What does the alternator do?

Alternator Replacement Image

A car’s alternator makes electricity to power the spark plugs, fuel injection, computers, and various other systems (headlights, radio, heater motor, heated rear window etc), as well as charging the battery. Without it your car won’t run for long.

The alternator is driven by the serpentine belt (or sometimes its own alternator belt), which is tunred by the engine’s crankshaft. It turns mechanical energy into electrical energy in the form of an AC current, which has to be turned into DC current via diodes for use in the car’s electrical systems.

Every car is different, so find specific instructions in the manual...

When to change your alternator

“The headlights will dim and other car electrics may start to malfunction - you should see a red warning light or other message on your dashboard

Alternators can last more than 100,000 miles without an issue, especially in a car that does not have a lot of electrical accessories, like power windows, heated seats, or a high powered stereo. When the alternator stops charging (you should see a red warning light or other message on your dashboard) the battery will go flat quickly. The headlights will dim and other car electrics may start to malfunction. You probably only have a few dozen more miles before it stops running completely

When alternators stop charging it is typically a matter of burned out diodes (which convert the alternating current to direct current) or the wire windings inside can short out, resulting in a lower output than is needed to charge the battery. Even when nothing fails, the carbon brushes will wear down until they just no longer make contact. As with anything that spins, there is also a chance the bearings inside can wear out or get play in them, causing it to make noise for a while before failing completely.

All cars are slightly different, so if it is time to change your alternator, use our before you begin checklist, and find your car for specific instructions.

How to change your alternator

This sample video and instruction is typical for vehicles with serviceable filter. Check your Haynes manual for specific instructions for your vehicle.

A very brief summary of the job:

  1. Disconnect the negative battery cable
  2. Depending on the car, you may need to remove other parts to gain access to the alternator. Your Haynes manual will have details
  3. Loosen and remove the drive belt turning the alternator
  4. Disconnect the alternator’s electrical connections
  5. Remove the alternator mounting bolts
  6. Removing the alternator may require moving or removing other components 
  7. Make sure the new alternator matches the old one including the pulley
  8. Installation is the reverse of removal
  9. Reinstall the drive belt and reconnect the battery

Why you should change your alternator

Alternator Image

An alternator is crucial to a car’s operation, especially in a modern car where electronics such as processors govern so many systems. The computers that control the engine, transmission, safety systems, and infotainment need a supply of stable 12 volt power to run properly. Years ago, a car with a carburetor and mechanical fuel pump would run for hours with a dead alternator, but not today.

Without the electricity generated by the alternator, a modern car won’t run for long because the fuel won't pump and the computers will start to falter.

Pay close attention when you first start the car, and you may catch a flicker of the charging system warning light when the alternator first begins to go bad. You can find a more detail explanation of how and why they fail here - When Good Alternators Go Bad (and Why).

Since replacing an alternator requires removing the drivebelt, you should plan on changing that as well if it hasn't been done in the past 5 years. Also, if you are working on an older car, the voltage regulator may need replacing as well.

Before you begin

Tools you will need

Mainly basic tools are required for this job, although you may need to raise the car to remove the splash shield or inner fender.

  • Floor jack and jack stands
  • Wrench set
  • Ratchet and socket set
  • Screwdrivers

Parts you may need

  • Alternator
  • Drive belt (if needed)
  • Voltage regulator (if needed)