How to Check If Your Alternator Is Bad

The fastest way to check an alternator is to measure battery voltage: about 12.6 V with the engine off and typically 13.8–14.5 V (sometimes 13.2–14.8 V on “smart” systems) with the engine running; if running voltage is low, wildly high, or unsteady—and AC ripple exceeds roughly 0.1–0.3 V—the alternator or its regulator/diodes is likely failing. Add a load test (lights, blower, rear defogger), inspect the belt and connections, and scan for charging-system fault codes to confirm.

Why the Alternator Matters

The alternator powers the vehicle’s electronics and recharges the battery once the engine is running. A weak or failed alternator can mimic a dead battery, cause dim lights and erratic electronics, and even stall the engine while driving. A careful check prevents unnecessary battery replacements and helps you avoid breakdowns.

Common Signs Your Alternator May Be Failing

Before grabbing tools, watch for typical symptoms that often point to a charging-system problem. These clues can focus your testing and save time.

• Battery/charging warning light (battery icon, “ALT,” or “GEN”) stays on or flickers.
• Headlights dim at idle and brighten with RPM; interior lights or screens flicker.
• Electrical accessories (blower, heated seats, audio) weaken or cut out, especially at idle.
• Engine stumbles, transmission shifts oddly, or vehicle stalls while driving.
• Repeated dead battery after driving, sulfur/burning smell, or whining/squealing from the belt area.
• After a jump-start, the car runs briefly then dies—classic sign of no charging.

These symptoms don’t prove an alternator is bad by themselves, but together—especially with a warning light—they strongly suggest a charging-system issue.

Tools You’ll Need

A few basic tools allow you to test most charging systems at home safely and accurately.

• Digital multimeter with DC and AC voltage ranges (and min/max if available).
• OBD-II scanner (for reading voltage-related codes and live data).
• Safety gear: eye protection and gloves; a flashlight for close inspection.
• Optional: clamp ammeter (for current draw), belt tension gauge, and contact cleaner.

Even an inexpensive multimeter is enough to diagnose most alternator problems, but a scanner adds helpful confirmation.

Step-by-Step Alternator Testing With a Multimeter

Follow these steps to determine whether the alternator, its regulator, or wiring is at fault. Work in a well-ventilated area and keep hands, hair, and clothing clear of moving parts.

1. Safety and setup. Set the multimeter to DC volts. Access the battery terminals. Do not disconnect the battery with the engine running—this can damage electronics.
2. Resting battery check (engine off). A fully charged battery should read about 12.6 V (AGM 12.7–12.8 V). Around 12.4 V ≈ 75% charge; 12.2 V ≈ 50%. If you see under ~12.2 V, charge or replace the battery before blaming the alternator.
3. Charging voltage at idle (engine on). Start the engine and measure at the battery posts:
   – Typical systems: about 13.8–14.5 V after 30–60 seconds.
   – Smart/temperature-compensated systems may range 13.2–14.8 V and briefly dip near 12.8–13.2 V during decel/fuel-saving modes.
   If you consistently see under ~13.2 V or over ~14.8–15.0 V (once stabilized), suspect the alternator or regulator.
4. Load test. Turn on headlights (high beam), blower on high, rear defogger, and heated seats. Voltage should generally stay above ~13.2 V at idle and recover above ~13.5 V when you raise engine speed to 1,500–2,000 RPM. If voltage is low at idle but normal at 2,000 RPM, check for belt slip or a weak alternator.
5. AC ripple test (diode health). Set meter to AC volts. Measure across the battery with the engine running and accessories on. Expect very low ripple: roughly 0.02–0.10 V AC is typical. Over ~0.3 V AC suggests a bad diode/rectifier; >0.5 V AC is a strong fail.
6. Voltage drop in cables (wiring check). With loads on, measure DC voltage from the alternator B+ post to the battery positive terminal; more than ~0.2 V drop indicates resistance in the cable, fusible link, or connections. Measure from the alternator case to the battery negative terminal; more than ~0.1–0.2 V suggests a poor ground.
7. Cranking voltage (battery vs. alternator). With the meter on DC, watch voltage while cranking a cold engine: it should stay above ~9.6 V at 70°F (21°C). A deeper drop points to a weak battery or high starter draw; fix battery issues before condemning the alternator.

