limp mode bypass. it works.

handypete

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Aug 23, 2021
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W211 E270 CDI
so your Mercedes 722.6 gearbox has gone into limp mode with no warning lamps or messages on the dash & your scanner gets no codes? it will only select 2nd gear in D position? don't worry, its not the end of the world & your car is not destined for the scrapyard. i figured a way to "bypass" limp mode & get the gears working in my own workaround fashion. i did this to 2 mercs without any problems.
i tried everything to get the box working properly, looking for fault codes,trying a different TCM swapping many parts,nothing worked so i tried this.
the TCM controls the shift solenoids in the gearbox, so i simply wired into them with 3 non locking (momentary) buttons & a few bits of wire.really simple but it works fine. i have used this for about a year with no damage or ill effects except to remember what gear your in, coz if you press the wrong button & change down instead of up it can be quite painful. anyhow i will explain the method of wiring.
1.find the TCM under the passenger side carpet against the bulkhead. its on a metal plate held by 3 plastic nuts. drop this down & see the TCM is the box with 2 long plugs running in.
2. remove it from the plate, held by 2 plastic nuts.
3. unplug the 2 plugs & look at the sides of them. you will see numbers for each pin.
4.firstly make a short thin wire (about 10cm should be enough) & bare each end.
5. connect this wire to link pin 29(TCM voltage supply) to pin 38(solenoid voltage supply).#29 is live with ignition on. #38 is dead since the ecu of the vehicle removes the power under limp mode. best way to connect the wire is to slide the bare end down the side of the pin in the plug from the bottom, when you replug hem in it will hold it tight.
6.join 3 longer wires,preferably different colours, no less than 100cm because it must reach from passenger footwell to centre console near the shift lever.
7.connect each of these wires seperately to pins 14(1-2/4-5 gearchange) 15(3-4 gearchange) & 16(2-3 gearchange) in the plug.
8. with all these wires secured, reattach the plugs onto the TCM, & replace it on the metal plate & back into the footwell.
9.route the 3 wires neatly behind the carpeting & up to the centre console.
mount your 3 momentary buttons side by side on something thin but solid, i used an old piece of printed circuit board with all the circuit lines scraped off.
10.attach each of the 3 wires to a button, keeping them in order from left to right, 1=pin 16, 2=pin 15, 3=pin 14.
11.connect another wire in line to the other side of each button & join the other end of this wire to negative ground earth.
12. mount your completed 3 button board to a handy place near the shift lever, either with double side tape or small screws, make sure it is secure & not likely to fall off.

now all of this work should enable you to change gear in a semi-automatic fashion.when you select D with the shift lever your car will select 2nd gear, this is fine for normal flat setting off. drive off in second & when you wish to go to 3rd simply press button 1 & release, voila, 3rd gear it is. when you want 4th, press button 2, & when you want top gear,5th, press button 3.each button press is a quick on/off action, don't hold it down. the changes are not as smooth as full auto but, hey, it works.to change down, like approaching lights or roundabout press the buttons in reverse order but only downshift button 1 to 2nd gear when almost stopped or its a bit violent. or if at standstill, shift lever to P and back to D which will reset everything to 2nd gear setoff.
now you may notice that button 3 controls both 1-2 & 4-5, if you need to select 1st for any reason, like pointing up hill or pulling heavy load, then pres button 3 after moving into D but before moving off,if you do this then you must press 3 again to get into 2nd then do as above.
this may sound complicated & the wrong way to drive your car but if you cant escape limp mode & you've tried everything its the next best thing & after a little while you get the hang of pressing the buttons.
i hope this is helpful to somebody but please don.t hold me responsible if something goes wrong.
 

Tuercas viejas

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You've got to be kidding !!!!!!!!!
Oh a latter day version of Chrysler's power/Torque Flight transmission if the `1960's then!
Coupled of course to a 460 cu in V8 Hemi Chrysler lump .
Watch this for a zany take on US motoring in the 1960's '
Tuercas viejas

P.S. I watched it, and expected at any moment to see Popeye pop up with corner of the screen and say "AND That's all Folks!!!" :p
 

Tuercas viejas

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landrover/rolls royce/MB AMG S600/Prius
We had a 2008 Sprinter in last week with all circuits dead after being struck by lightening, and had put the owner in hospital with concussion..
Eventually having worked through all the obvious dead modules, fried batteries & melted circuits the no start obstacle was the TCM module (little bugger ). Creating then a no start we simply removed it for a fire up. Then obtained a replacement and the van now runs and drives ..
Just need a bolt of lightening up the **** of the insurance company to write a cheque for many "million of dollars" of a repair tariff , the final repair part of the solution.
Tuercas viejas
 
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handypete

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Aug 23, 2021
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W211 E270 CDI
hi, thanks for your replies, very helpful. I have now fixed this. i got an ICARSOFT i980 scanner & it read everything from the car. some codes related to the TCM so i removed the power link i put on the TCM, cleared all the codes & now everything works fine. gearbox changes up n down, w/s switch, kickdown, cruise control, all good. this little gadget saved me heaps of cash & work. alls well with the world.
 

markjay

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so your Mercedes 722.6 gearbox has gone into limp mode with no warning lamps or messages on the dash & your scanner gets no codes?

..i got an ICARSOFT i980 scanner & it read everything from the car. some codes related to the TCM so i removed the power link i put on the TCM, cleared all the codes & now everything works fine...

So the bottom line is that the issue was that you used a generic OBDII scanner instead of an MB-specific OBDII scanner?

On one hand you could have saved yourself a huge amount of work by having the fault codes read with a proper scanner to begin with, but on the other hand you've learnt a lot about the electronics of the MB autobox, so it wasn't all in vain.
 
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handypete

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W211 E270 CDI
to scooby doo & trickythemerc, no i was not kidding/joking. it worked for me coz it was an electrical fault not mechanical so no fear of huge explosions with serious damage to life & limb. i knew it was electrical but with a cheapo scanner i couldnt find out the problem. i wasnt sure if it was just a broken wire coz i removed the old merc radio. anyhow, its now running perfect, yes i learned a lot, i also have a good scanner for future use & some of the parts i changed needed it anyway. job done.
 

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