I have a 97 E420 with 109K miles.
A month and a half ago transmission starts slipping between gearshifts and shifting harsh. Dealer says "gear selector switch " is bad. They replace it the switch , tell me that harsh shifting and slipping between gear shifts is solved but during test drive they found that transmission is slipping too much under load. They state transmission in worn and need replacement. I said no thanks, I'll pick up the car and drive it with this alledged slip.
When I drive the car off the lot, I notice it still slips in between gear shifts, even when accelerating slowly and it still shifts harsh ( with a jolt). I call the dealer, they invite me to come back and have it checked again. Due to time constraints I could not get to them right away so I drive the car around while babying it for a couple of days. At some point the car goes into electric emergency limp mode, I take it to the dealer, they check it out, try to reset the limp mode and say that the error codes keep coming back and that it is truly a worn transmission. I still don't believe it so I go to pick up the car and now it does not engage any gear at all anymore. I tow the car to my house and start troubleshooting. With my scantool I find codes p0743 p0753 p0763 po758 , all indicating some problem with the shift solenoids.
I read in some forum that oil can find it's way into the Transmission COntrol Unit and cause problems like this. I take out the TCU and sure enough its full of oil. I clean it meticulously and re-install it . I clear the codes using my scan tool and now at least the transmission goes back into Limp mode , meaning I have second gear and reverse available. But, car is not shifting up out of 2nd gear. I measure the wiring harness from TCU to transmission and determine it is fine. I measure the resistance of the solenoid valves in the transmission cover and they all read around 3 ohms which is in spec. I Measure the voltage on Pin 38 of the TCU, which is supposed to be the common 12V supply to the bank of solenoids in the tranny and I find that there is NO voltage there. ( Apparently the +12 V is common and the solenoids are energized by switching their grounds)
The p07codes keep coming back shortly after I clear them.
I figured the TCU must be damaged by the oil or by metallic particles suspended in the oil so I send it in for repair to autocraftdirect. The shop does a wholesale replacement of a bunch of components , they test the unit say it's good and I receive it back. I install it and I still have the same problem, the car is still in limp mode.
I still get codes P0743 P0753 P0763 P0758 and now also P0715 and P 0720.
I measure again the voltage on pin 38 of the TCU and find that there is still no 12volts there.
Now here are my questions.
Does the lack of 12 volts on pin 38 of the TCU tell me that the TCU is still bad or could it be that the TCU only puts the 12v out there if a bunch of conditions are met?
Could it be that the ECU is telling the TCU to stay in Limp Mode or does the TCU decide that for itself?
Could it be that although my scan tool is telling me that I succesfully cleared the codes, they really aren't cleared unless I use a MB "Hand Held Tester"?
I disconnected the battery for about 30 minutes, would that have cleared the codes and should it have reset limp mode?

Thanks for your insights,
Sancho
San Jose,
CA