Alternator Going Bad Symptoms
Alternator going bad symptoms appear gradually and can leave you stranded if ignored. The alternator charges your battery while driving — when it starts failing, every mile drains the battery a little more until the engine stalls completely.
Can I Drive?
Briefly. A fully charged battery gives you 30–60 minutes of runtime once the alternator fails. Turn off AC, audio, and all accessories to extend that window. Drive directly to a shop — do not turn the engine off until you arrive.
Most Likely Causes
- 1
Worn alternator brushes or diodes
The most common alternator going bad symptom cause. Internal brushes and diodes wear out after 80,000–150,000 miles. Output drops gradually before failing completely — a load test will reveal this before total failure.
High electrical loads (subwoofers, aftermarket lighting) accelerate wear.
- 2
Slipping serpentine belt
The alternator is driven by the serpentine belt. A worn, glazed, or loose belt slips on the alternator pulley, reducing output. You may notice a squealing sound on acceleration — a classic alternator going bad symptom caused by belt slippage.
Inspect belt tension and condition. Replace every 60,000–100,000 miles.
- 3
Failed voltage regulator
The internal voltage regulator controls alternator output. A failing regulator causes overcharging (above 14.8V) or undercharging (below 13.5V). Either condition damages the battery and electronics.
On most modern alternators the regulator is internal — replace the alternator as a unit.
How to Diagnose It
- 1
With the engine running, measure voltage across the battery terminals with a multimeter. A healthy alternator outputs 13.8–14.4V. Below 13.5V = alternator going bad symptom confirmed.
- 2
Rev the engine to 2,000 RPM and watch the voltage. If it rises toward 14V at higher RPM but drops below 13V at idle, the alternator is producing low output.
- 3
Have the alternator load-tested at any auto parts store (free). A load test applies a controlled electrical demand — a failing alternator will drop below 12V under load.
How to Fix It
Worn alternator brushes or diodes
Alternator replacement is intermediate DIY. Disconnect the battery, remove the serpentine belt, unplug the electrical connectors, unbolt the alternator (usually 2–3 bolts), and install a remanufactured unit with a warranty.
Slipping serpentine belt
Serpentine belt replacement is beginner DIY. Use the belt routing diagram under the hood. Rotate the tensioner to release tension, slip off the old belt, route the new one. $20–$40 in parts.
Parts & Tools
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Mistakes to Avoid
- Never disconnect the battery while the engine is running to "test" the alternator — this can damage the ECU on modern vehicles.
- Don't replace the battery without testing the alternator — a failing alternator will kill a new battery within days.
- Don't ignore a whining or grinding noise from the alternator — bearing failure causes the pulley to seize and snap the serpentine belt.