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

this seems to be a common problem on the 97 e420 since it is equiped with the early 722.6 trans. i am having a similar intermitent problem with my 97e420. mine was fine after i turned off the ignition , this seems to reset the computer. eveyone on this froum advised me to do an oil change first and then go from there. i have read about the tcu problem and i suspect that mine is having this problem too.
my question is, where is the tcu located and what do u use to clean it.?
i heard that you should measure the adaption values from the tcu to see if they are at or near max. if they are then your trans is toast and you will need a new one.

did your car manifest the problem at cold temperatures or during normal driving and warm temperatures?

here is a link to my thread
transmission acting very weird!

When in limp mode voltage is disconnected from all transmission solenoids until the fault is corrected and CLEARED FROM MEMORY. If the fault is not eliminated and erased from memory, the TCM will command limp mode. The TCM will also command maximum line pressure. You have to correct what is causing limp mode, most likely P0720 and or fluid in the TCM.

Dear e420A1
According to the information I have, there are two types of limp modes, see the following description for the two modes:

Electrical limp-home mode
In order to prevent damage to the automatic transmission in the event of an electrical fault, the gear currently engaged is held and the assigned diagnostic trouble code (DTC) is stored.

The limp-home mode comes into effect with the following procedure:


Stop vehicle.
Shift transmission selector lever to "P".
Shut off engine.
Wait at least 10 seconds .
Start engine.
After restarting engine, the vehicle can only be driven in 2nd or reverse gear.

NOTE: The engagement of "N -> D" and "N -> R" will be very harsh, since the electronic control of the automatic transmission has been turned off.
This type of limp-home mode can only reset by repairing the fault and erasing the DTCs with the Hand-Held Tester (HHT).

Mechanical-hydraulic limp-home mode
In order to prevent damage to the automatic transmission in the event of an mechanical-hydraulic fault,


the transmission shifts into 3rd gear and is held in this gear, or
the transmission shifts to the last "known good" gear and is held in that gear.
NOTE: This type of limp-home mode is reset by turning the Ignition OFF, and then ON again

It seems like your car goes into the Mechanical/Hydraulic limp mode, because you say it goes in third gear and can be re-set by simply turning your car on and off.

You are saying that your gear selector is at times difficult to move. As you probably know, there is no mechanical connection between the selector and the gearbox, the selector only actuates an electrical switch. The switch then tells the TCU which gear is selected and the TCU then energizes the appropriate solenoids to control the hydraulic flow in the transmission.

I would think that your shift lever problem is caused somewhere inside the console and is unrelated to your other problem.

My car is definitely in the electrical limp mode. The problem manifested itself both with the transmission and engine warm and cold.

The TCU is located under the hood, on the passenger side, close to the air intake under the windshield. If you stand in front of the car with the hood open, you can see a plastic cover, about 12x12 inches which is held down with 4 philips head screws. After removing the cover you will see a row of control units. The TCU is the second one from the bottom, it has two connectors on the top. I have attached a picture for you.
Once you have it out, you need to open the module, which you can do by sliding a two credit cards in between both sides of the housing and the connector base. I cleaned my board with a special PC board cleaner, which you can get at an electronics store. I also cleaned the wiring harness.


Dear C32AMG,

Thanks for your suggestions .The fluid is definitely gone from the TCM, and I have reset the codes with my generic scantool multiple times. The ones that keep coming back are the 0743 0753 0763 and 0758 codes. They all point towards problems with the solenoids. I have checked the resistance of the solenoids and they are in spec and I can actually hear all of them click when I power them externally.
You say you think that the 720 code which I had at the beginning is a likely cause, have you had this failure? and if so what did you do to solve it.
Thanks

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thanks ace for the detailed response
i appreciate you taking time to explain this (i'm new at this mercedes business)
i will check my tcu soon for any fluids. or maybe i should wait untill the temparutes are above freezing. last time i checked the temp was 14F here in michigan. i'm not looking forward to freezing my ass off in the dead winter cold.
i don't know which is worse having a hydraulic or electric limp mode, both sound severe to me.
by the way, where did you get hold of the technical info. do yo have a mb cd manual?
and one more thing, are you suppose to feel the shifts on the e420 when accelerating normally or is it supposed to be super smooth. i 'm trying to determine if there is slippage or if it is normal trans behaviour.
thank you in advance!

