To the casual observer, automatic transmissions seem like witchcraft once you open them up. All sorts of itty bitty pieces, solenoids, fluid channels, and wet clutches crammed into a casing and stuffed into a car so drivers can creep along in rush hour traffic, never so much as flexing their left legs. For the most part, modern iterations of these transmissions are reasonably reliable, although they get expensive when they go wrong. Today, we’re zeroing in on Mercedes-Benz automatic transmission conductor plate problems, poised to cause a little or a massive headache depending on which transmission they’re in.
Let’s start with the easy one: The five-speed 5G-Tronic 722.6 automatic transmission.
When the 5G-Tronic 722.6 automatic transmission works, it’s a marvel. Smooth and stout, it was a five-speed automatic deemed fit for the 996 Porsche 911 and Jaguar’s supercharged models around the turn of the millennium, and it still served duty in V12 Mercedes-Benzes long after its successor was installed in every new V8 model. From the range-topping Maybach 62 to the compact C-Class, pretty much every Mercedes-Benz made from the Y2K era used this properly luxurious transmission.
Mind you, when the 722.6 transmission isn’t working, it can be a bit of a headache. Thankfully, the clutches and brakes are remarkably strong, but some of the electronics inside the transmission can be fragile. Reports of failed conductor plates and the resulting second-gear-only limp mode can be found all over the internet from Reddit to Benzworld to the U.K. Mercedes-Benz Owners Forum.
However, before diving into conductor plates, we should back up and explain how an automatic transmission works. In a traditional planetary torque converter automatic transmission, planetary gears orbit around a sun gear, then a ring gear goes around the whole thing. Instead of changing ratios by engaging and disengaging different gears, an automatic transmission uses a series of clutches and brakes to halt components in their tracks, changing the effective drive ratio. As David Tracy summed it up so eloquently on the old lighting site:
Think of it this way: a planetary gear set lets you change gear ratios without having to engage different gears. They’re all already engaged. All you have to do is use clutches and brakes to change which components rotate and which stay stationary.
So how do those clutches and brakes engage and disengage? Well, automatic transmissions use hydraulic pressure to change gears, with a pump pressurizing the transmission fluid, a valve body providing potential paths for the fluid to take, and solenoids and valves regulating fluid flow. Aside from a period of Honda weirdness, the general principles of a planetary automatic transmission have stayed the same for more than 80 years. However, methods of controlling solenoids have changed in the electronic age.
Remember when automatic transmissions had governor gears? While older designs are mechanical in nature, you have to go way back to find transmissions like that. For the past few decades, automatic transmissions have been electronically controlled, and electric current requires conductors. This automatic transmission conductor plate contains the transmission’s speed sensors, conducts current to solenoids, and is a vital component of getting the 722.6 transmission to actually shift. After all, the older transmission control unit hitched to the 722.6 doesn’t know if a shift has actually happened unless it compares input shaft speed and output shaft speed.
Unfortunately, these speed sensors and conductors can fail, leading to a get-home failsafe mode condition. As OE automotive parts supplier Febi Bilstein notes in a bulletin:
Problems relating to gear change and selection can be due to the failure of the contacts in the conductor plate which carry the electrical sensing and control solenoids for the transmission. (two speed sensors and six solenoids, located on top of the valve body in the transmission), causing faulty signals and resulting in the engine management light (EML) illuminating and the vehicle going into ‘limp-home’ mode.
Simply put, if you own a Daimler and/or Chrysler product with the 722.6 five-speed automatic gearbox, or a turn-of-the-millennium supercharged Jaguar, or a Tiptronic 996 Porsche 911, and are experiencing transmission-related problems, the conductor plate is suspect number one.
Fortunately, for most 722.6 applications, conductor plate replacement is cheap. The part itself costs less than $175, and it is DIY-able, if a bit of a pain. It sits atop the valve body, as seen on our very own Ski-Klasse E320 wagon, so expect to dedicate a full day to conductor plate replacement at home. However, if you can’t wrench, the rock-bottom value of a clapped-out Mercedes-Benz combined with the labor cost to replace a conductor plate can mechanically total rustier, higher-mileage vehicles.
