Hi,
I recently had a problem with my ML320 and I wanted to share the experience, troubleshooting and final solution.
Read the below if you have one or more of the issues below and I'll cover each one to help you with your troubleshooting.
- Your truck is stuck in first gear and would not shift up
- You are unable to move the shift selector from P -> R\N\D
- You are unable to move the shift selector from N -> R\P, but you can from N -> D.
- You have to tap the brake gently while trying to shift out of the P.
- You have to insert a screw driver in the small hole just below the D selector (Its a small black cover that you can push down) in order to shift out of P -> R/N/D
- You don't see the shift selection position on the instrument cluster LCD display, in other words its just blank
- Your truck is overheating if you are have the AC running while idling for extended period of time
- You are driving your truck at 60 MPH at 2800 -> 3000 RPM
- You brake lights do not work
- You get "Check Engine" light comes on, but after some time it goes away or you turn off the engine and start it up and the check engine light goes away.
- You are in Limp-Home mode but no error codes available when you try to read with ODBII device.
Solution (Assuming at this point that all your connections are OK and electronic devices are grounded properly)
- First I tried reading the codes but the cheap ODBII reader could not find anything. After reading the post on the internet, people were suggesting that transmission control unit/module (TCU or TCM) and ECU keep the codes that only a Mercedes SDConnect system can read and that its causing the transmission to go into Limp-Home mode. Well I don't know if that is true or not, I have not asked MB but to me that seems "unlikely", so this ended being a dead end.
- Second, I decided to check the brake switch (A black switch with two plugs; 5 wires in total goes into it exacl above the brake pedal). I had the issue with my parking brake lights not working, so I replaced this switch, $25 at AutoZone. This seemed to solve the brake light issue but I still could not switch out of P on my shift selector. The pink wire goes from this switch to the shift select module. This pink wire seems to control a small solenoid in shift selector assembly that when you give it power it releases a small lever to allow you to shift out of P position. There is also another pink wire coming from the TCM/TCU that goes to this shift assembly, but I'm not sure which one controls the solenoid. The problem is still there, I tested the pick wires and I'm getting 12V. The shift assembly seems to be getting the power, so what the deal?
- Next I opened the fuse box under the hood and I've tested every fuse and relay (Fuse 22, transmission control) and they all looked ok, but there is something I noticed with relay K26 is that the connectivity was not good, it would spark everytime I plugged it in while I had the AC on. My father also notice that the two fans in front of the radiator and not coming on while AC is running. He said that they should come on while AC is running and he was right, I purchased the new relay and I fixed the connection on fuse box so it does not spark anymore. I've been driving for years like this and I never saw these two fans work so check the K26 relay connections. Well I just fixed a major problem with engine overheating while the AC is running so I suspected that transmission overheated and something burned out.
- Next I did some more reading and people were suggesting that the reason the car is in limp-home mode could be the automatic transmission conductor plate might be bad/cracked leaking or transmission fluid might be low. They were also suggesting that transmission fluid might get into the 13 pin adapter plug, its located on the corner of the transmission plate, with a small metal cover shielding it from the passenger side catalytic converter or exhaust heat. I was not sure about this one, this is a lot work and this does not seem to address the problem with my shift position not even showing up in instrument cluster, but since I wanted to change the fluid anyways I got the conductor plate (Above valve body) ($180), conductor plate adapter plug with O-Ring ($15), trans filter ($25), trans pan gasket ($20), trans dip stick measuring tool ($25), MB (Made in Germany) ATF fluid $20 per liter (5 L needed in this case but full flush will require 9 L, please get the fluid from MB, its worth every penny), trans drain plug ($5) and trans plate drain plug seal ($1). So I watched the video here (Mercedes 722.6 Transmission Shifting Problems Part 1: Do I Really Need a Complete Rebuild? - YouTube) or you can search you tube for Mercedessource 722.6 shifting problems and this guy has six videos on how to change the transmission fluid properly. After I completed this its as I suspected, truck is still in limp-home mode, not shift selection in instrument cluster, but I got the fluid changed! I have to admit, when I took the valve body and realized what I'm doing I immediately regret starting it, but it was too late, I had to keep going.
