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Case Study - Mercedes E240 Does not Start
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AuthorMike
Fri Jan 13, 2012 1:08 pm
Details: Andy Cape - Cape Auto Diagnostics |
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1. Vehicle Details:
- 1998 Mercedes E240
- W210
- Engine No M112
2. The Problems
Engine does not start!
3. Investigation with Scan Tool
No current faults logged in system (ME2-SFI), however a condition (live data) was stored: Safety fuel shut off is ON.
4. Investigation with PicoScope
Somewhere deep in the Mercedes Manuals I found a hint that a SAFETY Fuel Shut OFF is initiated by the ECU when there is something wrong with the throttle body. I then went ahead and carried out some checks. Firstly, to check the pedal position potentiometer is working correctly; both sensors can be seen below standard blue and emergency limp home sensor red.
 Position Potentiometer both Signal ok.psdata- (1.39 MiB) Downloaded 72 times
Next, I checked the throttle body end stop switches. Both seem to be correct (S1 & S2) for CTP (Closed Throttle Position) and WOT (Wide Open Throttle). One operates at 4.2 V the other at 0.8 V. I am not sure if the values are correct but I can tell it is switching at least.
- S1 & S2 Ignition off and on.psdata
- (1.07 MiB) Downloaded 31 times
The throttle valve position potentiometer output remains constant at 4.2 V (blue), with no change during re-programming or at ignition ON. I can hear the throttle motor working but the position is frozen at almost maximum. The Motor voltage (12 V green) is switched off at the moment I want to start the car or push the gas pedal. This is due to a safety fuel shut off being activated by the ECU. Red is showing the GND to body GND.
- No engine start but as soon gas pedal is pressed - motor 12V shut off.psdata
- (1.24 MiB) Downloaded 90 times
5. Result
After removing and inspecting the throttle body it was clear the throttle valve had heavy carbon deposits, causing the throttle valve flap to be stuck in the open position. The ECU then switched to a safety mode "fuel shut off".
After cleaning and re-programming (adaption reset) the car started without problems and the idle speed was fine.
Case Study Information
Detail: Andy Cape - Cape Auto Diagnostics
Author: Mike Valentine Mike Valentine
Assistant Technical Author - Pico Technology |
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