Here’s a video of our pal Danny from FCP Euro, who did the job on Ski-Klasse, going through a full 722.6 refresh.
Alright, so a relatively inexpensive part that can be replaced at home doesn’t sound like the biggest deal in the world, but that’s only the prelude. As you’d probably expect from German luxury cars, things got far worse when technology started to pile up. In 2003, Mercedes-Benz launched the 722.9 7G-Tronic, the world’s first seven-speed automatic transmission. Smooth, swift, and remarkably modern, it set the blueprint for the recent transmission arms race, but it didn’t solve the conductor plate issues found in its predecessor. In fact, it actually made things worse as the conductor plate in the 722.9 effectively took shadetree serviceability out behind the woodshed and shot it.
Not only does the conductor plate itself cost around $1,000 as seen on FCPEuro, each conductor plate for the 722.9 requires programming to the vehicle it’s installed in as it’s part of the anti-theft system and features an integrated transmission control module. Oh, but it gets worse — Mercedes-Benz also had three valve body iterations in this transmission, and if you have the first one, stamped VSG1, the valve body will also need changing to fit the currently-produced conductor plate. On, say, a 2004 to 2005 E500 or S500, this job can equal the cost of cheaper examples, mechanically totaling these cars with repairs worth more than 50 percent of the vehicle’s value. Our own Stephen Walter Gossin can confirm that conductor plate and valve body replacement on 722.9 cars can easily work out to north of $2,000. For later cars, such as late-aughts and early-2010s C-Classes, MLs, and S-Classes, expect a lower, yet still four-figure bill.
So, is there a known failure window for 722.9 conductor plates? Well, yes, but it’s wide and opens sooner than it should. Mercedes repair YouTube channel and blog Mercedes Medic notes that “These transmission problems may start developing as early as 50,000 miles.” Some allege failure to have arrived even sooner, like one 2006 ML-Class NHTSA complaint claiming “AT AROUND 32,000 IT NEEDED TRANSMISSION REPAIR.” At the same time, it’s not hard to hop on Facebook Marketplace and find models with the 7G-Tronic automatic transmission still plugging along with more than 100,000 miles on the clock. Conductor plate issues in these cars really are like a game of roulette, seemingly striking anytime, anywhere, at any mileage.
It’s one thing if a car’s Achilles heel can be prevented or planned around with cheap preventative maintenance or by bundling jobs, but nobody’s dropping a valve body periodically. At the same time, DIY maintenance, like what’s possible on the 722.6 transmission, can help mitigate cost. That’s the mildly scary part about the 722.9 transmission’s conductor plate — there’s no waypoint to prevent owners from being thrown to the wolves, no commonly agreed-upon replacement interval, and no easy DIY replacement since coding with factory diagnostic tools is required. Some of the cars equipped with the 722.9 are lovely, like the W221 S-Class, but cars with this transmission are to be bought with caution, money in the bank for repairs, and possibly a backup car in the stable.
(Photo credits: Mercedes-Benz, Amazon, eBay)
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Let’s not forget the differentials of our MB cars:
The owner’s manual and service requirements state that differential fluid must be changed periodically for AMG cars only – There’s never an indication of fluid change at any time for non-AMG cars.
Yet at just over 100,000 miles, the differential on my CLK350 had gone bad – making whining noises that were getting worse and worse – requiring a replacement at a cost of @$2000 for piece off a wrecked, low-mileage C Class (a remanufactured one would have cost @$4000 in parts/labor)
This is a case of a known issue that the MB dealership service departments completely ignore (the car was at the dealer for it’s annual service – and nobody said anything about it) while following service requirements to the letter. It was a visit to my independent shop (for a repair to the automatic roof mechanism) where the issue was diagnosed. I was informed that the fluid should have been changed every 50K (when the AMG cars are called out for fluid changes) at a minimum.
