handy pete
New Member
- Joined
- Aug 23, 2021
- Messages
- 7
- Reaction score
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- Your Mercedes
- 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.
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.