e420A1
I get my information from www.alldatadiy.com. It is a paid subscription. Minuses : I find it hard to understand the way the site is structured, they have a "search" function which hardly ever returns what you are looking for and a lot of the illustrations are miniscule. (you can get around this if you copy them and then blow them up using any picture editing software) On the plus side, it is reatively cheap ($25 per vehicle for one year and $15 to renew for subsequent years or to add another vehicle) and it does have a lot of information. I have never used the MB CD's so I can't compare to those.
Regarding your question about how it ought to feel. Based on my '97 420 and an '00 320 that I drive occasionally I would say the transmissions are smooth but not extremely smooth. You can certainly feel them shift.

There is a host of reasons why the transmission can go into limp mode, some are severe, some are benign, don't despair yet.

ACE
I must admit I did not read all of your posted information. D o you have a printout from MB with any codes or adaptation data read out. If the transmission is slipping it can only mean one of two things, worn clutches as they said, verifiable by the transmission adaptation value data, Or a broken control pressure regulator valve spring, low working pressure; do you know if you have the updated valve body and conductor plate.

The o-rings on the transmission harness and sleeve are leaking, that is how transmission fluid reached the TCM. The fluid in the harness connector, transmission side; could be causing the solenoid codes.

The OBDII scanners and transmission codes are useless as a diagnostic tool, on a lot of models you can’t read the other modules that are linked thru the can bus, which is a must in diagnosing a drivability problem. In my case P0720 was a speed sensor problem, but no slippage in the transmission.


E420A1
There are only four codes that I know of, which would allow a key reset




Code


8 Reverse lockout solenoid
9 Starter lock out relay module, ETC control module output negative
51 engaged gear implausible or transmission slipping; this code will default to 3rd gear
52 command valve sticking in pressure position. One of three valves in the Valve body.


Do you know if your have the updated valve body and conductor plate. Does your transmission slip; it could also be a broken control pressure regulator valve spring if you have the early design VB.

Thanks C32AMG.

You are correct about the o-rings, I have replaced those and so far the leaking seems to have stopped. I guess time will tell though.

I do not have the print out you are referring to and I do not know how I could get the read-out of the adaption values.
At this point it is hard to determine wheter the transmission is slipping or not , since it is not shifting. When I put it the selector in R it will go in reverse, P for park and N for neutral but regardless of whether I put it in 1,2,3,4 or D , the only forward gear it will go in to is 2nd gear. It does not shift up to 3rd, 4th or 5th, nor does it shift into 1st when I switch from the W to the S program. As long as it is in 2nd gear, it does not appear to be slipping but off-course it is most likely to slip in between gear-shifts, not when a gear is solidly engaged.

Here is the scenario. I disconnect the battery and/or I clear codes using my scantool (here is the link in case you are interested http://www.scantool.net/products/elmscan_iso.htm )
I start up the car, Check Enigne light will go off.
Now I would expect to see around 12 Volts when I measure on pin 38 of the TCM. Pin 38 is supposedly the common supply voltage for the solenoids (see attached schem1, wherein N15/3 is the connector on the TCM) but there is no voltage there.

This is what makes me think the TCM is not working right. At that point, I haven't put the car in drive or reverse yet, much less driven it since doing the reset. Even if there was something wrong with the Transmission (worn out or whatever) the TCM would not know it yet, at least that what I think.
Anyway , then I put the car in reverse or drive and immediately I can tell that it is shifting harsh. ( the remark about the TCM commanding maximum line pressure from your earlier post may support that).

I had the TCM rebuilt by autocraftdirect. I assumed it was bad because:

1- it had been full of oil, and I figured that metallic particles suspended in the oil could have shorted out a component.
2- I got a host of codes indicating problems with the solenoid valves. It seemed unlikely that all solenoids went bad at the same time, in addition, I measured the resistance of the solenoids and they are all in spec.
3-according to a troubleshooting chart I have, with the ignition on , pin 38 must carry 12 volts.