Don’t forget, the 722.6 was also used the mclaren slr. Did a couple of these in the .6. Definitely wouldn’t want to do one in the .9. I’ve had several cars with the .6. It’s actually a great trans. Never had a problem with them other than the plate. Wish every trans was as easy to repair.
LS and 4l80 swap…
All the more reason to just buy a car with a manual transmission (and by that I mean avoid Mercedes entirely unless you can find a manual unicorn). All the German autos from the early 2000’s have had some issues, like the ZF 5HP19 used in a bunch of BMW’s and Audis had a load of issues with torque converters and solenoids/clutches failing, the ZF 6HP had valve body + solenoid issues, and so on.
After this tale I’m thankful that the cheap beaters in our fleet use bomb proof early oughts Ford and GM 4 speed automatics and the good car has a Mazda 6 speed transaxle.
You gotta pull off that conductor plate to give the ‘ol Red Dragon a little more juice. It’s no exactly street legal, so keep it on the DL.
“Excuse me, do you have the time to talk about our lord and savior, eCVT?”
-Prius driver
Also, only dubious connection at best, but one thing that stunned me about my ’97 Econoline-150 and my family’s ’99 E-150 was that in 160,000 miles for the former and 240,000 for the latter, for all the problems we had with either, the transmissions weren’t one of them. Just seemed nuts that everything fell apart around them (although even the engine seemed to be immune to problems in the ’97; the 4.6l)
“Murica!”
Good topic, but, not even a side mention of the ‘pilot bushing’ that goes bad and tends to wick transmission fluid up into the Transmission Control Module, at least on 722.6 W211s? Had to replace that pos twice on my CDI, luckily the fluid didn’t make it up to the computer.
Current 722.9 owner, a very early one on a 2004 S430. Biggest problem with it is what a pain changing the fluid is. It shouldn’t require 3rd degree burns to add fluid to your transmission.
And this is why you don’t buy used German (or luxury cars of any kind, except 1st or 2nd gen LS400s) luxury cars. There are practically no new cars available that make for decent used cars; I am morbidly curious as to what our car-centric world will be like in ten-twenty years.
The pains of “used German” are way overstated. Especially if you can do basic wrenching.
My coworker, handy with a wrench, has sunk tens of thousands into his cheaply bought Audi A8. Replacing the rear bumper took nearly a month, as various sensors didn’t want to speak to each other. My BMW-loving friend (can disassemble a motorcycle after dinner) had to do something to the exhaust manifold in his BMW 528i Touring and it took him two-three weeks. Another one has an issue with the rear headrest sensors in his 3-series and he is still driving his wife’s Hyundai.
Give me a car with minimal equipment and fewer things to go wrong.
I am specifically against “used luxury” – it’s a great way to stay poor while trying to look wealthy – but it appears the Germans are that much more expensive in parts and labor as well, and since the 1990s they have dropped the ball completely on quality.
Well, as a person with an unreasonable affliction.. er, AFFECTION for old German vehicles, the struggle IS real. I mean, you have to take a wheel off, and remove the wheel well liner, on some of their sedans, just to change a low beam bulb. Which is unfathomably stupid.
However, as long as you are patient, and don’t become emotionally involved with things that, on the surface, are patently ridiculous and needlessly complex, you can own them without TOO MUCH heartache.
Your mind has to be in the right place though.
I’ve daily’d a 2000 530iT for over a decade. It’s extremely easy to fix. The parts are cheap. Guess I’m just lucky.
Yes. Never. It’s a terrible idea. Go buy new cars. Stop bidding up prices on used ones.
I would, if I could get a subcompact station wagon with a fabric interior, power windows, steering, and mirrors, 8 speakers, and nothing else. Oh, and a well-tested engine and transmission.
