How to Replace an Alternator

A working alternator is crucial to your car’s operation. As the producer of your car’s electricity — and as the mechanism that helps the battery hold its charge — it has a hand in everything from radio use to the car’s ability to start. When an alternator falters, your power comes directly from the battery instead, which isn’t engineered to provide power for an extended period of time. This means that if you wait too long to replace a broken alternator, you run the risk of being left stranded.

There are many signs of a bad alternator that may require you to replace it. Here are some of the most prominent indicators to look out for[1][2]:

  • Headlights dimming and brightening intermittently
  • “Low battery” warning light appearing on the dashboard
  • Difficulty starting the car
  • Car battery dying frequently
  • Growling, grinding noise
  • Strong smell of burning rubber

Fortunately, when it comes to car maintenance, changing your alternator is a relatively simple undertaking. Typically, alternators take about three hours to install and cost $130 to $250, depending on the quality and type you need.[3] Keep reading to learn all the details on how to test and how to replace an alternator.

How to test an alternator

Before you start digging too far under the hood, you’ll want to test your alternator to be sure it’s the true source of your car’s power problems. The best way to test an alternator is with a multimeter. Set the multimeter to the DC voltage setting, then follow these steps on how to test an alternator[1]:

  1. Make sure your battery is fully charged. With the vehicle at rest, check the battery’s voltage by touching the multimeter’s leads to their corresponding terminals on the battery (typically, the black lead is negative, the red lead is positive). A fully charged battery should read between 12.6 and 13.2 volts. If it’s at 12 or below, recharge your battery before continuing.
  2. Start the engine and check the battery’s voltage again. At idle, the voltage should be between 14.2 and 14.7 volts. If it reads 15 or higher, your alternator is providing too much power to the battery.
  3. Check the voltage after turning on car lights and electronics. If the voltage is in the proper range, check the battery’s voltage output again with some electrical accessories turned on, such as the headlights or radio. The voltage may start to dip here, but if it goes lower than 13, the alternator is not providing enough charge to the battery.
  4. If the voltage is below 13 volts. Ensure all connections between the battery and the alternator are in good condition — look for any possible kinks or corrosion. If all of the connections appear satisfactory, rev the engine to 1,500 rpm to see if the voltage output increases. If there’s no change, your alternator is faulty.

If you find that your current alternator is delivering too much or too little power to the battery, it’s probably time for a new alternator.

What tools are needed to replace an alternator

Now that you’ve determined that your alternator needs to be replaced, it’s time to collect the necessary equipment for the job. Here are the tools and parts you’ll want to have ready[4]:

  • Socket and ratchet set
  • Wrench set
  • Belt tensioner tool
  • Safety gear
  • Memory saver, if needed
  • New alternator

Replacing an alternator

With all of your gear prepared, you can finally get started. Here’s how to change an alternator[4][5]:

  1. If your vehicle requires it, use a memory saver to back up its preset electronic data. See your car’s user manual for more information.
  2. Disconnect the negative battery cable.
  3. Find the alternator. They’re normally mounted to the front or the side of the engine.
  4. Loosen the serpentine belt by springing back the belt tensioner with a belt tensioner tool. Pay close attention to how the belt is connected to the other components, because you’ll have to reroute the belt around all of the engine’s pulleys when you’re finished. Review the belt-routing diagram for your vehicle if possible.
  5. Remove the belt and scan it for any wear or tear, such as cracks or frays. Also check the belt tensioner to ensure that its roller still spins easily. If either component seems worn out, replace it.
  6. Disconnect any connectors or wiring from the alternator.
  7. Unbolt and remove the alternator. You may have to delicately maneuver it around other parts of your vehicle.
  8. Compare your old alternator to the new one. Make sure all of the connection ports and bolt holes match. If anything seems out of line, return the new alternator and get the correct replacement.
  9. Install the new alternator and reattach any connections.
  10. Reinstall the belt, shifting the belt tensioner as needed. Once the belt is back in place, check its tension; it should have about ½ inch of deflection.
  11. Reconnect the negative battery terminal.

After your new alternator is installed, it’s a good idea to test it with a multimeter to see if it’s working as it should. If the voltage output is in the right range, you’re ready to hit the road.

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