Alternators

The alternator generates the electricity that runs the vehicle while the engine is running and recharges the battery. Without it the vehicle runs only until the battery is flat, usually within an hour or two.

What it does

Driven by the ancillary belt, the alternator produces alternating current which internal diodes rectify to direct current. A voltage regulator holds output at roughly 14 volts under varying load and engine speed.

Signs a alternator is failing

A battery warning light while driving
Dimming headlights, especially at idle
A flat battery that returns after charging
A whining or grinding noise that follows engine speed
A burning smell from the drive belt area

When to replace them

Not a scheduled item. Replaced on failure, most often worn brushes, failed diodes, or a collapsed bearing.

What to confirm before ordering

Have these ready and the counter can usually confirm the part in under a minute.

Output rating in amps, from the label or case stamping
Pulley type, whether solid or a one-way clutch pulley
Mounting configuration and plug type
Whether the vehicle has a smart charging system managed by the ECU

Alternators by vehicle

Pick your vehicle for the years and engines covered, and what is specific to that application.

Alternators questions

How do I test whether the alternator is charging?

Measure battery voltage with the engine running. Roughly 13.8 to 14.4 volts indicates it is charging. A reading at or below resting battery voltage of about 12.6 volts means it is not.

Why does the alternator amp rating matter?

It must meet the vehicle’s electrical load. Fitting a lower-rated unit than specified leaves the battery undercharged whenever demand is high, which shortens battery life and creates intermittent faults.

Can a noisy alternator just need a bearing?

Frequently, yes. A whine or growl that follows engine speed is often the bearing rather than the windings, and a bearing costs a fraction of a replacement unit.