How To Replace the Alternator in Your Car

Updated: Feb. 01, 2024

Here's how to tell if your car needs a new alternator and how to install one yourself.

Next Project

1 to 3 hours




$200 to $350


If your car or truck won't hold a charge, you probably need a new alternator. Learn how to test it and how to replace it yourself.

Tools Required

  • Car battery charger
  • Digital multimeter
  • Rags
  • Socket/ratchet set
  • Wrench set

Materials Required

  • Alternator
  • Disposable gloves

What does an alternator do?

Think of your car’s alternator as a small but mighty electric generator. It charges the car battery and runs a vehicle’s electrical system while the engine is running. It’s one of three essential components of a vehicle’s electrical system, along with the battery and voltage regulator.

How does an alternator work?

Located near the front of the engine, your car’s serpentine belt wraps around a pulley attached to the alternator. As the engine runs, the alternator takes the mechanical energy generated by the serpentine belt and turns it into electrical energy.

The pulley turns a rotor shaft inside the alternator that spins magnets. This create an alternating current, which is then turned into direct current by diodes within. That direct current powers the electrical systems in your car when it’s running.

Signs of a bad alternator

If the charging light is lit on your dashboard or your battery won’t stay charged, chances are you’ve got a bum alternator. (For some cars, the check engine light might pop on if the alternator is failing.) Other signs of a bad alternator include dim or flickering headlights, or if your car dies after a jump start.

How much does it cost to replace an alternator?

It runs between $400 and $600, depending on the make and model of the vehicle, according to AAA. However, it’s not unusual for the job to cost more than $1,000.

What to know about buying an alternator

Instead of selling only to the carmakers, some manufacturers now sell new alternators through online stores. In many cases, you can buy a new alternator with a lifetime warranty for 30 percent less than the cost of a rebuilt one. So if you can live without your car for a couple of days, buy online.

How long do alternators last?

Alternators typically last between seven and 10 years, sometimes longer. The most common reasons alternators fail include damaged parts within, overuse, or engine oil or power steering fluid leaking into it.

Can I replace the alternator myself?

Yes. Replacing an alternator isn’t a complex task, but it does require a basic understanding of car mechanics. If you’re not comfortable doing it yourself, it’s best to take it to a pro to make sure it’s done correctly.

Even if you’re not confident replacing your alternator yourself, it’s a good idea to test it before taking it to a mechanic.

Project step-by-step (4)

Step 1

How to test an alternator

You’ll need a digital multimeter to perform the test. You’ll probably need a battery charger to bring the battery up to full charge.

  • Connect the multimeter to the battery, and set the multimeter knob to DC volts (20 or less).
  • Then touch the red lead to the positive battery post and the black lead to the negative post. Note the voltage reading.

How To Test An AlternatorTMB STUDIO

Step 2

Find the state-of-charge

Pro tip: You may need to charge the battery before testing the alternator if the battery is less than 50%.

  • Start by testing the battery’s engine-off voltage. Then refer to the battery voltage chart below to find your battery’s “state-of-charge.” If the battery is less than 50 percent charged, attach a battery charger to boost it to 100 percent before testing the alternator.
  • After you remove the battery charger, turn on the headlights (engine off) for four minutes. Then shut off the lights, start the engine and test the battery voltage again.
  • If the alternator is good, you’ll get a voltage reading of 13.5 to 14.5 volts. If it’s not that high, turn on the headlights and the blower motor and raise the engine speed to about 2,000 revolutions per minute (rpm). If the reading still doesn’t hit the mark, let the engine run for five minutes and repeat the test. If it fails this time, you need to replace the alternator.
    • If the alternator does produce the correct voltage, move on to the diode test.
  • The diodes are the electronic part of the alternator that convert AC voltage to DC. Switch your multimeter to the lowest AC setting and reattach the test leads to the battery. With the engine running, you shouldn’t see any AC voltage. If you do, you’ve got a bad diode and you need a new alternator.
Battery Voltage and Temperature tells State-Of-Charge
80° 60° 30°
100% 12.65 12.63 12.59 12.51
75% 12.45 12.43 12.39 12.32
50% 12.24 12.22 12.18 12.10
25% 12.06 12.04 11.99 11.92
0% 11.89 11.87 11.82 11.75
Step 3

How to Replace an Alternator

Disconnect the cables from the alternator

  • Start the alternator swap-out by disconnecting both battery cables from the battery. Do this carefully and in the proper order. Then remove the wires and cables from the back of the alternator.
  • Depress the latch clip on the electrical connector going to the voltage regulator and wiggle it out.
  • Then loosen the locknut to the “BAT” cable and remove the ring terminal.

Disconnect The Cables From The AlternatorTMB STUDIO

Step 4

Remove the alternator and install the new one

  • Rotate the belt tensioner (if equipped) or loosen the tensioning bolt near the alternator. Then slide the belt off the alternator pulley.
    • Pro tip: The bolts are really long and you’ll probably be cranking for a long time. If you have an air-powered ratchet, or access to one, this is a good time to use it.
    • The bolts are usually different lengths, so note where each bolt came from.
  • With the bolts removed, lift out the old alternator and drop in the new one. Reverse the procedure to reinstall.
    • Pro tip: While you’ve got the alternator off, examine your car’s serpentine belt. Look for cracks or fraying on both sides of the belt. Also check for glazed or slick spots on the edges, which can lead to the belt slipping.

Remove The Alternator And Install The NewTMB STUDIO