Starter vs. Alternator: How to Tell Which One Is Bad

If the engine won’t crank (or you hear a single, solid click) while the lights and electronics stay strong, it’s usually the starter; if the car starts, then dies shortly after, or the battery keeps going flat and system voltage is low while running, it’s usually the alternator. A quick multimeter check confirms it: a healthy battery rests near 12.6 V, voltage shouldn’t drop below about 9.6 V during cranking, and should hold around 13.8–14.7 V with the engine running. Here’s how to separate symptoms, run simple tests, and make the right call.

What These Parts Do—and Why It Matters

The starter is a high-draw electric motor that turns the engine over for a few seconds during start-up. The alternator is a generator that recharges the battery and powers the vehicle’s electrical systems once the engine is running. A bad starter causes cranking problems right now; a bad alternator causes charging problems that show up as dimming lights, stalling, and a battery that keeps dying over time.

Quick Symptom Check

The fastest way to narrow it down is to match what you’re experiencing with common, telltale signs. Below are the typical symptoms that point to a failing starter or a failing alternator.

Signs It’s Likely the Starter/Solenoid

These clues focus on what happens the moment you try to start the car. If power seems normal but the engine won’t physically turn over, the starter circuit is suspect.

• Strong dash lights, but a single loud “click” and no crank when you turn the key or push Start.
• No sound at all from the engine bay, yet interior electrics (radio, lights) are normal.
• Intermittent starting that improves if you gently tap the starter body (worn brushes/dead spot).
• Grinding or whirring during start attempts (failed starter drive/Bendix not engaging the flywheel).
• Heavy cable to the starter gets hot or there’s a burning smell near the starter during attempts.

If power is present and consistent but the engine won’t turn, the starter motor, solenoid, or the starter’s power/ground connections are the prime suspects.

Signs It’s Likely the Alternator/Charging System

Charging failures tend to show up while the engine is running or on repeat trips, as the battery isn’t being replenished.

• Battery warning light (or “ALT/GEN” icon) flickers or stays on while driving.
• Headlights or interior lights brighten with throttle and dim at idle, especially with accessories on.
• Whining or growling from the alternator, or a squealing belt on startup/under load.
• Car starts after a jump but dies shortly after removing the jump pack/cables.
• Repeated dead battery despite being new or recently charged.
• Electrical gremlins while driving—radio resets, ABS/airbag lights, erratic gauges—then stalling.

When a car runs only on a surface charge or jump assist, then fades, the alternator or its belt, wiring, or voltage regulator is the likely problem.

Simple At-Home Tests

With basic tools, you can run a quick diagnostic sequence to pinpoint the culprit. A cheap digital multimeter is the most reliable way to confirm what’s failing.

1. Safety first: Park in neutral/park with the brake on, keep hands and clothing clear of belts and fans, and avoid sparking near the battery.
2. Visual check: Inspect battery terminals and grounds for corrosion, looseness, or broken cables (especially the engine-to-chassis ground strap). Clean and tighten as needed.
3. Headlight test: Turn on headlights. Try to start. If lights stay bright but no crank (single thunk), think starter. If lights go very dim or die, think weak battery/charging issue.
4. Jump-start test: If it starts with a jump and then runs fine on its own, the alternator is likely charging and the battery may be weak. If it starts with a jump but quickly dies after removing the jump, suspect the alternator/drive belt.
5. Battery resting voltage: With engine off for 30+ minutes, measure at the battery posts. ~12.6 V is full; ~12.4 V is OK; ≤12.2 V is low/discharged.
6. Cranking voltage drop: Watch the meter while someone cranks. If it plunges below ~9.6 V (around 70°F/21°C), the battery is weak or the starter is drawing excessive current; check both the battery and starter circuit.
7. Charging voltage: Start the engine and measure across the battery. Expect roughly 13.8–14.7 V at idle on most vehicles. Below ~13.2 V or above ~15.0 V indicates a charging system fault (alternator, regulator, wiring, or belt slip).
8. Load test running: Turn on headlights, rear defogger, and blower. Voltage should generally stay above ~13.5 V. If it sags near battery voltage (~12 V), the alternator can’t keep up.
9. AC ripple check (optional): With the engine running, set the meter to AC and read across the battery. More than ~0.5 VAC often means a failed alternator diode.