This is a Hyundai Kona. Admittedly slightly less wagony than the previous generation, but still a wagon no matter what the marketing department says.
$26,785 before haggling. Get the orange one.
Ugh. I guess I can’t see what it looks like when I’m inside.
A $2000 repair on a complex german luxury vehicle doesn’t sound so bad. Let me introduce you to the GM 6L80 used in relatively simple pushrod v8 trucks and it’s frequent need for complete rebuild.
Just found out about this the really, really hard way. Torque Converter went out, took out the whole transmission with it. ’16 Sierra 5.3 with only 90k miles on it. Absolutely unacceptable.
Oooh do the VW DSG mechatronics next
“Remember when automatic transmissions had governor gears?”
That was during the war, when the style of the time was to tie an onion to your belt, amirite?
It was the fashion of the day if I recall
Can confirm, 722.9=expensive. Even for a completely normal transmission service, needing the fancy fluid means it is significantly pricier than a 722.6. We joke about “throw in a conductor plate” being the default Mercedes transmission repair, but when it’s a “fancy transmission” it becomes an impressive headache of extra labor plus co-ordinating with the dealer to have the car towed up the hill for programming. We can install TRP parts no problem, but Mercedes has that programming quite effectively locked down last we had to check. The good news is we haven’t seen a failure in at least 6 months. Naturally, that means I’ll get a call tomorrow.
The early .9s still use the ATF 134 from the .6s – I forget exactly when they switched to the ‘blue’ fluid, but, sometime around 2010 seems right. But, from a maintenance perspective, that’s about all they have going for them.
This is a good reason to avoid MBs and similarly over-complicated vehicles.
An AT going south is often the cause of an otherwise good vehicle going to the scrapyard. Manual transmissions are superior in many aspects.
Two words: Ballast Resistor.
sounds sexy. tell me more.
During the period of the infamous ballast resistor my family was exclusively Mopar, with a ’75 Valiant, ’77 Aspen, ’77 Dodge D100 pickup and ’82 Dodge D100 pickup (both an honest man’s single cab long bed 2WD work truck with no nonsense or frippery), and two K-cars in the 80s, an Aries Wagon and a Dodge 400 of indeterminate vintage (I forget which K-car it happened on but the camshaft snapped while not under any particular load). All this to say, I may not be any sort of great expert on malaise-era Mopar, but I probably have more experience than most.
In any case the ballast resistor was used in at least some of those and although you always wound up carrying a spare I don’t remember replacement taking more than a few minutes. My hazy recollection is that it was mounted on the firewall and relatively accessible, requiring only a couple bolts to unmount and mount, and a couple of wires to connect it. The ballast resistor was part of the electric/ignition system and they were notorious for constant failures. I remember my dad replacing a number of them during that era. I’m sure one can look up the term and find lots of articles on it and bust me appropriately if my memories are wrong.
It was admittedly a nuisance but a far cry from the car-killing conductor plate mess described in the article.
A ceramic rectangle found in the glovebox of all Mopar products I have owned, and in my toolbox today even though I only have Volvos at the moment
Anybody know what’s actually failing on the plate? Is anybody doing re-man parts? I had a video board go bad on an out of warranty Sony tv, Sony doesn’t sell replacement boards at any price, but found on eBay a guy in Cincinnati who did re-man boards. Like the Merc trans, this was a common failure.
It’s usually just a break in the sandwiched circuit paths or worn solenoid contacts. If you’re not careful, you can also damage the pins where the transmission harness connects to it (there’s a separate connector piece that joins the two.
I actually have seen enough about these to identify two specific failure modes. Modes that I’m quite certain THE QUALIFIED ENGINEERS POINTED OUT IN ADVANCE. The early conductor plate by the way, is basically just a collection of bus bars.
Item one, the contact arms are EXTREMELY thin copper. Which means, you guessed it, they break. Clean through. Resulting in intermittent or no continuity. Resulting in transmission go boom. This is very, very clearly a result of poor material selection combined with a high heat and high vibration environment. Do not use very thin copper under constant tension in environments full of vibrations and fluids.
