Engine Cranking But Not Starting : Case Study & Solution

Engine Cranking But Not Starting: A Diagnostic Case Study on Mercedes-Benz CLA W118

In this article, we’ll explore a real-world diagnostic case involving a Mercedes-Benz CLA W118 equipped with the M282 engine. The issue? The vehicle was cranking but not starting after the engine shut off due to the ECO Start/Stop function. This case study will walk through the diagnostic process, highlighting the steps taken to resolve the issue and restore the vehicle’s functionality.

Customer Complaint: Engine Cranking But Not Starting

The customer reported that their Mercedes-Benz CLA W118 was Engine Cranking But Not Starting after paying a highway fee, during which the engine had turned off due to the ECO Start/Stop function. Despite multiple attempts, the engine failed to start, leaving the vehicle stranded.

Initial Inspection:

Upon receiving the vehicle, the first step was to connect a battery charger and perform a quick diagnostic test. Surprisingly, there were no relevant fault codes from the engine control unit. However, the transmission control unit recorded a fault code indicating that the engine had malfunctioned.

Diagnostic Process:

1. Data Analysis:

• I thoroughly checked and read all actual values from both the engine and transmission control units. All values appeared normal, leading me to focus on the starting system.

2. Fuel System Check:

• High-pressure fuel: OK
• Low-pressure fuel: OK
• Fuel supply was confirmed to be functioning correctly.

3. Ignition System Check:

• Using the WIS wiring diagram, I checked the signal voltages of the ignition coils using a PicoScope and multimeter.
• 12V power supply: OK
• 5V signal voltages: OK
• Wave pattern: Consistent with expected values.

4. Injection Valves Check:

• I examined the signal voltages of the injection valves during cranking with a PicoScope. The readings were within normal parameters.

5. Camshaft Position Sensors:

• The power supply (5V) and signal voltages of both camshaft position sensors (exhaust and intake) were checked, and the wave patterns were correct.

6. Crankshaft Position Sensor Investigation:

• The power supply to the crankshaft position sensor was normal. However, upon checking the signal voltage during cranking, I noticed anomalies in the wave pattern.
• I removed the sensor from the engine, and when I attempted to crank the engine with the sensor removed, the vehicle started, though with prolonged cranking.

Deeper Inspection:

Suspecting an issue with the flywheel’s incremental wheel (pulse ring), I used an endoscope to inspect it and discovered damage to the pulse ring. This damage was likely causing the signal issues with the crankshaft position sensor.

Repair Process:

• The next step involved removing the engine and dual-clutch transmission, followed by separating the engine from the gearbox.
• Upon closer inspection, the flywheel (Dual Mass) was found to be damaged, with a piece of the arc spring broken and loose within the bell housing.
• The damaged flywheel was replaced with a new Dual Mass flywheel, which resolved the issue completely.

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:

1. 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.
2. 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.
3. 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.