These steps isolate whether you have a supply (battery/alternator) problem or a starter-motor problem. Numbers out of spec almost always point directly to the failing component or its wiring.

Multimeter Cheat Sheet (What the Numbers Mean)

Here’s a quick reference for typical readings on modern 12 V systems; temperature, vehicle design, and “smart” charging strategies can shift targets slightly.

• Battery resting (engine off): ~12.6 V (full), ~12.4 V (≈75%), ~12.2 V (≈50%), ≤12.0 V (very low).
• During crank: ideally ≥10.0 V; below ~9.6 V suggests weak battery, high resistance connections, or a dragging starter.
• Engine running (no load): ~13.8–14.7 V on most cars; some smart systems may briefly run 12.8–13.5 V to save fuel.
• Engine running (accessories on): typically ≥13.5 V; persistent ~12–13 V indicates poor charging.
• AC ripple while running: ≤0.3–0.5 VAC; higher often means bad alternator diodes.

Use these values as thresholds, not absolutes. If you’re close to the margins, repeat measurements, check connections, and consider temperature and vehicle-specific charging logic.

Decision Guide: Which One’s Bad?

Match your situation to the scenarios below for a practical verdict before you buy parts.

• No crank, lights stay bright, single “click”: Starter or starter solenoid (or its relay/wiring) is likely bad.
• Slow crank, lights dim hard, voltage dives: Weak battery or poor connections; if the battery tests OK but keeps dying, the alternator isn’t charging.
• Starts only with a jump, then stalls soon after: Alternator/charging circuit or a loose/slipping belt.
• Intermittent no-crank that responds to tapping the starter: Worn starter brushes or dead spot on the armature.
• Battery light on while driving, or lights brighten with revs and dim at idle: Alternator output or belt/ pulley tension issue.
• Strong crank but no start: Not a starter or alternator problem—look to fuel/ignition/engine management.

When symptoms straddle categories, test with a meter. It’s common for a weak battery to mimic starter failure, and for a failing alternator to be blamed on a new battery that’s simply being drained.

Common Pitfalls and Myths

A few widespread habits can mislead diagnosis or damage electronics. Avoid these mistakes to save time and money.

• Do not disconnect the battery while the engine is running—voltage spikes can fry the ECU and modules.
• Don’t assume a new battery is good; test it. New doesn’t guarantee charged or defect-free.
• Check grounds and terminals first; corrosion and loose clamps cause many “bad starter/alternator” misdiagnoses.
• Listen for belt squeal or inspect for glazing; a slipping belt can mimic a weak alternator.
• On vehicles with smart charging or stop/start, charging voltage may vary by design—consult specs.
• Hybrids/EVs use a DC-DC converter to charge the 12 V system; alternator diagnostics differ on those platforms.

A few minutes on connections and proper voltage checks can prevent unnecessary part swaps and pinpoint the real fault.

When to Call a Professional

Some situations warrant a shop visit, specialized tools, or programming after replacement.

• Repeatedly dead battery despite passing tests, or parasitic drain suspected.
• Charging voltage wildly fluctuates, or there’s electrical burning odor/smoke.
• Starter or alternator is buried (labor-intensive access), or immobilizer/security interlocks complicate starts.
• Late-model vehicles that require module coding after alternator replacement.

If basic tests don’t clearly point to one component, a technician can perform load testing, current draw measurement, and scan for fault codes in charging/start systems.

Summary

Starters cause immediate cranking failures; alternators cause ongoing charging failures. If the lights are strong but the engine won’t turn, think starter. If the car starts then dies or the battery keeps going flat and running voltage is low, think alternator. Confirm with a multimeter: healthy rest ~12.6 V, cranking ≥9.6 V, running ~13.8–14.7 V. Check cables and grounds first, and avoid risky “battery disconnect” tests. With a few simple checks, you can diagnose confidently and fix the right part.

What are the first signs of an alternator going bad?

