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Why is My Mercedes Benz Transmission not Shifting?
There are many reasons a Mercedes Benz transmission may stop shifting without notice, the problem could range from Shift control Module, Shift solenoid valve to RPM sensor. which ever might be the case the symptom is always the same, this happens because all modern Mercedes Benz cars are equipped with a fail-safe mode system, so when any major electronic in the drivetrain is faulty, the vehicle computer automatically activates the Limb Mode (Fail Safe Emergency System) to protect the occupant of the vehicle and also to prevent further damage to the vehicle.
If you are having difficulties in shifting your Mercedes C240 gear as a private owner we advise you to take it to the dealer for proper scanning and fault code analysis.
Having said that lets proceed, I am going to base this article on a job we did on C240 which came in with shifting difficulty.
With the help of Mercedes Star Diagnostic tool we got the first glimpse of the problem as a fault code P0715 "RPM Sensor 2 (Y3/6n2) to RPM Sensor 3 (Y3/6n3), implausible" and P0700 "Gear Implausible or transmission slips" with this information we went to work.
The diagram below shows the complete electronic that controls the transmission system, the label marked red, Y3/6n3 and Y3/6n2 are RPM (Revolution Per Minute ) sensors, these are the sensors the scanner identified as generating implausible signal.
To gain access to this sensor a minor disassembly of the transmission is required, first the transmission oil is drained to allow easy access and removal of the transmission cover.
After successful removal of the bottom plate of the transmission, the transmission hydraulic control module was removed as seen in the image below.
Pit view of the vehicle Transmission System after removing the entires EGS system, as you can see the oil was completely drained and the electrical connector linking the EGR to the vehicle was unplugged before a proper separation of the unit from the transmission.
The part labeled (y) is the transmission EGR housing, the RPM sensor is attached to it, to change the RPM sensor the conductor plate labeled (x) must be separated from the EGR housing. in other words, the part labeled (x) must be completely replaced which was what we did.
As you can see one of the sensors is cracked, this is the reason why the transmission was having difficulty in correlating signal generated as one of the RPM sensor was totally damaged.
The Sensor is attached to the conductor plate hence can not be separated, so we got a new conductor plate and replaced it and the vehicle came back to life.
This minor damage to the transmission in some cases leads to a complete transmission change, as you have seen there is no need to panic about the cost of changing a whole transmission as the problem is a common electrical fault.
Apart from Faulty Transmission RPM sensors, when the wheel speed sensors equally fail it goes a long way to activate limb mode, so if your vehicle is currently having stiff shifting the problem could be caused by any of these sensors or worse still the control module it's sending data to.
Disassembled ElectroHydrolic Controller Unit
However, we successfully brought the vehicle transmission back to life, but not without a tool, to identify the defective part we made use of a scan tool. So if you are currently facing this problem, your first line of action should be having the vehicle scanned to avoid assumptions that may not really offer the desired solution.
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