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GL X164 Trans Conductor Plate Speed Sensor Fix

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GL Class (X164) 2007-2012: GL320CDI, GL420CDI, GL450, GL550

GL X164 Trans Conductor Plate Speed Sensor Fix

 
Old 03-28-2019, 04:59 PM
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GL X164 Trans Conductor Plate Speed Sensor Fix

This is my experience with fixing my GL320's (X164) 722.9 Transmission Conductor Plate Speed Sensor.

I was experiencing the transmission going into limp mode (2nd or 3rd gear only, enough to get you to the side of the road), usually after a hard acceleration event, or sometimes at random. The issue started increasing in frequency as time went on. First once a month maybe, then once a week, then to where we would expect the transmission to go into limp mode at least once per drive.

Scanning for codes, I found two faults. Sometimes one or the other, sometimes both:
  • Y3/8N1
    • 0717 - The signal from component Y3/8N1 (Turbine Speed Sensor (VGS)) is not available
    • 0718 - The signal from component Y3/8N1 (Turbine Speed Sensor (VGS)) is defective
In the below diagram, Y3/8N1 is noted as the Turbine RPM sensor. This is the input speed sensor.



The faults indicated are a signal not present or intermittent. The speed sensors Y3/8n1 and Y3/8n2 (and Y3/8n3 output RPM sensor, for that matter) are hall-effect sensors, and sense tone-wheels (toothed rings) at these stages in the driveline. When the hall effect sensors sense a tooth they output (usually) a 5V signal, and a valley (no tooth) will be 0V. This creates a square wave as the tone wheel passes the sensor. The faster the changes in hi to lo, lo to hi, the faster the frequency detected by the sensors. These sensors are used by the Transmission Control Unit -TCU (which is integrated in the conductor plate) to determine revolutions per minute (RPM) at these stages in the transmission. The TCU uses these signals to determine if slip is occurring and to calculate when to shift and what gear to shift into, based on vehicle speed and torque input (accelerating hard, easy, decelerating, etc.). The issue with these sensors, I've learned, is that they either go bad after several years of use -which seemed to by my case, or they have a buildup of metal shavings from the clutch and other various transmission components over time that impede the ability to accurately read changes in tooth depth.

My issue was specifically with the input sensor Y3/8n1, but the same thing could happen to the Y3/8n2 internal sensor or even the Y3/8n3 output sensor. The output sensor I believe is less likely to these problems, I think this is because it is located in a different, less harsh environment that the first two are in. That's my speculation though. Interestingly, the output Y3/8n3 sensor seems to be a hard sensor to get a hold of or have repaired. If this sensor has gone bad for you, it might be able to be replaced still, but it may take a bit more research to find someone able to replace this, or even able to locate the sensor.

