Car Loses Power When Starting? Burnt Starter Commutator Might Be the Real Cause!

The burning of the car starter motor's commutator is a very typical automotive fault. Let's break down the symptoms, causes, and handling methods for a burnt commutator in a car starter motor.

I. Symptoms

When the starter motor's commutator becomes burnt, a series of very obvious symptoms will appear, usually not just one:

1️⃣ Weak Cranking or Complete Failure to Start (The core symptom)

✔️ Manifestation: When turning the key to the start position, you hear a ''click'' from the starter, but the engine crankshaft turns very slowly or not at all. Simultaneously, the dashboard lights will dim significantly or even go out.

✔️ Cause: The burnt commutator surface is uneven, leading to poor contact with the carbon brushes. This causes a sharp increase in contact resistance. The high starting current cannot flow smoothly, resulting in a significant voltage drop. The actual power reaching the motor is severely insufficient.

2️⃣ Smoke and Burnt Smell During Starting

✔️ Manifestation: When attempting to start, visible smoke and a distinct burnt smell of insulation lacquer may be noticed from near the starter motor (lower part of the engine bay).
✔️ Cause: Excessive resistance at the contact points generates intense heat according to Joule's law (Q=I²Rt). This burns the insulation between the commutator copper segments, carbon brush dust, and the insulation lacquer of the windings.

3️⃣ Excessive Sparking During Starting

✔️ Manifestation: In low-light conditions (or when testing the removed starter), abnormally large, ring-shaped strong sparks can be seen at the contact point between the commutator and carbon brushes, instead of the normal fine blue sparks.
✔️ Cause: The pits and bumps from the burning disrupt the smooth contact between the brushes and commutator segments. Intense arcing occurs at the moment of electrical contact make/break.

4️⃣ Unstable Starter Operation with Abnormal Noises

✔️ Manifestation: The starter motor makes ''chugging,'' ''clunking,'' or irregular grinding noises while turning. The speed is unstable, feeling ''jerky'' or uneven.
✔️ Cause: The burning causes the commutator surface to be out-of-round or uneven. The carbon brushes bounce violently on it at high speed, unable to transmit current steadily, leading to uneven torque output from the motor.

II. Causes

Commutator burning is not an isolated event; it is usually the result of other underlying problems. The root cause can be attributed to ''High Temperature,'' which originates from ''High Current'' and ''Poor Contact.''

1️⃣ Excessive Carbon Brush Wear or Insufficient Spring Pressure

✔️ This is the most common cause. When the brushes wear too short, the springs cannot provide enough pressure for tight contact with the commutator. This increases contact resistance, generating high temperatures and sparks, which then erode the commutator.

2️⃣ Starter Motor Overload

✔️ Excessive Engine Resistance: Such as viscous engine oil (especially in winter), or internal engine mechanical faults (e.g., bearing seizure, crankshaft seizure), requiring much higher than designed torque and current to start.
✔️ Prolonged Continuous Cranking: Each starting attempt is too long, or the interval between attempts is too short, preventing heat dissipation and causing cumulative heat buildup.

3️⃣ Poor Quality Commutator or Improper Maintenance

✔️ The commutator copper segments are of poor quality, or the mica insulation between them is not undercut as required (the mica should be slightly lower than the copper surface), leading to accelerated brush wear and sparking.
✔️ The commutator is out-of-round, or has surface contamination from oil or carbon dust buildup, affecting conductivity.

4️⃣ Solenoid Switch or Internal Motor Coil Failure

✔️ Solenoid Contact Welding: Causes the starter to remain engaged after the engine starts, forcing it to spin at extremely high speeds far beyond its design limits, damaging the commutator.
✔️ Internal Motor Coil Short Circuit: Causes abnormal current surge in localized areas.

III. How to Handle

Once commutator burning is confirmed, the handling method depends on the severity of the burning and the cost-effectiveness of repair.

Step 1: Diagnosis and Confirmation

✔️ Preliminary Check: Based on the symptoms above, first check if the battery is sufficiently charged (voltage should be above 12V) and if the battery terminals are tight and free of corrosion. Rule out battery issues.

✔️ Remove the Starter: If the battery is normal, the starter needs to be removed from the engine.

Visual Inspection:

✔️ Minor Burning: The commutator surface has a uniform dark purple or blue oxide layer, or slight pitting and burn marks, but the overall flatness is acceptable.

✔️ Severe Burning: The commutator surface is uneven, with severe pitting and molten protrusions. The copper segments are thinned or even missing from burning, and the color is blackened.

For Minor Burning:

✔️ Precision Lathe Turning: Disassemble the starter, remove the armature (rotor), and use a professional lathe to precision-machine the commutator surface, removing the burnt layer and restoring smoothness, flatness, and roundness.

✔️ Undercut Mica: After turning, the mica insulation between the commutator segments *must* be undercut by 0.5-1.0mm using a special saw to ensure the brushes contact the copper segments properly and don't ride on the mica.

✔️ Replace Brushes and Springs: This is mandatory! Regardless of how much life is left in the old brushes, a complete new set of carbon brushes and springs must be installed. The curvature of the new brushes should be sanded to match the commutator.

✔️ Clean and Assemble: Thoroughly clean all carbon dust and debris from inside. Check bearings for play. Reassemble.

For Severe Burning:

✔️ Replace Armature Assembly or Entire Starter: If the burning is very severe, turning the commutator would reduce its diameter below the minimum allowable limit, compromising structural integrity or proper brush contact. The most economical and reliable method in this case is:

✔️ Option A (Recommended): Purchase a replacement armature assembly (the rotor) of the same model for replacement. Simultaneously replace the carbon brushes, springs, and bearings.

✔️ Option B (Easier): Directly replace the entire starter motor assembly. Especially for older vehicles or where repair parts are hard to find, buying a quality remanufactured or reliable aftermarket unit is often a faster solution.

Step 3: Address the Root Cause

After repairing or replacing the starter, be sure to check and eliminate the root cause that led to the burning:

✔️ Check if the engine oil is suitable.
✔️ Check if the engine rotates smoothly (e.g., try turning the crankshaft pulley manually with a wrench).
✔️ Develop good starting habits: Each crank should not exceed 5 seconds, with intervals of at least 15 seconds between attempts.

Summary:

✔️ Commutator burning is the final manifestation of the starter motor being ''overworked'' and ''aged.'' For the average car owner, the most practical advice is: once symptoms suggest a starter problem, it should be sent to a professional auto electric repair shop for disassembly and inspection. Let the mechanic decide whether repair (turning the commutator) or replacement (armature or complete unit) is more cost-effective. Do not continue to force-start the car, as this can lead to complete failure or cause greater electrical system faults.

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