How to Tell If a Motor Coil Is Bad

You can tell if a motor coil is bad by isolating the motor, checking winding continuity and balance with a multimeter, testing insulation to ground with a megohmmeter, and looking for telltale symptoms like tripped breakers, overheating, or a burnt smell. In practice, equal resistance across identical windings, very high insulation resistance to ground, and normal running current indicate healthy coils; opens, shorts, low insulation resistance, or phase imbalance point to a bad coil. Below is a clear, step-by-step guide to diagnose motor windings accurately and safely.

Safety and Tools

Before testing, ensure the motor is safely isolated and you have the right equipment. These precautions prevent injury and protect sensitive electronics connected to the motor.

• Lockout/tagout the power source; verify zero voltage with a meter.
• Disconnect the motor leads from drives, starters, capacitors, and controls.
• Discharge any motor capacitors (single-phase motors) using a resistor-rated method.
• Tools: digital multimeter (ohms), megohmmeter/insulation tester (typically 500 V DC for 120–480 V motors), infrared thermometer or thermal camera, clamp meter for current, and optionally an LCR meter or surge tester.
• Work in a dry environment; moisture skews insulation readings.

Following these steps protects you and ensures test results reflect the motor’s condition, not external circuitry or unsafe setup.

Quick Field Checks Without Opening the Motor

Simple observations often reveal coil problems before you touch a meter. These checks can quickly indicate whether more in-depth testing is warranted.

• Breaker or overload trips immediately or after a short run.
• Motor hums but won’t start, or starts slowly and overheats.
• Burnt smell, discoloration, or melted varnish at the windings or leads.
• Uneven or excessive current draw compared with the nameplate (use a clamp meter).
• Unusual vibration or noise not linked to bearings or mechanical load.

While symptoms alone aren’t conclusive, they correlate strongly with winding faults such as shorts, opens, or ground faults.

Step-by-Step Electrical Tests with a Multimeter

Resistance and balance checks with a multimeter are the fastest electrical diagnostics for both single-phase and three-phase motors.

1. Isolate the windings: Disconnect the motor from all wiring. Label leads if not already identified.
2. Check for opens/shorts between winding terminals:
   

   • Three-phase: Measure between each pair (U–V, V–W, W–U). All readings should be similar, typically within 5% of each other.
   • Single-phase (split-phase/capacitor motors): Identify common (C), run (R), and start (S) by resistance. Expect R–C = lowest, S–C = higher, and R–S = sum (highest). If R–S ≈ (R–C + S–C), windings are continuous.
   • Very low near-zero ohms can indicate a shorted coil; infinite/open indicates a broken winding or connection.

3. Check winding-to-winding isolation (single-phase): There should be no continuity between start and run when measured through separate coils unless measured via their sum; unexpected low resistance between dissimilar points may indicate a short.
4. Compare to nameplate or service data if available: Absolute ohms vary by motor size; consistency between identical windings is the key indicator.

Consistent, balanced resistance suggests intact coils. Any open circuit, near-zero reading, or significant imbalance (>5–10%) indicates a likely winding fault or internal connection issue.

Insulation Resistance (Megohmmeter) and Polarization Index

Insulation testing checks for leakage paths from the windings to the frame (ground). This is the gold standard for finding ground faults and moisture-damaged coils.

1. Preparation: Disconnect the motor from all cables, drives, and electronics. Ensure it’s clean and dry.
2. Select test voltage:
   

   • 120–480 V motors: 500 V DC.
   • Up to 1 kV motors: 1000 V DC.
   • Small fractional-horsepower motors may be tested at 250 V DC.

3. Measure insulation resistance (IR): Test each winding (or phase group) to the frame. Record the value at 1 minute.
4. Evaluate results (typical field guidance for low-voltage motors):
   

   • IR > 100 MΩ: typically very good (dry, healthy insulation).
   • IR 2–100 MΩ: acceptable to good, consider environment and history.
   • IR 1–2 MΩ: marginal; investigate contamination or aging.
   • IR < 1 MΩ: suspect/bad; likely ground leakage or moisture.

5. Polarization Index (optional but informative): Measure IR at 1 minute and 10 minutes; PI = IR(10 min) / IR(1 min).
   

   • PI ≥ 2: good.
   • PI 1–2: marginal; drying or cleaning may help.
   • PI < 1: poor; likely deteriorated insulation.

High, stable IR and a strong PI indicate sound insulation. Low IR or PI suggests moisture, contamination, or breakdown of varnish—conditions that can trip protection and damage the motor under load.

Advanced or Confirmatory Tests

When results are borderline—or the application is critical—advanced tests pinpoint internal faults like shorted turns that basic meters can miss.

• Surge comparison test: Detects shorted turns between coils/slots; a service shop test for definitive winding integrity.
• Inductance/LCR test: Phase-to-phase inductance balance; significant deviation flags turn shorts.
• Growler test (DC armatures and some AC rotors): Reveals shorts in armature coils using a magnetic field probe.
• Thermal and current profiling: Infrared scan and phase-current measurements under no-load/loaded conditions to find hotspots and unbalance.

These methods are often used by motor repair centers to confirm whether rewinding is required or if issues stem from wiring, drives, or installation.

What Different Results Mean

Interpreting the findings correctly helps distinguish a bad coil from external problems like supply issues or mechanical faults.

