How to Tell if a Motor Is Bad: Symptoms, Tests, and What to Do Next

You can tell a motor is bad if it won’t start, trips a breaker or fuse, hums without turning, overheats, smells like burnt insulation, shows excessive vibration or grinding noise, or fails basic electrical checks (abnormal current, unbalanced winding resistance, or low insulation resistance). In practice, confirming a failed motor means separating motor faults from issues in power supply, controls, and the driven load, then verifying with simple tests (visual, mechanical, multimeter) and—when available—advanced diagnostics (insulation/megger, current balance, thermal imaging).

Quick Symptom Checklist

The following are common real-world signs that point to a motor nearing failure or already failed. Use them to quickly triage before deeper testing.

• Motor won’t start, or starts then stalls
• Breaker/fuse trips immediately or after a few seconds of running
• Loud humming without rotation (especially on single‑phase motors)
• Overheating, hot casing, or a burnt varnish/electrical smell
• Excessive vibration, grinding, or rattling (often bearings or imbalance)
• Visible damage: burnt leads, swollen/bulged capacitor, contamination, cracked fan
• Abnormally high or uneven current draw compared with the nameplate
• Sparking at brushes (brushed DC) or signs of commutator wear
• Slow speed, low torque, or erratic speed control under constant load
• Intermittent running or frequent thermal overload trips

Any one of these symptoms is a flag; multiple symptoms together usually indicate the fault is inside the motor or its start/run components rather than elsewhere.

Step-by-Step Diagnostics You Can Do

A methodical approach saves parts and time: stay safe, eliminate external causes, then test the motor electrically and mechanically before deciding on repair or replacement.

Safety First

Before touching a motor or panel, protect yourself and the equipment.

• Disconnect and verify power is off; lockout/tagout where applicable
• Discharge capacitors (single‑phase and DC systems) and beware stored energy
• Use insulated tools, PPE, and a properly rated meter
• Keep clear of rotating parts; remove belts/couplings before test runs when possible

These steps reduce shock, arc, and entanglement risk and help ensure the readings you take are reliable.

Rule Out External Causes

Many “bad motor” calls turn out to be power, control, or load problems. Check these first.

1. Verify supply: measure line voltage at the motor terminals (under start and run). Low or missing phase/leg can mimic a bad motor.
2. Check breakers, fuses, overloads, contactors, and relays; look for loose or burnt connections.
3. Confirm control signals: start/stop circuits, pressure/limit switches, PLC/VFD commands, emergency stops.
4. Inspect the driven load: jammed pump/impeller, seized gearbox, tight belt, misalignment. Decouple if possible and try again.
5. Spin the shaft by hand (power off). It should turn smoothly without rough spots or scraping.
6. If using a VFD/ESC, check fault codes, carrier frequency settings, and motor nameplate parameters.

If the motor behaves normally when uncoupled, the problem is likely in the load. If supply and controls check out but symptoms persist, proceed to motor testing.

Basic Electrical Checks (Multimeter/Clamp Meter)

These quick checks catch most failures without specialized tools.

1. Nameplate vs. current: With a clamp meter, compare running current to the nameplate Full-Load Amps (FLA). Significantly over FLA suggests overload, single-phasing, or internal faults.
2. Continuity/open winding: With power off, check for continuity between winding leads. An open circuit usually means a burnt winding or failed internal connection.
3. Ground fault check (basic): Measure resistance from each winding lead to the motor frame. A very low reading indicates a short-to-ground. Note: a proper insulation test uses a megohmmeter.
4. Winding balance (three-phase): Measure resistance phase-to-phase. Values should be similar; a difference >5–10% indicates trouble.
5. Capacitor test (single-phase): Visually inspect for bulge/leak. Measure capacitance; readings typically should be within the capacitor’s tolerance (often ±5–10% for run caps, ±10–20% for start caps).

These measurements help differentiate wiring/control issues from internal motor failures like shorted turns, open windings, or deteriorated insulation.

Mechanical Checks

Mechanical health is as important as electrical integrity and often the root of “bad motor” symptoms.

• Bearings: Listen for rumble or grinding; feel for play or roughness when rotating the shaft
• Endplay and shaft wobble: Excessive movement points to bearing or housing wear
• Rotor/fan rub: Scrape marks or melted shrouds indicate contact and overheating
• Alignment: Misalignment with the load raises current and vibration
• Contamination: Water, oil, dust, or conductive debris inside the frame can cause shorts

Mechanical faults often show up as heat, vibration, and noise before they escalate into electrical failures.

Advanced Tests (When Available)

If you have access to specialized tools or a motor shop, these tests confirm borderline cases and inform repair decisions.

• Insulation resistance (megger): For low-voltage motors, healthy windings typically read in the tens to hundreds of megohms to ground; below about 1 MΩ is a red flag, especially under humidity.
• Polarization index (PI): Compares 10-minute to 1-minute megger readings; low PI suggests contamination or aging insulation.
• Surge/impulse testing: Detects turn-to-turn shorts not obvious on a simple ohms test.
• Current signature analysis: Finds broken rotor bars, eccentricity, or bearing defects.
• Thermography: Hot spots indicate poor connections, overload, or localized winding damage.
• Scope checks on VFD systems: Looks for DC bus issues, excessive ripple, or wiring/shielding problems causing nuisance trips.

