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Vehicle: Mercedes-Benz W205 C300. · Reported issue: unusual noise from the engine bay and complete loss of battery charging. · Identified cause: wear and failure of the alternator overrunning pulley mechanism. · Work performed: alternator removal, professional bench testing, carbon brush replacement and installation of a new overrunning pulley. · Warranty: 12 months. · Date: 30 October 2026
A Mercedes-Benz W205 C300 arrived at our service center with unusual noise coming from the engine compartment, followed by a complete loss of battery charging. The charging system warning indicator was illuminated on the dashboard.
Mercedes-Benz W205 C300 — Diagnosing and Repairing the Alternator
During the initial inspection, we focused on the alternator drive system. Our diagnostics identified a failure in the overrunning pulley mechanism, which was affecting the proper operation of the alternator.
The overrunning pulley is an important component of the alternator drive system. Its function is to reduce dynamic loads caused by changes in engine speed and minimize drive belt vibrations. Wear or internal damage can result in abnormal noise, vibration, and impaired alternator operation.
Our Diagnostic and Repair Process
- Initial Diagnostics: We inspected the battery charging system, drive belt, and alternator pulley operation. The inspection revealed a malfunctioning overrunning pulley that was directly affecting alternator performance.
- Alternator Removal and Bench Testing: The alternator was removed from the vehicle and installed on our specialized diagnostic test bench. We inspected its electrical components, including the rectifier bridge, voltage regulator, and windings, to assess the overall technical condition of the unit.
- Carbon Brush Replacement: During disassembly, we also inspected the carbon brushes. Due to significant wear, we replaced them with new ones. This allowed us to address an additional worn component while the alternator was already disassembled, helping reduce the likelihood of a repeat repair in the near future.
- Overrunning Pulley Replacement and Assembly: The damaged overrunning pulley was replaced with a new unit. After reassembly, the alternator underwent another professional bench test under different load conditions.
Final result: after reinstalling the alternator, we checked the vehicle's charging system. The abnormal noise disappeared, and the charging voltage returned to normal operating specifications.
Work Performed — Step by Step
| Stage / Component | Work Performed |
|---|---|
| Charging diagnostics | Voltage measurement — no charging detected. |
| Belt and pulley | Overrunning pulley fault confirmed. |
| Alternator removal | Removal from the vehicle and bench testing. |
| Disassembly and inspection | Inspection of brushes, regulator, rectifier bridge, windings and bearings. |
| Overrunning pulley | Removal of the faulty pulley and installation of a new one. |
| Brush replacement | Significant wear — replaced with new ones. |
| Bench testing | Alternator test under load. |
| Reinstallation and final checks | Noise eliminated, charging restored. |
| Warranty | 12-month warranty on the work performed. |
Expert Advice — How to Recognize a Faulty Alternator Overrunning Pulley
On Mercedes-Benz W205, W213, and other models, the overrunning pulley plays an important role in the alternator drive system. It helps absorb rotational fluctuations, reduce belt stress, and maintain stable operation of the engine's auxiliary drive components. Over time, the internal clutch mechanism can wear out, resulting in two main types of failure:
- Pulley seizure: the internal mechanism can no longer rotate freely in the required direction. This may cause squealing, grinding noises, vibration, and unstable belt tensioner operation. Battery charging may initially remain functional.
- Pulley slippage or internal clutch failure: the pulley can no longer transfer engine rotation to the alternator shaft correctly. As a result, alternator speed and electrical output may decrease or charging may stop completely. This was the type of failure identified on this Mercedes-Benz W205 C300.
Important: if you notice unusual noise around the alternator, drive belt vibration, or a battery warning light on the dashboard, arrange a diagnostic inspection promptly. Early detection can help prevent further damage to the drive system.
Safety recommendation: do not keep driving without charging — the car runs on the battery alone, and as voltage drops electronic systems may shut down and the engine may stall. A seized or disintegrating pulley can also damage the drive belt. Do not replace the battery before diagnostics.
Recommendations for Vehicle Owners
- Do not ignore noise: grinding, whistling or vibration near the alternator can be an early sign of pulley wear.
- Check the whole alternator: when replacing the pulley it makes sense to bench-test the unit and inspect the brushes.
- Do not replace parts by guesswork: the pulley, brushes and regulator can cause similar symptoms — diagnostics establish the cause.
Frequently Asked Questions
Why did charging disappear on the Mercedes W205 C300?
One common cause is a faulty alternator overrunning pulley: the belt turns but the alternator shaft does not, so no current is produced. Other possible causes include the brushes, regulator, rectifier bridge or wiring, so diagnostics are required.
What is the difference between a seized and a slipping pulley?
A seized pulley typically produces noise and vibration while charging may continue. If the pulley slips or breaks, loss of charging is added to the noise.
Can only the pulley be replaced without replacing the alternator?
Yes, provided diagnostics confirm that the alternator's other components are serviceable. On this car the alternator was bench-tested and the pulley replaced, and the brushes, worn close to the limit, were replaced proactively.
Is on-site service available?
Yes. In Tbilisi a technician can be dispatched to diagnose the car and remove the alternator. The repair is done in the workshop and the unit is then reinstalled. See our “On-Site Service” page.
ALTERNATOR GEORGIA — Alternator Diagnostics and Repair in Tbilisi
We specialize in automotive alternator diagnostics and restoration. Every unit is inspected using specialized test equipment to identify the actual cause of failure. Worn or damaged components are replaced when necessary. Visit our service center or arrange our mobile service in Tbilisi.
All completed work and replaced components are covered by a 12-month warranty.
Alternator noise or no charging?
Accurate diagnostics
Price agreed upfront
12-month warranty