How to Check if an Alternator Is Putting Out

The fastest way to confirm your alternator is producing power is to measure battery voltage with a multimeter: expect roughly 12.6 V with the engine off and 13.8–14.7 V (often 13.5–15.0 V on modern smart-charging systems) with the engine running; if running voltage does not rise above the engine-off reading, the alternator likely isn’t charging. Below is a clear, step-by-step guide—including deeper diagnostics for newer vehicles with variable charging—to verify alternator output accurately and safely.

Tools and Safety

You don’t need specialty equipment to perform basic checks, but having the right tools improves accuracy and safety. The following items help you test output and isolate faults.

• Digital multimeter (with DC volts, AC millivolts, and ideally min/max capture)
• Clamp ammeter for DC (optional but useful to see alternator current)
• Scan tool or OBD-II reader (helpful for smart-charging systems and codes)
• Protective gloves and eye protection
• Service manual or wiring diagram for your vehicle (recommended)

With these tools, you can safely determine whether the alternator, wiring, or control system is responsible for charging issues.

Quick Driveway Test with a Multimeter

This basic procedure verifies whether the alternator raises system voltage as it should and how it responds to electrical loads.

1. Engine off: Measure battery voltage across the terminals after the car has sat 30+ minutes. A healthy, fully charged battery reads about 12.6–12.8 V; 12.4 V is ~75% charge; 12.2 V is ~50%; 12.0 V or less is discharged.
2. Start the engine and let it idle: Measure at the battery again. Typical charging voltage is 13.8–14.7 V once stabilized. Many late-model vehicles with “smart” charging may vary between ~12.5 and 15.0 V depending on temperature, load, and battery state.
3. Add electrical load: Turn on headlights, rear defroster, blower on high, and seat heaters. Voltage should generally remain above ~13.5 V on conventional systems; smart systems may dip briefly then recover. If voltage falls toward battery-only levels (below ~13.0 V) and stays there, output is suspect.
4. Rev lightly to 1,500–2,000 rpm: If voltage rises meaningfully from a low idle reading, you may have a weak alternator, slipping belt, or an underdrive situation at idle.
5. Check for overcharge: If voltage exceeds ~15.2 V warm (lead-acid), the regulator or sensing circuit may be faulty, risking battery damage.

If the running voltage is clearly higher than engine-off voltage and remains stable under load, the alternator is generally “putting out.” If not, continue with deeper checks below.

Interpreting the Readings

As a rule of thumb, a stable 13.6–14.8 V at warm idle indicates healthy charging on most vehicles; modern systems can legitimately target lower or higher numbers transiently. The key is that system voltage should rise above static battery voltage and respond to load and rpm changes in a controlled way.

Deeper Diagnostics for Certainty

Load and Ripple Tests

To distinguish a weak alternator from a wiring or diode issue, look at performance under load and AC ripple (alternators create AC that’s rectified to DC; bad diodes increase ripple).

1. With the engine running and major accessories on, note voltage stability at the battery.
2. Switch your multimeter to AC millivolts and measure at the battery. Healthy ripple is typically under ~50–100 mV AC; >300 mV AC often points to failed diodes or poor connections. Note: many basic meters read ripple imperfectly; use this as a trend, not an absolute.
3. If voltage sags excessively with load and ripple is high, suspect internal alternator faults.

Stable DC voltage under load with low ripple indicates the alternator and rectifier are functioning properly.

Voltage-Drop Testing (Cables, Grounds, and Connections)

If running voltage at the alternator is good but low at the battery, the issue may be wiring, the battery negative or positive cables, or a fuse link. Voltage-drop tests find hidden resistance.

1. Positive side drop: With engine running and loads on, place the meter leads from the alternator B+ post to the battery positive post (meter on DC volts). A reading above ~0.2 V indicates excessive resistance in the positive path (cable, fuse link, or connections).
2. Negative side drop: Place leads from the alternator case (good metal ground) to the battery negative post. More than ~0.2 V indicates poor grounds or corroded connections.
3. Total system drop across both sides under load should typically stay under ~0.5 V.

Excessive voltage drop means your alternator may be fine, but power isn’t reaching the battery efficiently due to wiring or ground issues.

Measure Alternator Output Current (If You Have a Clamp Meter)

Current confirms what voltage alone can’t: how much the alternator is actually supplying under demand.

1. Clamp around the alternator output cable with the engine running.
2. Turn on high loads and observe amperage. At idle, many alternators output a fraction of their rating; output should rise with rpm.
3. If current remains low despite low battery voltage and high load, the alternator may be weak or the belt is slipping.

A healthy alternator will increase current as loads rise and as rpm increases, within its design limits.

Smart-Charging Systems: What’s Different

Since the mid-2010s, many vehicles use ECU-controlled alternators (via LIN/BSS or similar). These systems vary voltage intentionally—sometimes running as low as ~12.5–13.0 V to reduce fuel use, then spiking to 14.8–15.0 V for battery maintenance or coasting charge. A low reading at idle isn’t automatically a failure if the system responds to load or driving conditions. If in doubt, check “desired generator voltage” or duty cycle with a scan tool; the commanded value should correlate with your meter.

Scan Tool Checks and Common Trouble Codes

Electronic diagnostics can confirm whether the control system is requesting charge and whether faults are stored.

• Look for codes like P0562 (System Voltage Low), P0563 (System Voltage High), P0620/P0622 (Generator Control Circuit), or P2503 (Charging System Voltage Low).
• Check data for alternator duty cycle/field command and battery current/voltage from the Intelligent Battery Sensor (IBS), if equipped.
• If the ECU isn’t commanding charge, the issue may be software, a failed control wire/LIN line, or a faulty IBS rather than the alternator itself.

If the ECU reports normal commands but voltage stays low, the alternator or wiring is more likely at fault.

Don’t Overlook the Basics

Several simple issues can masquerade as alternator failure.

• Battery condition: A sulfated or shorted battery can drag system voltage down; load-test or swap in a known-good battery for confirmation.
• Belt and pulley: A loose or glazed serpentine belt, weak tensioner, or decoupler pulley failure can cause low output or chirping/squeal.
• Fuses/fusible links: Many cars protect the alternator B+ with a high-amperage fuse; a blown or cracked link will block charge to the battery.
• Ground straps: Corroded or broken engine-to-chassis grounds create charging and starting issues.

Verifying these fundamentals often resolves charging complaints without replacing the alternator.

When to Bench-Test or Replace

If on-vehicle tests suggest alternator failure, parts stores can bench-test units. Replacement is warranted when the alternator cannot maintain voltage under load, exhibits excessive ripple, or fails to respond to proper ECU commands. Always fix wiring and belt issues first to avoid damaging a new alternator.

What Results to Expect

In normal conditions with a healthy 12 V lead-acid system: expect 12.6–12.8 V engine off; 13.8–14.7 V warm engine running on conventional systems; variable 12.5–15.0 V on smart systems depending on load and temperature; AC ripple generally under 100 mV; voltage drops under 0.2 V per side under load. Any persistent deviation warrants further checks as outlined above.

Summary

To confirm an alternator is “putting out,” compare engine-off vs. running battery voltage and observe how it holds under load; healthy systems rise above static voltage and typically maintain roughly 13.6–14.8 V, with smart systems varying more widely by design. If voltage remains low, test ripple, voltage drops, and output current; inspect belts, grounds, and fuses; and use a scan tool to verify commanded charging on modern vehicles. This layered approach separates alternator faults from wiring, battery, or control-system issues—and prevents unnecessary parts swaps.