First signs of a failing alternator include the battery warning light on the dashboard, dim or flickering headlights, slow-starting or stalling engines, strange smells like burning rubber, and odd noises from under the hood. You may also notice other electrical issues, such as slow power windows or a dead battery.
 
Warning Lights and Electrical Issues 

• Dashboard light: A red battery-shaped warning light, or a light with “ALT” or “GEN” on it, often indicates a problem with the charging system. 
• Dim or flickering lights: Headlights, dashboard lights, and interior lighting may appear dimmer than usual or flicker. Lights may also get brighter as you increase the engine’s RPMs. 
• Slow or malfunctioning electronics: Power windows, the radio, or other electrical components may work slowly, intermittently, or stop functioning altogether. 

Engine Performance Problems 

• Slow start or frequent stalling: The engine may crank slowly when starting, or the car might stall while driving or idling because the alternator is not supplying enough power. 
• Dead battery: A failing alternator can’t recharge the battery, leading to a dead battery, even after being charged or replaced. 

Unusual Sounds and Smells 

• Burning smell: A smell of burning rubber or wires can signal an alternator issue. 
• Strange noises: Worn bearings in the alternator can produce a growling or grinding sound, while a loose or worn serpentine belt can cause a squealing noise. 

If you notice these signs, it’s best to have your vehicle’s charging system tested by a professional.

How to check battery alternator and starter?

You can use a multimeter to measure the voltage at your battery terminals. If the voltage is above 14.7 volts or below 13.7 volts when the engine is running, it may indicate a failing alternator or voltage regulator. It’s important to replace or repair a dying alternator as soon as possible.

How do you tell if you have a bad starter or alternator?

A bad starter typically results in clicking or grinding noises when you turn the key, with the engine refusing to crank, or the engine cranking slowly. A bad alternator usually manifests as a car that starts but then dies shortly after, dimming lights, or a battery warning light on the dashboard, as it cannot recharge the battery while the engine is running. 
This video demonstrates the sounds you might hear from a bad starter: 59sproclaimliberty2000YouTube · Dec 2, 2016
Signs of a Bad Starter

• No Crank, No Start, Maybe a Click: You turn the key, and you hear a single click, multiple rapid clicks, or a grinding noise, but the engine doesn’t turn over. 
• Slow Cranking: The engine cranks very slowly, struggling to start. 
• Lights On, Engine Off: The dashboard lights, headlights, and radio work, but nothing happens when you try to start the car. 
• Failed Jump Start: If you try to jump-start the car and it still doesn’t crank, the starter is a strong possibility, as a working battery is available, but the starter can’t engage. 

Signs of a Bad Alternator

• Battery Warning Light: Opens in new tabThe battery warning light on your dashboard is illuminated, indicating a charging system problem. 
• Car Dies While Running: Opens in new tabThe car starts initially but dies shortly after, or it stalls frequently, because the alternator isn’t supplying enough power to keep the engine running. 
• Dim or Flickering Lights: Opens in new tabYour headlights and other lights become dim, or they might be overly bright, due to inconsistent electrical output from the alternator. 
• Dead Battery: Opens in new tabThe battery frequently drains, leading to trouble starting the car. 
• Unusual Noises or Smells: Opens in new tabYou might hear a whining or growling sound from the engine area, or a smell of burning rubber or wires. 

This video demonstrates how to test for a bad alternator by removing the battery cable: 1mproclaimliberty2000YouTube · Apr 21, 2018
How to Test Your Car

1. Jump Start: Attempt to jump-start the car. 
2. Analyze the Result:
   • If it doesn’t crank after a jump, your starter is likely the issue. 
   • If it starts but then dies shortly after, or fails to restart after sitting, the alternator is likely the culprit, as the battery wasn’t able to stay charged. 
3. Check the Battery Light: If the battery light stays on after the car has been running, it’s a strong indicator of a failing alternator. 

How do I know if it’s the alternator or starter which is gone bad in my Lincoln MKX 2013?

Common Signs of a Failing Alternator in Lincoln Cars
Warning light: The most common sign of a failing alternator is a warning light on the dashboard, usually shaped like a battery. Dimming headlights: A failing alternator may cause the headlights to become dim or flicker, especially at low speeds or when idling.