lead to the alternator case. Then, raise the engine RPMs to around 1500 and check the voltage reading. A reading of 0.05 volts is acceptable.

What should you do if you find a substantial voltage drop when testing the alternator's ground connection?

If you find a substantial voltage drop, check the connection between the alternator bracket and the engine block. Clean any corrosion or debris from the connection to ensure a good ground connection.

What is the recommended procedure if a voltage drop is found on the positive side of the battery during the test?

If a voltage drop is detected on the positive side, check the connection at the nut and stud on the alternator. Clean the connection if necessary. Then check the wire connection to the alternator and clean any bad contacts.

What is the importance of the alternator belt in the charging system?

The alternator belt is crucial as it drives the alternator, allowing it to generate electricity. A loose or worn belt can prevent the alternator from charging the battery efficiently, leading to a malfunctioning electrical system.