Now, in order to fix my Y3/8n1 (input turbine sensor), I had a few options:
  • First, I could have elected to take it to someone else (or the dreaded dealer) and get reamed over the coals for service (somewhere north of $2k, if I remember, sheesh! According to what MB says, the conductor plate, which is the plastic tray that sits above the transmissions valve-body, and contains sensors, shift solenoids, and the TCU, would have to be replaced in its entirety. Yes, this would mean a new TCU, because they decided to locate the TCU to the conductor plate inside the transmission. And yes, this would mean that the new TCU would have to be mated to the Engine Control Module (ECM), or Unit, ECU, whatever acronym you want to use here. And yes, since each module has security features to prevent theft (or DIY work!) this requires an MB star diagnostic tool that's licensed and has a link to Mercedes-Benz's database, so that they can be officially paired. See the rabbit hole here?? This is over the top rediculous. Finding even a certified mechanic (that has this diagnostic tool and the license to use it) that's not explicitly a MB dealer would cost such a great amount. -And rigtfully so; the poor mechanic must recoup his license costs somehow.. So, this is out. I'd say this isn't necessary even if you don't plan on doing this yourself, because of option two; see below.
  • Second, I could send the conductor plate into someone to replace the sensor. There are a few places I've found around the interwebs that offer such service. They will usually replace both internal sensors (the Y3-8n1 input and Y3-8n2 internal sensors). I've found a good and reputable business charging $300 for this service. This is much easier to swallow.
  • Third, I could take the self-service a step further and do it myself, purchasing the sensors and a punch-removal tool from ebay for ~$40. That seemed great. So actually, that's what I tried first. Verdict: I wouldn't recommend it for the faint of heart. I received the new sensors and a punch tool that was supposed to be used to cut the plastic around the sensor so it could be extracted. Sounds good in theory, and it did work in the ONLY TWO videos I could find of someone using this tool. One was in Chinese and the other in Russian I think. Both were so shaky you could barely tell what they were doing. But it seemed simple enough: Desolder and detach the ribbon cable and use the punch to remove the sensor. After a few wacks with the punch and a hammer the punch started to fold on itself, loosing its sharp edge, and I figured if I kept at it I would crack the conductor plate and then be up a creek, trying to find a new conductor plate (and TCU) and being forced to go with option one above.
So, I decided to go with option two. I drained my transmission oil, removed the pan, removed the valve body, and then detached the conductor plate from the top of the valve body once on the bench. I then called circuitboardmechanics, told them my story, and sent the conductor plate to them. I paid upfront over the phone and they emailed me a shipping label. I received it back in 6 or 7 days and plopped everything back in, threw on a new filter, gasket, and drain poppet, and filled with fluid. It has been operating perfectly for just over six months now with a few ~5klbs trailer tows, many short trips, and 6 or 8 long (550 mile) road trips. I even noticed that the clunking issue is gone, which would happen usually when the vehicle is cold, and coming to a stop the transmission will shift too early into 1st and give a bit of a jolt, just before the brakes actually bring the vehicle to a stop. So that's gone too. It drives now how I imagine it drove when new.

So there's the victory story, but now for the helpful part.
First, here is the transmission fluid, gasket, filter, aluminum oil-pan bolts, and transmission overflow plug kit I bought from FCPeuro:
722.9 Transmission Service Kit
The kit was $82.99 when I bought it, but at the time (2019 March 26) of this post it is now a hefty $108.92. Still not horrible, but maybe you can find such a kit cheaper somewhere else. Maybe not.

Here's the most important first step: Be SAFE about it! Put the car on solid jack stands so you aren't squished! I found another clever use of the air-ride suspension for this service: Before you start the job, raise the vehicle to its highest level, then put jack stands under the frame rail. Be sure to prop the car up and not to rely on the suspension holding up. Remember that this vehicle does the auto-leveling thing even when the car is turned off? And that sometimes it levels by squatting down a bit? -And that it could try to squat right on your chest. So I played it safe here and used jack stands and always kept one door open. The auto-leveling won't work if one of the four doors are open. The tailgate isn't baked into this equation, unfortunately).
So, the actual first step here: Remove the harness from the transmission. Before Valve body is removed, after oil pan is dropped, remove the connector and then remove that connector insert with a 7mm hex bolt:
I can't remember if mine had the bolt or not. It's a good idea to check though. If there is a bolt holding the harness on, remove it.

Here is a good guide to draining (and refilling) the transmission oil and replacing the filter, gasket, and bolts:

Below is the Valve body Removal/Installation guide I used when removing it.
722_9 VB Removal-Installation Guide Edited.pdf

Here is a video of removal and installation. It should help you understand how you need to wiggle and pull the valve body. Unfortunately the video is a bit simplistic, cutting off some of the finer details, but between this video and the instructions above you should have no issues.

Lastly, here are a couple of documents on the 722.9 transmission, just in case you were interested in more intricate details and interworkings:
722.9 a.pdf
722.9 b.pdf

I will have to come back to this post to add my personal photos once I get Flikr figured out, or a new place to host images. Not much else to add though, the above information should be great resources to get this job done.