These methods provide a deeper look at winding condition and rotor integrity and are commonly used by repair shops to validate rewind decisions.

What the Results Mean

Linking symptoms to likely causes helps you act quickly and avoid repeated failures.

• Immediate breaker/fuse trip: shorted winding, short to ground, or miswired leads
• Hums but won’t start (single‑phase): failed start capacitor, start winding, or centrifugal switch
• Runs hot with low torque: overload, misalignment, blocked ventilation, or missing/low voltage phase
• High, uneven current on phases: voltage imbalance, winding damage, or rotor bar issues
• Excessive vibration/rumble: failing bearings, imbalance, bent shaft, or soft foot
• Sparks at brushes (DC): worn brushes, dirty/ovalized commutator, armature faults
• Starts only if spun by hand: bad start circuit or capacitor (single‑phase), or controller fault (BLDC)
• Intermittent operation: thermal overload trips, loose connections, moisture ingress

When multiple indicators point to internal windings or bearings, the motor is likely bad; if indicators point to power or load, focus repairs there first.

Special Cases by Motor Type

Single‑Phase AC (Capacitor‑Start/Run)

These are common in HVAC, compressors, and appliances; start circuits are frequent failure points.

• Bulged/leaky capacitor or out‑of‑tolerance µF value
• Faulty centrifugal/start switch (pitted contacts, stuck mechanism)
• Open start winding; motor hums and overheats without rotation

Swapping a known‑good capacitor and cleaning/adjusting the switch often resolves issues without replacing the motor.

Three‑Phase AC Induction

Industrial workhorses that suffer from supply issues and mechanical wear.

• Single‑phasing or voltage imbalance causes overheating and low torque
• High vibration with healthy electrical tests suggests bearing or alignment problems
• VFD misconfiguration (wrong base frequency, voltage, or motor data) mimics motor failure

Confirm balanced supply and correct VFD parameters before condemning the motor.

Brushless DC (BLDC/PMSM)

Common in e‑bikes, drones, servers, and modern appliances; controller and sensors are critical.

• Hall sensor faults or broken phase lead cause cogging or no‑start
• Controller errors, overcurrent trips, or desync at load changes
• Stator contamination leading to phase‑to‑ground faults

Test coils phase‑to‑phase, verify sensor outputs, and substitute a known‑good controller if possible.

Brushed DC

Simple to diagnose and repair, but brushes and commutators wear.

• Short brushes, weak springs, or uneven wear cause arcing and loss of torque
• Dirty or grooved commutator needs cleaning or turning
• Armature shorts show as high no‑load current and overheating

Brush replacement and commutator service often restore performance unless the armature is damaged.

Automotive Starter Motors

High current devices where voltage drop and solenoid function matter.

• Single click but no crank: low battery, corroded cables, or stuck solenoid
• Rapid clicking: insufficient battery voltage or poor ground
• Bench test shows slow/no spin: worn brushes or shorted armature

Perform a voltage drop test across positive and ground paths during crank; large drops indicate cable/connection issues rather than a bad starter.

Repair or Replace?

When a motor is bad, weigh cost, downtime, and efficiency before choosing a path.

• Size and cost: Small fractional‑horsepower motors are usually cheaper to replace than rewind
• Age/efficiency: Upgrading to a premium‑efficiency unit can cut operating costs
• Lead time: Rewinding may be faster than sourcing specialty motors
• Environment: Motors exposed to moisture/chemicals may need higher IP/TEFC ratings
• Root cause: Fix load, power quality, or controls to prevent repeat failures

For large or specialty motors, a shop evaluation (electrical and mechanical) can determine if a rewind is economical and reliable.

When to Call a Professional

Some scenarios call for certified technicians and test equipment you may not have.

• High‑voltage systems, hazardous locations, or explosion‑proof equipment
• Repeated trips/failures after basic checks, or signs of severe insulation breakdown
• Critical production equipment where downtime risk is high
• Need for megger/surge testing, vibration analysis, or laser alignment

A motor shop can provide definitive diagnostics, warrantyable repairs, and root‑cause reports.

Tools and Reference Values

Having the right tools speeds accurate diagnosis and prevents guesswork.

• True‑RMS multimeter and clamp meter
• Megohmmeter (insulation tester) with appropriate test voltage
• Non‑contact tachometer and IR thermometer/thermal camera
• Mechanic’s stethoscope and dial indicator for mechanical checks
• Basic hand tools and pullers; brush/capacitor service parts for applicable motors

With these, most field checks can be completed quickly and safely.

Typical benchmarks help interpret your readings correctly.

• Running current: about 90–110% of nameplate FLA at rated load; much higher indicates trouble
• Insulation resistance: aim for tens to hundreds of megohms to ground on low‑voltage motors; below ~1 MΩ suggests cleaning, drying, or rewind
• Phase resistance balance (3‑phase): within 5% is desirable; >10% is problematic
• Temperature: bearings typically should stay below about 90°C casing temperature in service
• Capacitors: run caps typically ±5–10% of rated µF; start caps often ±10–20%

Always compare measurements to the motor nameplate and manufacturer guidance for the most accurate go/no‑go decisions.

