Car Shuts Off While Driving – Case Study and Diagnostic Process
Experiencing a car shuts off while driving or would suddenly stop from time to time can be alarming and dangerous. This article delves into a real-life case study where a customer faced this issue. We will explore the diagnostic process undertaken to identify the root cause of the problem and the steps taken to resolve it.
Customer Complaint
The customer reported that their Mercedes C-Class equipped with M272 Engine would suddenly stop from time to time. This intermittent issue posed a significant safety concern, necessitating a thorough investigation to identify and fix the problem.
Initial Diagnosis
Fault Code Identification
- Upon connecting the diagnostic tool, a fault code P0335 indicating an “inaccurate signal from the crankshaft sensor” was found. Initially, this fault was stored and not current.
Xentry Wave Pattern Analysis
- The Xentry diagnostic tool showed an alright wave pattern, even though the fault code persisted after erasing. Upon rechecking, the fault code was current, indicating an ongoing issue.
Sensor Socket Values
- Using a multimeter, the sensor socket values (voltage) were checked and found to be accurate according to the guided tests.
Continuity Check
- The continuity from the sensor socket to the ECU pins was tested and confirmed to be “OK.”
Fuel Pressure Check
- The fuel pressure was checked as a precaution and found to be within acceptable limits. Additionally, the engine was not misfiring, eliminating these as potential causes.
Advanced Diagnosis
Pico Scope Analysis
- To get a more detailed view, a Pico scope was used to analyze the crankshaft sensor signal. Although the sensor showed a faulty signal once, it was not conclusive enough to pinpoint the sensor as the primary issue.
ECU Examination
- The engine control unit (ECU) was not original from the manufacturer, raising suspicions about its integrity. Given the intermittent nature of the fault, this aspect required further scrutiny.
Solution
Sensor Replacement
- The crankshaft sensor was replaced with a new one of the same part number. However, the same fault persisted, indicating that the sensor was not the root cause of the problem.
ECU Replacement
- Based on the recurring issue and the non-original ECU, it was concluded that the ECU might be defective. The control unit was replaced with a new, manufacturer-approved unit.
The random and intermittent car shutdowns were ultimately traced back to a faulty engine control unit. Despite initial indications pointing to the crankshaft sensor, thorough diagnostics revealed that the non-original ECU was causing the fault. Replacing the ECU resolved the issue, ensuring the vehicle operated smoothly and safely.
What is the Position Crankshaft Sensor?
In the Mercedes-Benz M272 engine, the crankshaft position sensor is typically located at the rear of the engine, near the transmission bell housing. Here’s how you can generally locate it:
Rear of the Engine
The sensor is mounted on the engine block, close to the flywheel or flexplate. This positioning allows it to monitor the rotation of the crankshaft accurately.
Near the Transmission
Specifically, it is located where the engine meets the transmission. You might need to access it from underneath the vehicle, depending on your specific model and the space around the engine bay.
Sensor Appearance
The crankshaft position sensor usually has a small, cylindrical shape with an electrical connector and a mounting bolt.
How To Change Crankshaft Position Sensor ?
Changing the crankshaft position sensor on a Mercedes-Benz M272 engine involves a few steps. Here’s a general guide to help you with the process:
Tools and Materials Needed:
- New crankshaft position sensor
- Ratchet and socket set
- Screwdriver
- Jack and jack stands
- Wheel chocks
- Safety gloves and glasses
Steps to Change the Crankshaft Position Sensor:
- Remove the air filter housing
- Remove the vacuum line from the brake booster and set it aside
- Remove the hot film mass air flow sensor
- Remove the air guide housing
- Remove the vacuum pump. On vehicles with a vacuum pump
- Unlock the electrical connector of the Hall sensor (B70) and disconnect it
- Unscrew the bolt (1) and remove the clamp on the Hall sensor (B70)
- Remove the Hall sensor (B70) from the engine block
- Place the new sensor in the same position as the old one.
- Perform the initial calibration and then the adaptation of the transmitter with STAR DIAGNOSIS
Crankshaft Position Sensor Test
How to Test a Crankshaft Position Sensor?
The crankshaft position sensor (CKP) is a critical component in modern engines, providing vital information about the position and rotational speed of the crankshaft. This data is used by the engine control unit (ECU) to manage ignition timing and fuel injection. A faulty CKP sensor can lead to engine misfires, poor performance, and even failure to start. Testing the crankshaft position sensor is a crucial step in diagnosing engine problems. Here’s a brief guide on how to perform this test.
Tools You’ll Need
- Multimeter (preferably digital)
- Osciloscop
- Vehicle service manual (for specific sensor location and specifications)
- Basic hand tools (screwdrivers, socket set)
Steps to Test the Crankshaft Position Sensor
Locate the Sensor:
The CKP sensor is typically located near the crankshaft pulley, flywheel, or on the engine block. Refer to your vehicle’s service manual for the exact location, See the location above.
Inspect the Sensor:
Before testing, visually inspect the sensor and its wiring for any obvious signs of damage, such as cracks, corrosion, or loose connections.
Test for Resistance:
- Disconnect the sensor from the wiring harness.
- Set your multimeter to the ohms (Ω) setting.
- Measure the resistance across the sensor terminals. Compare the reading to the specifications in your service manual. A significantly higher or lower reading indicates a faulty sensor.
Check for Voltage:
- Reconnect the sensor to the wiring harness.
- Set your multimeter to the voltage (V) setting.
- Back-probe the sensor connector (if accessible) to measure the voltage while cranking the engine. The sensor should produce a small AC voltage if it’s working correctly. Refer to your service manual for the expected voltage range.
Signal Output Test:
- For a more thorough test, you may need an oscilloscope to check the sensor’s signal pattern.
- Connect the oscilloscope probes to the sensor output and ground.
- Crank the engine and observe the waveform. A consistent, regular waveform indicates a functioning sensor, while irregularities suggest a problem.
Conclusion
Testing the crankshaft position sensor involves checking resistance, voltage, and signal output. These tests help determine if the sensor is functioning correctly or needs replacement. Always refer to your vehicle’s service manual for specific instructions and specifications. If you’re unsure about performing these tests, consider seeking help from a professional mechanic. Ensuring your CKP sensor is in good working order is vital for the smooth operation of your engine.