Results recap: low or unstable charging voltage, excessive AC ripple, and normal battery performance together point to a failing alternator or regulator; high cable drops indicate wiring issues rather than the alternator itself.

Visual and Mechanical Checks

Mechanical problems can mimic a bad alternator. A quick inspection can uncover easy fixes.

• Belt condition and tension: Cracks, glazing, contamination, or looseness cause slip and low output, especially at idle. Listen for squeal on startup or with loads.
• Decoupler (one-way) pulley: A failed overrunning alternator pulley can chirp, vibrate, or reduce charging at idle.
• Connectors and grounds: Look for corrosion at the battery, alternator B+ terminal, grounds, and the engine-to-chassis ground strap.
• Fuses/fusible links: A blown fuse in the charging circuit or a failed fusible link will prevent charging even if the alternator spins.
• Smell and noise: Hot electronics smell, whining bearings, or grinding indicate internal alternator issues.
• Warning lights behavior: Battery light should illuminate with key-on/engine-off and go out after start. If it never lights, the bulb/circuit may be bad, affecting excitation on some systems.

If the belt drives multiple accessories, correcting tension or replacing a worn belt/pulley can restore normal charging without replacing the alternator.

Scan for Fault Codes and Live Data

Modern vehicles monitor charging and may log diagnostic trouble codes. A quick scan adds clarity, especially on vehicles with smart charging.

• Relevant codes: P0562 (System Voltage Low), P0563 (System Voltage High), and manufacturer-specific generator/alternator codes.
• Live data: Look for “System Voltage,” “Generator Command/Feedback,” battery temperature, and current. Discrepancies between commanded and actual voltage suggest alternator or control faults.
• Smart charging behavior: Expect controlled voltage dips or surges during deceleration, stop-start, or battery reconditioning. Correlate with loads and RPM before declaring a fault.

Codes plus abnormal live data make a strong case; normal data alongside low terminal voltage points back to wiring or mechanical issues.

Alternator vs. Battery vs. Parasitic Draw

Many no-starts are batteries, not alternators. Distinguish the root cause to avoid replacing the wrong part.

• Alternator likely: Vehicle dies while driving; runs only while jump-started; lights brighten with RPM but system voltage stays low.
• Battery likely: Slow cranking after sitting; passes charging test once running; cranking voltage dips below ~9.6 V; old battery (4–6+ years).
• Parasitic draw: Battery drains overnight even though charging voltage is normal while running. With engine off and modules asleep, key-off draw should typically be <50 mA (0.05 A); a failed alternator diode can cause higher key-off draw.
• Confirm with a full recharge: Charge the battery fully and retest. If charging numbers look good afterward but the battery won’t hold a charge, the battery is suspect.

Separating these scenarios prevents unnecessary alternator replacements and helps you fix the true cause of repeated dead batteries.

What Not to Do

A few old-school tricks are risky on modern vehicles and can cause expensive damage.

• Do not disconnect the battery with the engine running. Voltage spikes can fry ECUs and infotainment modules.
• Don’t short terminals or “spark test.” This risks fires and damages rectifiers and wiring.
• Don’t rely on a jump-start “holding” test alone. Always verify with a meter; some cars can run briefly on battery even with a failed alternator.

Using proper test methods protects you and the vehicle’s electronics while yielding reliable results.

When to Seek Professional Help—and Costs

If your tests point to alternator failure but you’re unsure, professionals can confirm quickly and safely.

• Parts-store bench test: Many retailers will test the alternator off the car for free.
• Shop diagnosis: Expect 0.5–1.0 hour labor for a charging-system test with load and ripple analysis.
• Replacement costs: Typical alternators range roughly $200–$700 for parts; labor can vary from 0.7 to 3+ hours depending on access. Some vehicles require reprogramming or battery registration after replacement.

Given labor and potential coding needs on newer cars, a definitive test can save time and money.

Special Cases: Hybrids, EVs, and Stop-Start Vehicles

Not all vehicles use traditional alternators; procedures vary accordingly.

• Hybrids/EVs: They use a DC–DC converter to charge the 12 V battery. Test procedures focus on converter output and high-voltage interlocks; follow factory guidelines.
• Stop-start systems: Often use AGM/EFB batteries, smart regulators, and intelligent battery sensors (IBS). Battery registration and software updates may be required after service.
• Temperature compensation: Cold weather can raise target charging voltage (up to ~14.8–15.0 V); hot conditions lower it. Interpret readings with ambient temperature in mind.