• Open winding (infinite resistance): Broken coil or internal connection; motor will fail to start or run weakly.
• Shorted turns (normal-looking resistance but abnormal inductance or surge test): High no-load current, overheating, low torque.
• Ground fault (low IR to frame): Immediate trips, shock risk, severe overheating; do not energize.
• Phase imbalance (three-phase resistance/current mismatch >5–10%): Uneven heating; may be coil damage or poor connections.
• Single-phase motor mismatch (R–S not equal to R–C + S–C): Miswired or damaged windings; faulty capacitor or centrifugal switch can mask as coil issues.

Matching symptoms and measurements provides a clear diagnosis; when in doubt, retest after drying/cleaning and verify all external components.

Common Pitfalls and Safe Practices

A few frequent mistakes can lead to false failures or damaged equipment. Avoid these to get reliable results and prevent harm.

• Testing through a VFD, soft starter, or contactor: Always isolate the motor leads before insulation tests.
• Not discharging capacitors: Single-phase motors can hold dangerous charge; discharge safely with a resistor.
• Moisture and contamination: Wet windings read low IR; bake or dry before condemning.
• Temperature effects: IR drops as temperature rises; correct readings to 40°C if trending results.
• Assuming absolute ohms: Compare identical windings for balance rather than relying on a single resistance value.

Careful setup and awareness of environment and temperature ensure that your tests reflect the motor’s true condition.

Repair or Replace?

Once a bad coil is confirmed, decide whether to rewind or replace based on motor size, criticality, and service conditions.

• Rewind if the motor is large, special-purpose, premium efficiency, or part of a matched system; a shop can also upgrade insulation class.
• Replace if the motor is small, commodity-grade, or has multiple issues (severe core damage, contaminated laminations, rotor defects).
• Address root causes: Overloads, voltage imbalance, poor ventilation, incorrect protection, or incompatible VFD settings can cause repeat failures.

Economics and uptime typically drive the decision; a reputable motor shop can quote both options and advise on reliability improvements.

Summary

To tell if a motor coil is bad: isolate the motor, verify winding continuity and balance with a multimeter, test insulation to ground with a megohmmeter, and corroborate with symptoms and, if needed, advanced tests. Opens, shorts, low insulation resistance, or significant phase imbalance confirm winding faults. Following safe, structured diagnostics will distinguish genuine coil failures from external electrical or mechanical issues—and guide whether to repair or replace the motor.

How do you tell if you have a bad engine coil?

Symptoms of a bad ignition coil include the illuminated check engine light, rough or erratic engine idling, engine misfires, backfiring, difficulty starting the engine, reduced power and poor fuel economy, and even engine stalling. A foul smell of gas from the exhaust can also be a sign of unburned fuel due to a misfiring cylinder. These symptoms indicate the ignition system is not consistently creating the spark needed for combustion, which can lead to more severe engine damage if left unaddressed. 
Here are the common symptoms of a faulty ignition coil:

• Check Engine Light: The most common initial sign of an ignition coil problem is a glowing check engine light on your dashboard. 
• Engine Misfiring: The engine may cough, sputter, or vibrate due to a cylinder not firing correctly because the ignition coil isn’t providing enough voltage for the spark plug. 
• Rough Idling: The engine’s smooth rumble at idle will turn into a juddering, faltering, and noisy experience, especially when the car is stationary. 
• Difficulty Starting: A failing ignition coil can make it harder for the engine to start, requiring the ignition system to work overtime. 
• Engine Stalling: In severe cases, a misfire from a faulty ignition coil can cause the engine to stall while driving. 
• Reduced Power and Fuel Economy: You might notice a significant loss of power and slower acceleration because the engine isn’t combusting fuel efficiently. 
• Backfiring and Gas Smell: When fuel doesn’t ignite in a cylinder, it can exit the exhaust and ignite, causing loud pops (backfiring) and a strong smell of gas. 

How to tell if a coil needs replacing a car?

You may need to replace a car’s ignition coil if you notice symptoms like the Check Engine Light coming on, the engine misfiring, rough idling, difficulty starting, stalling, slower acceleration, poorer fuel economy, or a backfiring sound from the exhaust. These issues indicate the coil isn’t providing sufficient voltage for proper spark plug ignition, disrupting the engine’s normal operation. 
Common Symptoms of a Failing Ignition Coil

• Check Engine Light: An illuminated Check Engine Light is a common indicator of a problem, often accompanied by a diagnostic code that can point to a misfiring cylinder. 
• Engine Misfires: You may feel a stuttering or jerking sensation, particularly during acceleration, as the coil fails to deliver a consistent spark to the spark plug. 
• Rough Idle: The engine may run unevenly or vibrate more than usual when idling. 
• Difficulty Starting: A bad coil can make it harder for the engine to start, or in some cases, prevent it from starting entirely. 
• Engine Stalling: The engine may cut out unexpectedly, sometimes during acceleration or at a stop. 
• Loss of Power: The car can feel sluggish and slow to respond to the accelerator pedal. 
• Poor Fuel Economy: With incomplete combustion from inconsistent sparks, the engine becomes less efficient, leading to increased fuel consumption. 
• Backfiring: Unburned fuel can enter the exhaust and ignite, causing a popping sound from the tailpipe. 
• Strong Fuel Smell: The presence of unburned fuel in the exhaust can also create a distinct fuel odor. 

What to Do Next
If you experience these symptoms, it’s a good idea to have the issue diagnosed. A mechanic can use an OBD-II scanner to pinpoint the exact cylinder with the misfire, helping to confirm a failing ignition coil.

How to test a motor coil?

Windings. You’re looking for burns melting or breaks in the motor windings. If you do see burns your motor’s likely been overloaded. And will need to be repaired or replaced.

How do I test an engine coil?

And the secondary. Side of an old canister style ignition coil what you’ve got is positive and negative terminals. And then you have your output which is where your coil wire.