A 2007 Mercedes-Benz S350, powered by Mercedes-Benz M272 engine, mileage 43 000 km. When driving the car or the engine idling sometimes stalled, and after stalled a period of time, the engine can not start.
Maintenance staff connected to the fuel pressure gauge continued to idling observe the vehicle. After about 30min vehicle automatically turn off again, this time fuel pressure gauge measured fuel pressure is about 50kPa, the value show that fuel system pressure is too low, the provisions of the fuel pressure is around 1000kPa. Fuel pressure created by the fuel pump, if the fuel pump does not work, the vehicle fuel system can not be established oil pressure. So maintenance staff check fuel pump related lines, not found signs of damage or corrosion, and then query the circuit diagram by workshop information system, turn on the ignition switch and measure the fuel pump connector 1st terminal (ground terminal) and 3rd terminal (supply-side) between the voltage is 12.2V, standards-compliant, fuel pump relay can be closed normally, it appears that the vehicle has returned to normal at this time.
Maintenance staff through the above diagnostic process analysis, think the direct reason of this failure is fuel pump intermittent not working. May be fuel pump, fuel pump relay intermittent failures, or engine control unit failure. In order to find the point of failure as soon as possible, maintenance staff decided to replace the fuel pump and the fuel pump relay, if after replaced fault cleaned, that means the fault point among the two, otherwise check the engine control unit. After replaced to test, the engine after running 20min at idle speed, again appeared automatic flameout phenomenon, at this time directed at the engine control unit and related lines.
Maintenance staff use Mercedes Star Diagnose
connection engine control unit and harness, when the vehicle is turned off failure occurs, measured the voltage between 101 terminal and 113 terminal of diagnosis harness box 050, the result is 0, that mean the circuit have disconnection. In order to find the disconnection point, maintenance staff have also measured the voltage between 101 terminal and 113 terminal and the ground of diagnosis harness box, found the voltage between 113 and the ground is 0, that mean the corresponding engine control unit N3/10 17 terminals and circuit disconnection, through measuring determined the external lines without failure, it can be concluded that the engine control unit internal fault, causing fuel pump relay works abnormal, then leading to the vehicle turn off fault.
After replaced the engine control unit, maintenance staff programmed upgrade the control unit and transport SCN code (control software calibration coding) by mb star c3
, finally after repeated test, the fault does not appear.
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