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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