Good luck!
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Old 03-28-2019, 09:26 PM
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Good man. After many hours of research and staring at this problem from a clueless beginning you have effectively crystallized what needs to be done to fix this 722.9 transmission problem. Let this be a solid guide for those in the future that will run into this issue. I'd rather pay $500-600 than $2300 as I was quoted by the stealership!

Last edited by 86ortega; 03-28-2019 at 09:34 PM.
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Old 03-28-2019, 11:17 PM
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E550, Gl550,
I actually had similar experience.

My 2011 GL550 decided to not shift anymore, so RPM's would increase without the car's speedometer actually increasing. This happened during a road trip of course, so we took it to the nearest dealer to have it looked at and also received a loaner. They diagnosed it as a bad wheel speed sensor on the left front wheel and wanted $800, including labor to replace that speed sensor. I declined, towed it to a friends house and replaced it myself. Part no. 1649058200. Car was back running.

Fast forward a couple of weeks: with me knowing the left front speed sensor had failed, I decided to replace the other 3 wheel speed sensors myself. Part no. 1x 1649058200 and 2x 1649058300.

Car was driving smooth one day until same thing happened again! Checked codes this time and found the turbine speed sensor faulty code. Cleared codes and continued driving.
Couple of weeks later, GL had the same symptoms, RPMs increase without speed increasing. Checked codes with Autel Maxisys, and found turbine speed sensor faulty. Did some more research and found that the 722.9 transmissions must have the conductor plate replaced due to the faulty sensors vs the 722.5 transmissions where the sensor is replaceable DIY. Further research concluded that the 722.9 transmissions conductor plates must be "NEW" because they must be "VIRGINIZED" with the car.

We took the GL to our local dealer and had them diagnose it to confirm it was the conductor plate. However, my service advisor could not figure out what was wrong since the problem was intermittent, he even requested for the GL to go home with the technician through the weekend. Monday, I come back and the technician says "everything is fine with the car and I can't find any codes on there......". I show them my code that I got through my scanner and they still claim the car is fine. Literally, 4 days later, I'm driving home, about to turn on the local roads, and same symptoms occur AGAIN!

We then called around the area with different dealers regarding quotes for replacing the conductor plate. Cheapest I quote I received was $1500 including parts and labor. We proceed with that quote and drove 1 hour and 30 minutes to drop the car off. Rental was given to us. 2 days later, the service advisor calls saying he can't do the repair because he can't find any actual code, so Mercedes-Benz does not authorize repairing the conductor plate if no actual codes are found......, even if I wanted to pay out of pocket.......

Did further digging and digging and found some had relief of the transmission fluid being flushed and replaced. The reason behind this is the "metal shavings" OP mentioned may what be causing the sensors to go bad. If fluid was flushed and refilled, then there could be a chance you fix this problem. Some said that the reason the transmission conductor plate sensors fail is because of neglected maintenance and following the maintenance schedule of the truck. I went to a local indy and did the job. $300. It has almost been 8 months without the conductor plate symptoms . Knock on wood........
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Old 03-30-2019, 07:12 PM
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Smash that Thanks button, guys. This is a serious quality post.
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Old 05-02-2019, 12:30 AM
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Two GL450's - one with EORP, one without.
Very nice writeup.

If one had a Star system, could one reprogram one's new VSS using the resident coding in that system - or does it HAVE to be connected online to and MB database?

