New Case Study Jan 2012 - Mercedes E240

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New Case Study Jan 2012 - Mercedes E240

Post by PicoMike » Fri Jan 13, 2012 12:08 pm

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.png
Position Potentiometer both Signal ok.psdata
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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.png
S1 & S2 Ignition off and on.psdata
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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.png
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
Direct Sales Manager: Pico Technology

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Re: New Case Study Jan 2012 - Mercedes E240

Post by tom.denton » Thu Jan 19, 2012 9:38 am

Good case study that shows the importance of knowledge about the systems and great test equipment. It also illustrates that a logical process in diagnostics is essential.

I wonder how many places would have fitted a new ECU to see if that fixed it...

These case studies are great, keep them coming.


Tom Denton
Automotive Textbooks

Posts: 5
Joined: Wed Mar 30, 2011 6:29 pm

Re: New Case Study Jan 2012 - Mercedes E240

Post by Andy-Cape » Thu Jan 19, 2012 5:09 pm

Hello Tom,
thanks for your comments - I am honored. Of course I read your book "Advanced Automotive Fault Diagnostics".


There was a question from Ron about : where to measure WOT and CTP signals. Look at the wiring diagram of the Bosch throttle body. It has 16 wires. Round 10 wires are for feedbacks to the ECU. In the body are 2 control switches . When the flap opens the switch must leave close position for example. It can only be one switch in close - either WOT or CTP. If both are ON or OFF than the ECU would go in limb mode.
The task was to check why the ECU is in limb mode! And also
The pedal potentiometer has 2 tracks. one for normal acceleration and one for limb (50%).

Finally only the position of the flap itself did not leave the top mark. That was not logical .


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