Now that I received it back from autocraft, the 12V is still missing. I called them , they really don't seem all that knowledgeable. They claim the TCM only provides ground, but that is not what the schematic indicates. They also could not tell me exactly which components they had replaced . I am wondering if they just replaced some of the most likely failing components of the board on the assumption that those would be the culprit.


So, my suspicion is that either :
-the TCM is still defective
-or somehow the limp mode is never cleared (does removing the battery clear the limp mode or is there some sort of non volatile memory that stores the codes and error conditions in spite of removing the power to the electronics?)
-or the limp mode is re-enabled as soon as I start up the car because there is some hard failure in the transmission.( this last option seems unplausible to me since as I said the 12 v are missing before I even take it out of Park).
-Can anyone confirm if the 12 v is supposed to come on to pin 38 as soon as ignition is turned on?

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A coffee spill seems to have triggered electronic limp home mode (2nd gear only) in a 99 C230 Kompressor. Working the gearshift while driving plus interior heat (it is below freezing here) seems to have rejuvanated the "D" light, which was not illuminating.

Wondering if there is anything to try today (Sunday) before taking to the dealer Monday. I am hoping the problem is one of mechanical contact rather than electrical damage. Earlier post suggests even if the shifter has returned to normal the car will remain in limp-home mode until the fault is reset.

Is is feasible to get at this mechanism to see that it is cleaned out?

....Electrical limp-home mode ...In order to prevent damage to the automatic transmission in the event of an electrical fault, the gear currently engaged is held and the assigned diagnostic trouble code (DTC) is stored.
...MOST likely! some liquid has been spilled on the gear shifter! IT is a computer & computers don't like liquids!...MERCEDES BENZ MASTER GUILD TECHNICIAN (6 TIMES)...
...the vehicle can only be driven in 2nd or reverse gear.
NOTE: The engagement of "N -> D" and "N -> R" will be very harsh, since the electronic control of the automatic transmission has been turned off.
This type of limp-home mode can only reset by repairing the fault and erasing the DTCs with the Hand-Held Tester (HHT).

Pin #38 on the 14 pin connector of the TCM is the solenoid voltage supply which becomes pin #6 on the transmission harness.

This is the limp mode strategy, once the TCM detects a transmission operating fault or a network input fault from the engine and chassis management systems, the transmission control module turns off (pin #38) all power to the solenoids in the valve body. That’s why you have NO VOLTAGE at pin #38. Working pressure is set at maximum value; the transmission is locked into the gear it was in before the fault occurred.

If you stop the car, turn if off, wait at least 10 second then restart the car move the lever into Drive, the transmission will fail safe into 2nd and reverse gear. The transmission will stay in Limp mode until the malfunction is fixed and erased from memory. Some codes reset automatically or by key reset.

Unique shift pattern:

The 722.6 transmission shift solenoids are an on /off solenoid and stay OFF to control the gear they are in. If You’re driving and the transmission control module commands a 3rd to 4th gear up shift, the y3/6y4 solenoid is switched from OFF to ON and OFF again to make the shift. While in 4th gear the y3/6y4 solenoid is OFF, no voltage.

This explains why, when the transmission control module detects a fault while the car is being driven, the transmission is locked into the last gear it was in before the fault occurred until the car is turned off.

I would remove the pan and check for any signs of a transmission failure and while you are in there get all the numbers from the valve body. To diagnose

boblane,

Do not turn the car on

Try drying the spill with a blow dryer. That all you can do, tomorrow when you strat the car you will know if it worked. If it set a code it has to be erased with a scanner.

Good luck.

You cannot get the car out of limp mode without a Mercedes HHT or a Snap-on scanner with the right module.

I learned this the hard way with a 1997 SL320 that had a bad speed sensor on the conductor plate. Even after you repair the fault, the system will still remain in limp mode. You'll have to find a friend that has a Mercedes shop or go to the dealer and ask them to reset the TCM.

Your TCM is probably fine. It just needs a reset.

Is this THE Herbert Phillips of Memphis?