Thanks.
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Old 05-02-2019, 10:40 AM
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If you are just replacing the speed sensors in the conductor plate, no programming is necessary. If you are replacing the whole conductor plate (which includes the Transmission Control Module), then you will have to pair/marry the new TCM to the ECM, which will require STAR, and as far as I have researched, must be connected to Mercedes Benz's server/database for "security". My feelings are more toward making this a pain in the @$$ for anyone trying to do a repair, but that's my silly opinion.
This is why I'd suggest the much cheaper and much easier option of simply replacing the speed sensors on the conductor plate. This way, you are still using the original TCM and the ECM/vehicle is none the wiser.
You will likely not even need to clear codes, as they will clear as they see a good signal from the speed sensors once the fix is made. If the codes are lingering after the fix, you can use any simple off the shelf OBD tool to clear codes. I use an 'el cheapo $10 bluetooth OBD 2 adapter and clear it with an app on my phone, but you could more easily drive to most any car parts store and ask them to clear your codes for you. They will do that for no charge.
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Old 06-19-2019, 06:55 PM
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Originally Posted by 4loops View Post
This is my experience with fixing my GL320's (X164) 722.9 Transmission Conductor Plate Speed Sensor...
Thanks very much for the write up. I was curious to know if you may have had any issues with the transmission prior to it going into limp mode?

I have a CLK550 (different car but same trans), and I have a rough shift in 1st and 2nd gear mostly when cold (and sometimes when warm). Did you have any such issues?

My car has no codes when scanned with STAR, but there's obviously something going on. I'm very tempted to try this fix to see if it might improve anything.

Thanks for any info you may be able to provide.
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Old 06-21-2019, 10:54 AM
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Hey nkx1,

Yea, I do believe I had experienced intermittent hard shifts prior to replacing the speed sensors, yes usually when cold. I'm remembering the most notable thing was when slowing down and coming to a stop, when the trans would shift too early into first gear. My theory here is that if (in my personal experience) I was seeing codes for intermittent/implausible signal from the trans input speed sensor, (or any speed sensor I'd assume could cause this), then the TCU isn't seeing correct speed, and if it thinks the vehicle is moving slower than it actually is it might think its time to downshift. This could also be the issue with yours, and that the signal is not bad enough to throw a fault. That's my theory though. My guess is that your thought process is bang on, I'd also suspect speed signal(s) to be providing a less than perfect signal. Unfortunately, there's no easy way to throw a mobile scope on the output of the hall-effect speed sensors, as they go directly to the TCU, which is inside the transmission. That is, unless STAR has a graphical monitor for speed sensors, but I doubt it. Not having a STAR, I can't be for sure on this though. I'd hate to recommend you go through this process as it's basically throwing money and effort at it and hoping that'll fix it. However, without a good method to monitor signal quality from the speed sensors there might not be many other troubleshooting options.

Good luck bud.
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Old 06-21-2019, 11:44 AM
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Originally Posted by 4loops View Post
Hey nkx1,

Yea, I do believe I had experienced intermittent hard shifts prior to replacing the speed sensors, yes usually when cold. I'm remembering the most notable thing was when slowing down and coming to a stop, when the trans would shift too early into first gear. My theory here is that if (in my personal experience) I was seeing codes for intermittent/implausible signal from the trans input speed sensor, (or any speed sensor I'd assume could cause this), then the TCU isn't seeing correct speed, and if it thinks the vehicle is moving slower than it actually is it might think its time to downshift. This could also be the issue with yours, and that the signal is not bad enough to throw a fault. That's my theory though. My guess is that your thought process is bang on, I'd also suspect speed signal(s) to be providing a less than perfect signal. Unfortunately, there's no easy way to throw a mobile scope on the output of the hall-effect speed sensors, as they go directly to the TCU, which is inside the transmission. That is, unless STAR has a graphical monitor for speed sensors, but I doubt it. Not having a STAR, I can't be for sure on this though. I'd hate to recommend you go through this process as it's basically throwing money and effort at it and hoping that'll fix it. However, without a good method to monitor signal quality from the speed sensors there might not be many other troubleshooting options.

Good luck bud.
Right on, thanks for the reply, 4loops. I was thinking the same thing, that there's an issue with the speed sensor(s) but it's just not significant enough to generate a code.

That's a good idea, to potentially evaluate the quality of the speed sensors with STAR. I also doubt this capability exists, but it can't hurt to ask (I'll ask my mechanic).