Knowing your vehicle’s specific system prevents misdiagnosis and ensures correct repair steps.

Summary

Check resting battery voltage, then confirm running voltage under idle and loaded conditions. Healthy systems hold roughly 13.8–14.5 V (or 13.2–14.8 V on smart systems) with minimal AC ripple (<0.1–0.3 V). Inspect belts, pulleys, fuses, and grounds; scan for voltage-related codes; and distinguish alternator faults from weak batteries or parasitic draws. When in doubt, get a bench test or professional diagnosis before replacing parts.

How do I test if my alternator is bad?

To test a potentially bad alternator, connect a multimeter to your car battery; with the engine off, it should read around 12.6 volts. Start the car, and the voltage should rise to 14.2 to 14.7 volts, indicating the alternator is charging the battery. If the voltage stays low (below 13.2v) with the engine running, or if it drops significantly when accessories are turned on, the alternator may be failing. 
This video demonstrates how to test an alternator with a multimeter: 1mClassic Car MaintenanceYouTube · Apr 1, 2024
Symptoms of a Bad Alternator
Before testing, look for signs of a faulty alternator, such as: 

• Dim or flickering headlights
• Difficulty starting the car after it has been running
• Slow or weak power accessories (power windows, locks, etc.)
• A warning light on the dashboard

Steps to Test Your Alternator

1. Gather your tools: You will need a multimeter set to DC volts (VDC) with a range above 15 volts. 
2. Check the battery voltage (engine off): Connect the multimeter’s red lead to the positive (+) battery terminal and the black lead to the negative (-) terminal. Note the reading, which should be around 12.6 volts for a healthy, charged battery. 
3. Start the engine: With the car running, check the multimeter reading again. 
   • Healthy Alternator: The voltage should increase to between 14.2 and 14.7 volts, indicating the alternator is charging the battery. 
   • Bad Alternator: If the voltage does not increase and stays near 12.6 volts, the alternator is not charging the battery. 
   • Overcharging Alternator: If the voltage goes above 14.7 volts, the alternator’s voltage regulator may be malfunctioning, which can damage the battery. 
4. Test under load: Turn on multiple accessories (headlights, radio, blower motor) while the engine is running. The voltage should ideally stay above 13 volts and not drop significantly. If it drops below 13 volts, the alternator may be failing. 
5. Inspect the alternator: Visually inspect the alternator and its wiring. Check the belt for tightness and ensure all connections are secure and free of corrosion. 

You can also watch this video to see how to perform the load test: 55sAaron HinesYouTube · Mar 23, 2018

How do I know if it’s the battery or the alternator?

To tell if the problem is the battery or the alternator, check the car’s battery voltage with the engine off and running; with the engine off, a reading below 12 volts indicates a bad battery, while a healthy engine-running voltage should be between 13.5 and 14.5 volts, and if it doesn’t rise, the alternator is failing. A battery will typically show signs like slow cranking or clicking when starting, whereas a failing alternator often leads to frequent stalling, dim or flickering lights, and a “battery” warning light on the dashboard.
 
Symptoms to Watch For
Bad Battery Symptoms:

• Clicking sound: When you turn the key, you hear a clicking noise but the engine doesn’t start. 
• Slow cranking: The engine turns over slowly. 
• Dim or no lights: Headlights, dashboard lights, and other electronics are dim or don’t work properly. 
• Swollen battery case: The plastic case of the battery may look bloated. 

Bad Alternator Symptoms:

• Frequent stalling: The car starts but then dies shortly after, even when driving. 
• Warning lights: A battery or check engine light illuminates on the dashboard. 
• Dim or bright lights: Headlights and other lights may flicker, dim, or become overly bright. 
• Strange noises: A whining, screeching, or growling sound may come from the engine. 
• Burning smell: A smell of burning rubber or wires can indicate a problem with the alternator. 