Item two, the contact arms stretch out and no longer make proper contact with the solenoids. Which, you guessed it, leads to intermittent or no signal. Because when installed properly, as you see, those arms get are trying to push downward – continuously – to ensure ‘good contact’ with the solenoids. And what happens if you hold metal bent in position for a very long time? That’s right: it stays there and doesn’t try to bend down. You know, the mechanical action necessary to maintain good contact with the pads on the solenoid. Which are way too damn small.
It is a deeply stupid design decision. Understand, designs like the conductor plate can and DO work, when engineered correctly. That generally means instead of relying on vertical pressure, they rely on a broad contact patch to accommodate wear, and avoid high vibration areas. It’s a good thing to do when you want to avoid having wires (which yanno, dangle about) in the vicinity of say quickly rotating parts (like transmissions or very powerful computer fans.) Using a spring contact design for these just… why? That’s an honest question. I do not understand why this was chosen other than to OEM the solenoids by cutting off the leads.
an intelligent assassination of poor engineering. well done
I’m a mechanical engineer working on OEM powertrain stuff.
The stuff that falls through the gaps when electrical engineers start designing mechanical bits is deeply worrying.
I don’t design black boxes or spaghetti because I’m not qualified, but everyone and his dog can design shit that bolts up, right? Harness brackets are the worst.
That said, if this sort of failure is getting in to production they need to change their durability/validation testing. They almost certainly have based on service parts supply alone.
Yeah, so you KNOW things like vibration/harmonic calculations in that environment way better than I do. (And yeah, I know it sometimes boils down to “fuck it, just use tape.” Just stop letting them cheap out on the tape, goddamnit. Proper gaffer tape at least.)
But TBQH, I’m 100% certain that this stupidity was called out by the engineers. And they just changed the validation testing around until it showed a sufficient number of warranty claims could be denied of the ones that got through. As long as it lasts 36,000 miles or they can use the ‘abuse’ excuse to deny claims, that’s “good enough” as far as the accountants are concerned.
I worked somewhere once where the validation test was 73 hours long. Normally it’s a round number in some unit (miles, km, seconds, something), so I asked around and was told that was supposed to be a 100 hour test, but parts started failing after 73 hours, so…
Yeah, that most DEFINITELY tracks… or they just waive it and insist it’ll be fine because they’ve used that design before, it’s fiiiiiiiiiine.
The most embarrassing part about the conductor plate though? It’s literally nothing more than a zero component PCB in a plastic housing, because nobody could be assed to design a filter provision.
Excellent explnation. This situation smells a lot like they had components on a shelf that they decided to use instead of mandating the correct item be made.
Great take. Can it be solved? I guess the required OEM programming makes it impossible anyhow.
Glad I missed this one when I had a 1999 Jag XJR. I only had to deal with the “lifetime” transmission fluid deciding it needed a change of scenery while out for a drive in the country. Amazingly, the transmission survived, though the 1-2 shift was a little rough while coming up to temperature afterwards.
Lifetime transmission fluid, the essential oils of the automotive world
Ah yes. They love to say lifetime. However, their definition of ‘lifetime’ is the end of the warranty, not the life of the car.
Ah yes, the 5G-Tronic 722.6 automatic transmission, benevolent predecessor to the NAG1 used in the JK Wrangler and LX platform vehicles. I’m familiar with these plates because I dropped the transmission pan in my JK in order to swap out the failing TCC (Torque Converter Clutch) solenoid. While I was in there, I inspected the conductor plate, cleaned any contact surfaces, and replaced the o-rings on the other solenoids. The Jeep certainly shifted more smoothly afterward and it all but eliminated any TCC cycling when cold and driving uphill.
Achtung, Baby. Yet another reason not to buy a used modern-day Benz. Just stick to the leased versions.