Summary

A motor is likely bad if it shows consistent start failures, trips protection, overheats, smells burnt, vibrates excessively, or fails electrical tests. Start by verifying power, controls, and load; then check current draw, windings, insulation, and bearings. Map symptoms to causes—capacitors and start circuits on single‑phase, imbalance and bearing issues on three‑phase, and brush/commutator or controller faults on DC/BLDC. Use data, not guesswork, to decide on repair or replacement, and involve a professional when safety, scale, or repeated failures demand deeper diagnostics.

How do you check if a motor is bad?

To test if a motor is bad, first disconnect power and perform a visual inspection for damage. Next, use a multimeter set to ohms to check the motor’s winding resistance for shorts or open circuits. You should also check for grounds by testing the windings against the motor’s casing. You can perform a quick bearing check by manually spinning the motor shaft to feel for free, quiet rotation.
 
1. Visual Inspection 

• Look for damage: Check for signs of overheating, contamination, or physical damage to the motor housing and components.

2. Winding Resistance Test (with Multimeter) 

• Set the multimeter: Set the multimeter to the ohms function (Ω). 
• Disconnect power: Ensure the motor is completely disconnected from the power source. 
• Test the windings: Measure the resistance between the various motor leads (e.g., T1 to T2, T2 to T3, T1 to T3). 
• Check against specifications: The readings should fall within the range specified in the motor’s manual, generally a low resistance (e.g., 0.3 to 2 ohms). 
• Watch for shorts: A reading of 0 ohms indicates a short circuit, and a complete lack of reading (infinite ohms) may indicate an open circuit. 

3. Ground Test (with Multimeter)

• Set the multimeter: Set to the ohms function. 
• Test against the casing: Connect one probe of the multimeter to each winding lead and the other probe to the motor casing. 
• Check for continuity: You should get an infinite (no) reading, as there should be no conductivity between the windings and the grounded motor casing. 

4. Manual Shaft Check 

• Spin the shaft: With the motor disconnected, manually try to spin the shaft or rotor.
• Listen and feel: The shaft should spin freely and quietly.
• Check for issues: Any scraping, grinding, or a lack of free movement can indicate bad bearings or other internal problems.

What is the diagnostic test for motors?

Motor Circuit Analysis (MCA) testing: Instruments using values of resistance, impedance, inductance, phase angle, current:frequency response, and insulation testing can be used to troubleshoot, commission and evaluate control, connection, cable, stator, rotor, air gap and insulation to ground health.

How do you know if your motor needs to be replaced?

You may need a new engine if your car consistently overheats, experiences a significant loss of power, or is accompanied by other symptoms like loud knocking noises, excessive smoke from the tailpipe (white, blue, or black), or metal shavings in the engine oil. Other signs can include frequent stalling, rough engine idling, odd smells, and a persistent “check engine” light. If you notice these issues, it’s essential to have a qualified mechanic inspect the vehicle to determine the cause and the necessary course of action, as some problems can be repaired, while others may require a complete replacement.
 
Signs of Engine Failure

• Constant Overheating: Persistent overheating, even after cooling system maintenance, can indicate major issues like a cracked head gasket or block. 
• Loss of Power and Performance: A noticeable decline in engine power, difficulty accelerating, and struggling to climb hills are strong indicators of a failing engine. 
• Excessive Smoke: A large amount of black, white, or blue smoke from the tailpipe suggests the engine is burning oil, fuel, or coolant, a sign of serious damage. 
• Metal Shavings in Oil: Finding metal flakes in your engine oil is a red flag, as it means engine parts are disintegrating and causing extensive damage. 
• Loud Knocking Noises: Continuous knocking sounds from the engine can signal a problem with the pistons or rod bearings, leading to damage within the cylinder block. 
• Stalling or Rough Idle: An engine that stalls frequently or runs rough, with significant vibrations, indicates a problem with the combustion process. 
• Odd Smells and Unusual Sounds: Smells like burning oil or coolant, or unusual noises like hissing or popping, can point to a developing engine problem. 
• Check Engine Light: A constantly illuminated “Check Engine” light can signal a variety of engine-related problems, some of which may be severe. 

This video explains some common symptoms of a failing engine: 46sScotty KilmerYouTube · Jun 9, 2021
What to Do
If you observe these symptoms, especially multiple ones, take your car to a qualified mechanic for a thorough inspection. A mechanic can diagnose the exact problem and advise you on whether an engine rebuild or a complete engine replacement is necessary. They will consider the severity of the issue, the age of your vehicle, and the overall cost-benefit of repair versus replacement. 
You can watch this video to learn how to inspect a used engine and look for potential problems: 44sDave’s Auto CenterYouTube · Mar 3, 2025

What are the symptoms of a faulty motor?

Common signs a motor’s efficiency has been compromised, or that it may be time for a replacement, include abnormal noises or vibration, below-normal performance of equipment or output, and overheating. If any of these signs are present, it might be possible to fix the problem with some troubleshooting.