How to Test Your Car’s System

1. Check Battery Voltage (Engine Off): With the engine off, use a multimeter to test the battery’s voltage. A healthy battery should read over 12 volts. 
2. Check Voltage (Engine Running): Start the car. The battery voltage should rise to between 13.5 and 14.5 volts. 
3. Turn on Accessories: Turn on your car’s headlights, radio, and other accessories. The voltage may drop slightly for a moment but should quickly return to the 13.5-14.5 volt range. 
4. Interpret the Results:
   • If the voltage does NOT rise: with the engine running, the alternator is likely bad and not charging the battery. 
   • If the car still won’t start: after the battery is fully charged, even if the alternator is good, the battery itself is likely the issue. 
   • If the car starts but dies quickly, the alternator is failing to charge the battery. 

What are the signs of a faulty alternator?

Signs of a faulty alternator include a battery warning light, dim or flickering lights, dead batteries, strange noises (like whining or growling), a burning rubber or electrical smell, slow or malfunctioning electrical accessories, and trouble starting or frequent engine stalling. If you notice any of these symptoms, have your alternator checked by a professional, as a failing alternator can lead to being stranded.
 
Electrical Signs

• Battery Warning Light: Opens in new tabThe most direct sign is a warning light on your dashboard, often shaped like a battery, indicating a problem with the charging system. 
• Dim or Flickering Lights: Opens in new tabHeadlights, dashboard lights, and interior lights may appear dim, flicker, or change intensity with engine RPMs. 
• Electrical System Malfunctions: Opens in new tabPower windows, the radio, air conditioning, or the entertainment system may work slowly or stop functioning correctly. 
• Frequent Dead Batteries: Opens in new tabThe alternator may not be providing enough charge to the battery, leading to frequent dead batteries. 

Auditory & Olfactory Signs 

• Strange Noises: A grinding, whining, or growling sound coming from under the hood can indicate a failing alternator or a worn or misaligned belt turning the alternator.
• Burning Smell: An alternator overheating can produce a smell of burning rubber or wires.

Performance & Driving Issues 

• Difficulty Starting or Stalling: The engine may struggle to start, or the car could stall while driving because the alternator isn’t supplying enough power.
• Sluggish Accessories: Electrical accessories like power windows and wipers may move slowly due to insufficient power.

What to Do
If you suspect a bad alternator, it’s best to have it checked by a professional mechanic. They can test the alternator’s output with a voltmeter and determine if the issue is with the alternator, the belt, or the battery.

How can I test my alternator without a tester?

To test an alternator without tools, observe your car’s electrical system: if headlights, interior lights, or dashboard gauges are dim, flickering, or erratic, the alternator may be failing. You can also try a risky method by disconnecting the negative battery cable with the engine running; if the engine stalls, the alternator is bad. However, this method can damage modern vehicles and should only be used in an emergency, as the safest method is to have a tool-equipped technician test the charging system. 
Symptoms of a Failing Alternator
Before attempting a test, watch for these signs: 

• Dim or Flickering Lights: Headlights, interior lights, and dashboard lights may appear dim or flicker when the alternator isn’t producing enough power.
• Battery Warning Light: The battery or charging system warning light on your dashboard often indicates an alternator problem.
• Strange Noises: A whining or grinding sound from the engine can be a sign of a worn-out alternator bearing.
• Burning Smell: An electrical burning smell could signal an internal electrical issue within the alternator.

The Risky Battery Disconnect Test (Use with Caution)
This method is not recommended for newer cars with complex electronics, but can be used in an emergency: 

1. Start the Engine: Ensure the car is running at idle. 
2. Disconnect the Negative Terminal: With the engine still running, carefully loosen and remove the negative (black) battery cable from the battery terminal. 
3. Check the Engine: 
   • If the engine stalls: The alternator is failing to produce enough power to run the engine by itself. 
   • If the engine continues to run: The alternator is still charging and working properly. 
4. Re-Secure the Cable: Promptly reconnect the negative battery cable to the battery terminal. 

Why This Test is Risky

• Damage to Electronics: In a modern car, disconnecting the battery with the engine running can overload the voltage regulator and fry expensive electronic components. 
• Not Foolproof: Some older alternators or vehicles with less complex systems might not be damaged by this test, but it’s a dangerous gamble. 

Professional Testing

• Auto Parts Stores: Many auto parts stores offer free charging system testing. 
• Professional Diagnostics: A mechanic can perform a proper and safe test with a voltmeter to accurately diagnose your alternator’s health.