To test an alternator, set a multimeter to DC volts and measure the battery voltage with the engine off (about 12.4–12.7 volts), then with the engine running (roughly 13.5–14.5 volts), then again with the lights, blower and rear heater on; a running reading that stays near the resting battery voltage, or that falls below about 13 volts under load, points to a charging fault. It builds on the quick diagnosis in is it the battery or the alternator, and the figures are explained in normal car battery voltage.
What You Need
- A digital multimeter with a DC volts range of 20 V, and an AC volts setting for the diode check.
- A fully charged or healthy battery. If the battery is flat, charge it or jump it first — see how to jump-start a car safely — otherwise the figures mislead.
- Eye protection and gloves, and a clear, level working area.
With the engine running, keep the meter leads, your hands, hair and clothing away from the belt, pulleys and cooling fan. Clip the leads on before starting the engine, and never disconnect the battery to ‘see if the engine keeps running’ — it can damage the electronics.
Step by Step
- Inspect first. Check the drive belt is present and tight, and the battery terminals are clean — see serpentine belt inspection and how to tell if your car battery terminals need cleaning.
- Resting voltage. With the engine off for at least an hour (or after switching the headlights on for a minute to clear any surface charge), put the red lead on the positive terminal and the black on the negative. A healthy, charged battery reads about 12.4–12.7 volts.
- Running voltage at idle. Start the engine and measure again. Healthy charging is typically around 13.5–14.5 volts.
- Running voltage at about 2,000 rpm. Hold the engine at a steady 2,000 rpm for a minute. The reading should stay in the same range, and not climb above about 15 volts.
- Load test. Switch on the headlights, blower on high and the rear window heater. The voltage should stay above roughly 13 volts, and recover when you rev slightly.
- Diode (ripple) check. Switch the meter to AC volts and measure across the battery with the engine running. More than about 0.5 volts AC suggests failing alternator diodes.
Reading the Results
| Reading with engine running | What it suggests |
|---|---|
| 13.5 – 14.5 V, steady under load | Charging system healthy |
| Near the resting battery voltage (about 12.5 V) or lower | Alternator not charging — check belt, fuse, wiring, then the alternator |
| Falls below about 13 V under load | Alternator weak or belt slipping |
| Above about 15 V | Overcharging — faulty regulator; can damage the battery |
| High AC ripple (over about 0.5 V AC) | Failing diodes in the alternator |
| Good running voltage, but the battery goes flat overnight | Parasitic drain or a worn-out battery — see battery keeps dying overnight |
Symptoms That Point to a Charging Fault
- A battery warning light — see battery light on while driving.
- Dim or flickering headlights — see headlights dim or flicker while driving.
- Electrical gremlins, slow windows, a dying stereo.
- A battery that keeps going flat — and a new battery that fails the same way. See how often to replace your car battery and how to read a car battery date code.
- Stalling once the battery is drained — see car stalls while driving.
Possible Costs
| Item | Typical cost |
|---|---|
| Digital multimeter | $15 – $40 |
| Free charging-system test at many parts stores | $0 |
| Drive belt or tensioner | $80 – $350 |
| Replacement alternator | $350 – $900 |
Can I Keep Driving With a Weak Alternator?
Only for short trips, close to home, and with non-essential loads switched off. A failing alternator runs the battery down, and when it is empty the engine can cut out — see our battery light guide. If you can’t have it tested promptly, don’t take the car on a long journey.
When to See a Mechanic
If your readings are outside the ranges above, the belt and wiring look fine, or you’re not comfortable working near a running engine. Many parts stores test charging systems for free. More in our electrical section and maintenance section.
Frequently Asked Questions
What voltage should an alternator put out?
Roughly 13.5–14.5 volts with the engine running. Much lower means it isn’t charging; above about 15 volts suggests overcharging.
How do I test an alternator with a multimeter?
Measure battery voltage engine-off, then running at idle and at about 2,000 rpm, then with lights, blower and rear heater on, and compare with the expected ranges.
Is it safe to disconnect the battery to test the alternator?
No. It’s an old trick that can damage modern electronics. Test with the battery connected.
What does high AC voltage across the battery mean?
More than about 0.5 volts AC with the engine running suggests failing diodes in the alternator.
How much does it cost to replace an alternator?
Typically $350–$900, depending on the car.
