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Engine Cranking But Not Starting: A Diagnostic Case Study on Mercedes-Benz CLA W118
When your Mercedes-Benz engine cranks but doesn’t start, it can be frustrating especially when no fault codes appear. This case study on a Mercedes-Benz CLA W118 with the M282 engine explores a real-world scenario where the engine failed to start after the ECO Start/Stop function activated.
Through systematic diagnostics, we discovered a surprising cause a damaged dual mass flywheel (DMF) affecting the crankshaft position signal.
Customer Complaint: Engine Cranking But Not Starting
A customer reported that their CLA W118 wouldn’t start after paying a highway toll. The ECO Start/Stop system had turned the engine off as usual, but the car failed to restart, even after multiple attempts. The dashboard showed no specific warning, leaving the vehicle immobilized.
Initial Inspection
Upon receiving the car, we:
- – Connected a battery charger to ensure stable voltage.
- – Performed a quick diagnostic scan using XENTRY.
No engine control unit (ME) faults were detected, but the transmission control unit (TCU) recorded an error suggesting the engine had malfunctioned during a previous start attempt.
This pointed us toward an engine synchronization or signal issue rather than a fuel or ignition problem.
Step-by-Step Diagnostic Process
1. Data Analysis
Using live data from both engine and transmission control units, all measured values (voltage, RPM, fuel pressure) appeared normal during cranking prompting deeper mechanical and sensor checks.
2. Fuel System Verification
- High-pressure fuel: OK
- Low-pressure fuel: OK
Fuel delivery was consistent. No sign of fuel starvation or leak-back.
3. Ignition System Check
Using the WIS wiring diagram, we tested ignition coil signals with a PicoScope and multimeter:
- – 12V power supply : OK
- – 5V signal voltage : OK
- – Wave pattern : Stable and consistent
4. Injection Valve Testing
Signal voltages of all injection valves were measured during cranking; the waveforms matched the expected pulse timing.
→ Injection control system was functional.
5. Camshaft Sensor Evaluation
- Intake and exhaust camshaft sensors received correct 5V reference signals.
- Signal waveforms showed proper synchronization.
No fault here either.
6. Crankshaft Position Sensor Investigation
While the crankshaft sensor’s power supply was normal, the signal waveform during cranking showed irregular amplitude spikes.
As a test, we removed the crankshaft sensor and attempted to crank again the engine started briefly after extended cranking, indicating signal distortion from mechanical interference rather than an electrical fault.
Deeper Inspection: The Hidden Culprit
Using an endoscope, we inspected the flywheel’s incremental (pulse) wheel and found visible damage.
This damaged section was preventing the crankshaft position sensor from reading accurate rotation signals.
Repair Process
To confirm and repair:
- 1. Removed the engine and dual-clutch transmission.
- 2. Separated the engine from the gearbox.
- 3. Inspected the dual mass flywheel (DMF).
A fragment of the arc spring had broken loose and was moving within the bell housing — physically disturbing the pulse ring.
➡ Solution: Replace the Dual Mass Flywheel (DMF).
After reassembly and testing, the vehicle started flawlessly, with no further fault codes or synchronization issues.
Understanding the Dual Mass Flywheel (DMF)
A Dual Mass Flywheel is designed to absorb engine vibrations and provide smoother drivetrain operation. It’s made of two masses connected by torsional springs.
Main Components
| Component | Function |
|---|---|
| Primary Mass | Connects to crankshaft and stores rotational energy. |
| Secondary Mass | Transfers torque to the transmission. |
| Torsional Springs | Absorb engine pulses to reduce vibration and drivetrain stress. |
How It Works
When the engine rotates, minor vibrations occur at low RPMs.
The DMF isolates these vibrations allowing smoother torque transfer between engine and gearbox.
However, if the internal springs or ring gear fail, it can disrupt sensor readings, leading to no-start or misfire conditions.
Pros & Cons of a Dual Mass Flywheel
| Advantages | Disadvantages |
|---|---|
| Reduces drivetrain vibration | Higher replacement cost |
| Improves comfort & shifting | Springs can fail over time |
| Extends gearbox life | Can cause hard-to-trace starting faults |
Conclusion
This case study demonstrates why thorough diagnostics are critical when facing an Engine Cranking But Not Starting issue.
Even though electrical and fuel systems appeared normal, the real fault stemmed from mechanical damage to the flywheel’s pulse ring.
Replacing the Dual Mass Flywheel fully resolved the no-start condition.
Related Diagnostic Resource
If you’re facing similar starting problems, explore our full troubleshooting hub:
Mercedes No Start Issues: Causes, Fixes & Case Studies — a complete guide covering electrical, fuel, and ECU-related no-start scenarios across multiple Mercedes models.
Conclusion:
This case study highlights the importance of thorough diagnostics when dealing with an Engine Cranking But Not Starting issue. In this instance, the problem was traced back to a damaged flywheel, a component that might not immediately come to mind when diagnosing starting issues. By systematically checking each potential cause, the root issue was identified and resolved, restoring the vehicle to full functionality.
What is dual mass flywheel ?
A Dual Mass Flywheel (DMF) is an advanced type of flywheel designed to reduce vibrations and improve the smoothness of a vehicle’s drivetrain. Unlike a traditional single-mass flywheel, which is a single solid disc, a dual mass flywheel consists of two separate masses connected by a series of springs. Here’s a breakdown of what it is and how it works:
Components of a Dual Mass Flywheel:
- Primary Mass: This part is connected directly to the engine’s crankshaft. It functions similarly to a traditional flywheel, providing inertia to smooth out engine pulses.
- Secondary Mass: This part is connected to the transmission and is responsible for transferring power to the gearbox. It moves independently from the primary mass.
- Torsional Springs: Located between the primary and secondary masses, these springs absorb and dampen the torsional vibrations produced by the engine. This reduces the shock to the transmission and drivetrain, resulting in a smoother driving experience.
How a Dual Mass Flywheel Works?
When the engine runs, it generates rotational forces that can produce vibrations, especially at low RPMs. In a traditional setup, these vibrations would be transmitted directly through the drivetrain, leading to noise, harshness, and potential wear on components.
The dual mass flywheel, however, isolates these vibrations by allowing the primary and secondary masses to move slightly out of phase with each other. The torsional springs between the two masses absorb the vibrations, smoothing out the engine’s power delivery. This reduces stress on the transmission and improves the overall driving comfort.
Benefits of a Dual Mass Flywheel:
- Reduced Vibrations: By absorbing engine vibrations, a DMF provides a smoother and quieter ride, particularly at low RPMs.
- Enhanced Drivetrain Longevity: By reducing shock loads, the DMF protects the transmission and other drivetrain components, potentially extending their lifespan.
- Improved Gear Shifting: The smoother power delivery can result in smoother gear changes, enhancing the driving experience.
Drawbacks of a Dual Mass Flywheel:
- Higher Cost: DMFs are more complex and expensive to manufacture than traditional single-mass flywheels, making them costlier to replace.
- Potential for Failure: Over time, the springs within a DMF can wear out or break, leading to issues like rattling noises or drivetrain vibrations. This can require replacement of the entire unit.
Conclusion:
The dual mass flywheel is a sophisticated component that plays a crucial role in modern vehicles by reducing vibrations and improving the smoothness of the drivetrain. While it offers significant benefits in terms of driving comfort and component longevity, it also comes with higher costs and potential maintenance challenges. Understanding how it works and its role in the vehicle can help in diagnosing related issues, such as those seen in the case study of the Mercedes-Benz CLA W118.