- Next I had to figure out what electronic part was responsible for displaying the shift (P/R/N/D) in the instrument cluster? Is it TCM/TCU or ECU or Shift assembly? I took the shift assembly apart, I brok few plastic pieces (Don't take it apart!) but it all looked ok, nothing obvious, no damage and no weak connections. The shift lever assembly was getting the power. If there was issue with TCM/TCU I would have to see some kind of error code or not even start the car. So I purchased the TCM from ebay (Wasted $70), I tried it, all lights in dash light up but the engine would not turn. So this tells me that my old TCM was ok, because if it was not, the car would not start. Hmm.
- I did some more reading on line, and I came across only youtube video where some kid that had the same issue I had with car being in limp-home mode and no shift position on instrument cluster. So I've decide the spend $580 for automatic transmission shift lever assembly, I also asked MB if I need to reprogram it and they said NO, it should be plug-and-play. (Bty some people on internet are saying that you have to re-program it and that is not true!; You might have to do it on newer cars but not on the older models). I replaced this and I'm so happy that it fixed the issue. I was able to shift immediately without issues, I saw the shift positions (P/R/N/D -+ 1,2,3,4) are all displaying in dash. Problem solved!
I took the car to the street and run it fast on highway and I noticed it shifts transparently, I also noticed that at 60 MPH engine was at 2200 RPM ( Before at 60 MPH it was 2800 RPM). I've only changed the fluid once, but I will get another 5 L of fluid and change it again. Mixing old and new oil is OK, unless you want only new but that might be expensive and you might have to take it to the dealer to get it flushed out. Conductor plate has bunch of sensors, 2x RPM sensors, temp sensor, speed sensor so in my case some sensor did not work for long time, so replacing the conductor plate is worth it.
But sometimes, you just have to take the car to the dealer as they are equipped with tools to diagnose the problem more quickly so you don't throw money away, I could have been lucky in this case, it might have been worse. I spent $1000 so far and was running out of options, so if the shift assembly didn't resolve the issue I would have to take it to MB for diagnostics which could have been another $1000 on top of mine.
Hope this helps someone out there!
thanks
Dejan
I recently had a problem with my ML320 and I wanted to share the experience, troubleshooting and final solution.
Read the below if you have one or more of the issues below and I'll cover each one to help you with your troubleshooting.
- Your truck is stuck in first gear and would not shift up
- You are unable to move the shift selector from P -> R\N\D
- You are unable to move the shift selector from N -> R\P, but you can from N -> D.
- You have to tap the brake gently while trying to shift out of the P.
- You have to insert a screw driver in the small hole just below the D selector (Its a small black cover that you can push down) in order to shift out of P -> R/N/D
- You don't see the shift selection position on the instrument cluster LCD display, in other words its just blank
- Your truck is overheating if you are have the AC running while idling for extended period of time
- You are driving your truck at 60 MPH at 2800 -> 3000 RPM
- You brake lights do not work
- You get "Check Engine" light comes on, but after some time it goes away or you turn off the engine and start it up and the check engine light goes away.
- You are in Limp-Home mode but no error codes available when you try to read with ODBII device.
Solution (Assuming at this point that all your connections are OK and electronic devices are grounded properly)
- First I tried reading the codes but the cheap ODBII reader could not find anything. After reading the post on the internet, people were suggesting that transmission control unit/module (TCU or TCM) and ECU keep the codes that only a Mercedes SDConnect system can read and that its causing the transmission to go into Limp-Home mode. Well I don't know if that is true or not, I have not asked MB but to me that seems "unlikely", so this ended being a dead end.
- Second, I decided to check the brake switch (A black switch with two plugs; 5 wires in total goes into it exacl above the brake pedal). I had the issue with my parking brake lights not working, so I replaced this switch, $25 at AutoZone. This seemed to solve the brake light issue but I still could not switch out of P on my shift selector. The pink wire goes from this switch to the shift select module. This pink wire seems to control a small solenoid in shift selector assembly that when you give it power it releases a small lever to allow you to shift out of P position. There is also another pink wire coming from the TCM/TCU that goes to this shift assembly, but I'm not sure which one controls the solenoid. The problem is still there, I tested the pick wires and I'm getting 12V. The shift assembly seems to be getting the power, so what the deal?
