Owner's Problem Description
The owner reports that the engine of their 2006 Mercedes Benz S350 shuts off automatically after about 35 minutes of driving. After the first shutdown, they have to wait approximately 5 minutes before the engine will restart, but on the second occurrence, it only runs for about 10 minutes before shutting off again. Diagnostic tests show various errors, including ABS, ESP, gearbox, battery current, and sensor issues, all of which are erasing codes. The owner has already replaced the battery, crankshaft, and fuel pump, but the problem persists. Notably, when the engine shuts off, the odometer continues to increase without the vehicle being driven.
Professional Technical Analysis
This 2006 Mercedes Benz S350, operating with a 3.5L V6 engine and approximately 130,000 miles, is exhibiting a critical intermittent stalling issue. The vehicle shuts down after roughly 35 minutes of operation, followed by a second occurrence occurring within 10 minutes. Notably, during the stalling, the odometer continues to increase, suggesting potential electrical faults. The owner has already replaced the battery, crankshaft position sensor, and fuel pump, but the symptoms persist. Initial diagnostics indicate multiple error codes related to the ABS, ESP, gearbox, and battery current, most likely triggered by low voltage conditions. The front battery shows 12.5V when off and 13.8V when running, while the rear battery presents concerning readings of 11.8V off and 13.5V running, indicating possible charging issues. Voltage under load must be verified to ensure that the charging system is operational. The PCM (Powertrain Control Module) is likely affected by these voltage fluctuations, leading to erratic behavior in fuel delivery and ignition timing. The current condition of the vehicle raises safety concerns due to the potential for stalling in traffic, necessitating immediate comprehensive diagnostics of the electrical and charging systems.
Possible Causes
Most common causes (ordered by frequency):
- Charging System Failure: The rear battery's voltage of 11.8V when the engine is off raises concerns about the charging system's functionality. The ideal charging voltage should be between 13.8-14.4V under load. If the alternator is not providing sufficient voltage, it can lead to low voltage conditions that trigger erroneous fault codes in multiple systems, including the PCM, ABS, and ESP. A complete charging system evaluation should include inspecting the alternator output, B+ terminal connections, and wire integrity. - Observed voltage readings of 12.5V (off) and 13.8V (running) for front battery, 11.8V (off) and 13.5V (running) for rear battery, indicating potential charging issues.
- PCM Voltage Sensitivity: The Powertrain Control Module (PCM) is highly sensitive to voltage fluctuations. When operating below 12V, the PCM may trigger multiple fault codes due to insufficient power supply, particularly affecting critical functions such as fuel injection and ignition timing. This sensitivity can lead to erratic vehicle behavior and stalling. - Multiple error codes present for ABS, ESP, and gearbox likely due to low voltage affecting PCM operation.
- Faulty Sensors: The potential failure of key sensors, including crankshaft position and MAP sensors, may contribute to erratic engine performance. If these sensors are providing incorrect readings due to electrical issues or internal failures, the PCM may not operate correctly, leading to stalling. - Symptoms of stalling and increasing odometer readings without vehicle movement suggest sensor malfunction.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Diagnosis: Begin with a thorough scan using a professional OBD-II scanner or the GeekOBD APP to retrieve any stored fault codes. Pay attention to the freeze frame data which can provide insight into environmental conditions at the time of code setting. This helps in understanding if low voltage is a recurring issue.
- STEP 2 - Charging System Evaluation: Test the charging system under load by running the engine with all electrical accessories turned on (lights, A/C, etc.). The voltage should be between 13.8-14.4V at the battery terminals. If voltage drops below this range, inspect the alternator output, battery condition, and connection integrity.
- STEP 3 - Sensor Functionality Testing: Utilize the GeekOBD APP to monitor live data from critical sensors, including the crankshaft position sensor and MAP sensor. Ensure that these sensors are providing accurate readings during operation. Check for any erratic behaviors or voltage drops that could indicate failure.
- STEP 4 - Road Test for Verification: Conduct a controlled road test while continuously monitoring live data through the GeekOBD APP. Document any voltage fluctuations and performance issues. Pay particular attention to the odometer reading behavior during engine shutdowns to analyze patterns that could indicate electrical issues.
