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Fix a car alternator that will not charge

We'll confirm the symptom with a multimeter, check belt tension and terminals, then isolate the cause—or tell you when to call a mechanic for alternator rebuild or replacement.

Troubleshooting · Home appliances
15–30 min
What you'll need
  • Multimeter (DC volt setting)
  • Wire brush or terminal cleaner (optional)
  • Replacement belt (if loose or worn)
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Steps

Goal: Confirm the alternator is not charging, then isolate the cause—belt, terminals, or internal fault.

  • Run the engine and check the battery light. If it stays on, the alternator is not charging. Or test battery voltage with a multimeter: engine running should show 13.5–14.5 V.
  • Good: Battery light on or voltage below 13.2 V when running—alternator fault. Proceed to Check voltage.
  • Bad: Battery light off and voltage 13.5–14.5 V—alternator is charging; if the battery dies, check the battery or parasitic drain.

Check voltage

Goal: Confirm the alternator is not charging with a multimeter.

  • With the engine off, test battery voltage at the terminals. A healthy battery reads 12.4–12.6 V. Below 12.2 V means charge the battery first.
  • Start the engine. Test voltage at the battery again. Charging voltage should be 13.5–14.5 V. Below 13.2 V means the alternator is not charging. Above 15 V means overcharging—call a mechanic.
  • Good: Voltage below 13.2 V when running—alternator not charging. Proceed to Check belt and terminals.
  • Bad: Voltage 13.5–14.5 V—alternator is charging. If the battery still dies, check the battery.

Check belt and terminals

Goal: Rule out belt tension and terminal corrosion before assuming alternator failure.

  • Check the drive belt. Press the longest span between pulleys—it should deflect about 1/2 inch (12 mm). If it deflects more, the belt is loose. Check for cracks, glazing, or fraying. A loose belt slips and does not spin the alternator properly.
  • Inspect the battery terminals and the wire connections at the alternator. Corrosion (white or green buildup) increases resistance. Disconnect the battery negative first, clean the terminals, then reconnect. Check the alternator output terminal (B+ or BAT) and ground for corrosion.
  • Good: Belt tight and terminals clean. If voltage still low when running, the alternator is faulty internally. See When to get help.
  • Bad: Belt loose or terminals corroded—tighten or replace the belt, clean the terminals, then retest.

When to get help

Call a mechanic if:

  • The belt and terminals are good but voltage stays below 13.5 V when running.
  • The alternator makes grinding or growling noise (bearings failing).
  • You are not comfortable working with electrical components.

Alternator rebuild or replacement (worn brushes, bad diode, faulty voltage regulator) is a common repair.

Verification

  • Battery voltage with engine running is 13.5–14.5 V.
  • The battery light is off when the engine is running.
  • The drive belt is tight and shows no cracks or glazing.
  • Battery and alternator terminals are clean and tight.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the battery light is on or voltage is low when running.
  2. Voltage test Test battery voltage with engine running—expect 13.5–14.5 V.
  3. Belt and terminals Check belt tension and condition; clean terminals and connections.
  4. Alternator output If belt and terminals are good but voltage stays low, alternator is faulty.
  5. Call a pro Alternator rebuild or replacement—call a mechanic.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Battery voltage with engine off and running
  • Belt tension and condition
  • Terminal and connection condition
  • Steps already tried

Is the battery light on or charging voltage low?

Run the engine. Check the battery light or test voltage at the battery with a multimeter.

Run the engine. If the battery light stays on, the alternator is not charging. Or test with a multimeter: battery voltage with engine running should be 13.5–14.5 V. Below 13.2 V means the alternator is not charging. Good: battery light on or voltage low—alternator fault. Bad: battery light off and voltage 13.5–14.5 V—alternator is charging; if the battery dies, check the battery or parasitic drain.

Yes No

You can change your answer later.

Is battery voltage 13.5–14.5 V with engine running?

Test at the battery terminals with a multimeter on DC volts. Engine running.

Start the engine. With the multimeter on DC volts, touch red to positive and black to negative terminals. 13.5–14.5 V: alternator is charging—if the battery still dies, check the battery or parasitic drain. Below 13.2 V: alternator is not charging—proceed to belt and terminals. Above 15 V: overcharging—call a mechanic. Verify you should see 13.5–14.5 V when charging.

Yes (charging) No (not charging)

You can change your answer later.

Battery or alternator wiring

If you see 13.5–14.5 V when running, the alternator is charging. If the battery still goes dead, check the battery or parasitic drain.

Done

Is the belt tight and the terminals clean?

A loose belt or corroded terminals can prevent charging.

Check the drive belt. Press the longest span—it should deflect about 1/2 inch. If loose, tighten or replace. Check battery and alternator terminals for corrosion. Clean tight connections. Good: belt tight and terminals clean. Bad: belt loose or terminals corroded—fix those first.

Yes No

You can change your answer later.

Fix belt and terminals, then test

Tighten or replace the belt. Clean the battery and alternator terminals. Reconnect and test voltage with engine running. If you now see 13.5–14.5 V, the fault was belt or terminals. If still low, the alternator is faulty—call a mechanic.

Done

Alternator faulty internally

Belt and terminals are good but voltage stays low. Worn brushes, bad diode, or faulty voltage regulator.

If the belt is tight, terminals are clean, and voltage at the battery stays below 13.5 V when running, the alternator is faulty internally. Worn brushes, a bad diode, or a faulty voltage regulator require alternator rebuild or replacement. Call a mechanic. If you hear grinding or growling from the alternator, the bearings may be failing—call a mechanic.

Done

Call a mechanic or check battery

If the battery light is off and voltage is 13.5–14.5 V when running, the alternator is charging. If the battery still goes dead, check the battery or parasitic drain. Call a mechanic if you are not comfortable with electrical work.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a car alternator not charge?
Common causes: loose or slipping drive belt, corroded terminals, worn brushes, bad diode, or faulty voltage regulator. Check belt tension and terminals first; internal faults often need a pro.
Can I fix a car alternator that will not charge myself?
Yes, for belt tension, terminal cleaning, and connection checks. Alternator rebuild or replacement (brushes, diode, voltage regulator) usually requires a mechanic or specialist.
When should I call a mechanic for an alternator that will not charge?
Call a mechanic if the belt and terminals are good but voltage stays below 13.5 V when running, the alternator makes grinding or growling noise, or you are not comfortable working with electrical components.

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