- Next I opened the fuse box under the hood and I've tested every fuse and relay (Fuse 22, transmission control) and they all looked ok, but there is something I noticed with relay K26 is that the connectivity was not good, it would spark everytime I plugged it in while I had the AC on. My father also notice that the two fans in front of the radiator and not coming on while AC is running. He said that they should come on while AC is running and he was right, I purchased the new relay and I fixed the connection on fuse box so it does not spark anymore. I've been driving for years like this and I never saw these two fans work so check the K26 relay connections. Well I just fixed a major problem with engine overheating while the AC is running so I suspected that transmission overheated and something burned out.
- Next I did some more reading and people were suggesting that the reason the car is in limp-home mode could be the automatic transmission conductor plate might be bad/cracked leaking or transmission fluid might be low. They were also suggesting that transmission fluid might get into the 13 pin adapter plug, its located on the corner of the transmission plate, with a small metal cover shielding it from the passenger side catalytic converter or exhaust heat. I was not sure about this one, this is a lot work and this does not seem to address the problem with my shift position not even showing up in instrument cluster, but since I wanted to change the fluid anyways I got the conductor plate (Above valve body) ($180), conductor plate adapter plug with O-Ring ($15), trans filter ($25), trans pan gasket ($20), trans dip stick measuring tool ($25), MB (Made in Germany) ATF fluid $20 per liter (5 L needed in this case but full flush will require 9 L, please get the fluid from MB, its worth every penny), trans drain plug ($5) and trans plate drain plug seal ($1). So I watched the video here (Mercedes 722.6 Transmission Shifting Problems Part 1: Do I Really Need a Complete Rebuild? - YouTube) or you can search you tube for Mercedessource 722.6 shifting problems and this guy has six videos on how to change the transmission fluid properly. After I completed this its as I suspected, truck is still in limp-home mode, not shift selection in instrument cluster, but I got the fluid changed! I have to admit, when I took the valve body and realized what I'm doing I immediately regret starting it, but it was too late, I had to keep going.
- Next I had to figure out what electronic part was responsible for displaying the shift (P/R/N/D) in the instrument cluster? Is it TCM/TCU or ECU or Shift assembly? I took the shift assembly apart, I brok few plastic pieces (Don't take it apart!) but it all looked ok, nothing obvious, no damage and no weak connections. The shift lever assembly was getting the power. If there was issue with TCM/TCU I would have to see some kind of error code or not even start the car. So I purchased the TCM from ebay (Wasted $70), I tried it, all lights in dash light up but the engine would not turn. So this tells me that my old TCM was ok, because if it was not, the car would not start. Hmm.
- I did some more reading on line, and I came across only youtube video where some kid that had the same issue I had with car being in limp-home mode and no shift position on instrument cluster. So I've decide the spend $580 for automatic transmission shift lever assembly, I also asked MB if I need to reprogram it and they said NO, it should be plug-and-play. (Bty some people on internet are saying that you have to re-program it and that is not true!; You might have to do it on newer cars but not on the older models). I replaced this and I'm so happy that it fixed the issue. I was able to shift immediately without issues, I saw the shift positions (P/R/N/D -+ 1,2,3,4) are all displaying in dash. Problem solved!
I took the car to the street and run it fast on highway and I noticed it shifts transparently, I also noticed that at 60 MPH engine was at 2200 RPM ( Before at 60 MPH it was 2800 RPM). I've only changed the fluid once, but I will get another 5 L of fluid and change it again. Mixing old and new oil is OK, unless you want only new but that might be expensive and you might have to take it to the dealer to get it flushed out. Conductor plate has bunch of sensors, 2x RPM sensors, temp sensor, speed sensor so in my case some sensor did not work for long time, so replacing the conductor plate is worth it.
But sometimes, you just have to take the car to the dealer as they are equipped with tools to diagnose the problem more quickly so you don't throw money away, I could have been lucky in this case, it might have been worse. I spent $1000 so far and was running out of options, so if the shift assembly didn't resolve the issue I would have to take it to MB for diagnostics which could have been another $1000 on top of mine.
Hope this helps someone out there!
thanks
Dejan