In the end, I'm probably going to have to just try the speed sensor fix to see if it improves anything. Thanks again!

Last edited by nkx1; 06-21-2019 at 12:06 PM.
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Old 06-21-2019, 01:10 PM
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To help rationalize the decision to get the speed sensors fixed, consider that it might be close to time to do a Trans flush and filter change anyway, and since you're in there already... the conductor plate is just right there on top of the valve body. So at that point it's just asking for it, really

As a reminder to anyone following this thread, simply changing the speed sensors does not require any reprogramming. Just swap and go, the TCU and ECM is none the wiser. Nkx1, I see you deleted this part after posting initially -I'm sure you reread and saw reprogramming wasn't required, but I thought I'd just mention it again here for others reading.

Cheers!
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Old 06-21-2019, 02:15 PM
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Originally Posted by 4loops View Post
To help rationalize the decision to get the speed sensors fixed, consider that it might be close to time to do a Trans flush and filter change anyway, and since you're in there already... the conductor plate is just right there on top of the valve body. So at that point it's just asking for it, really

As a reminder to anyone following this thread, simply changing the speed sensors does not require any reprogramming. Just swap and go, the TCU and ECM is none the wiser. Nkx1, I see you deleted this part after posting initially -I'm sure you reread and saw reprogramming wasn't required, but I thought I'd just mention it again here for others reading.

Cheers!
That's what I was thinking (it's not much more work). However, I'm paying someone to do the work because I don't have a lift and I currently live in an apartment. I wanted to buy a refurbished conductor plate and just have it ready to go to pop in, but that, combined with programming, is proving to not be very cost effective.

Therefore, I'm probably going to have to find a local place to replace the sensors the same day. I contacted a local place that does the speed sensor replacement, and they said that my issue (rough 1-2 and 2-3 shifting when cold) is likely more of a hydraulic issue and not an electronic one given the absence of error codes. Given that my mechanic didn't have any strong guesses without tearing apart the transmission, I'm thoroughly confused about what to do. Since the speed sensors are almost certainly going to eventually fail no matter what, maybe I can use that as a justification to give this experiment a whirl.

My mechanic seems to think that the trans fluid has already been changed because the drain plug has been removed before (or some similar rationale), but I'm not entirely convinced of this. Man, what a hassle lol.
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Old 06-21-2019, 02:35 PM
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You're right though, a giant hassle. Sorry to hear you having so many troubles with it currently. But also, right you are: It's not a matter of if the speed sensors will need replacing, but of when. (at least that's what I keep reading on similar forums)

I hope you're able to find a local place that can swap 'em on the spot and get you taken care of quickly.
You should start a thread about your struggles and post your solutions (when they come, hopefully soon!). We can keep growing this knowledge base and help the next poor sucker that's stuck in a similar situation! If you do that, do post a link to it here so folks can jump to your thread.

Concerning your trans fluid.. It could have been changed recently, before you took ownership.. but there's no way of telling that the right fluid was used. I've learned in the past that this can make a big difference. -Just a thought.

I may get flack for saying such things as this, but when most EV range is above 250 miles and a bit more affordable I'm totally looking forward to jumping ship. At least for the commuter vehicle. This maintenance stuff is just starting to eat up too much time.. especially now that there's a bazillion extra engine and emissions controls to keep working. I enjoy wrenching, but geez!
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Old 06-21-2019, 03:06 PM
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Yeah, I'll post a thread if/when I ever get this sorted out. Good point about the trans fluid.

Regarding EVs, I'm in full agreement with you. No reason to hassle with an overly-complicated ICE-based vehicle when a comparatively simple EV will out perform the ICE-based vehicle in almost all regards. I've read about up-and-coming technology that should provide 80% charging in 5 minutes. The coming years should be interesting.

If I could go back in time, I probably would have avoided buying this car a few months ago. PPI didn't reveal any transmission issues, and I didn't notice it when I bought it. I have better things to do and spend money on than something as basic as ensuring my car works!
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