Full Speed Ahead…All 7 of Them
722.9 7G-Tronic Transmission
The 722.9, also known as the NAG2 or 7G-Tronic with a 7-speed transmission, made its debut on top-line Mercedes-Benz cars in 2004. Now, almost all Mercedes passenger vehicles are equipped with this gearbox. The transmission is the fifth generation of Mercedes Benz automatic gearboxes, also known as the W7A400 and W7A700. These transmission designations represent two power classes. The first class of transmission is for engines with up to 400Nm of torque input, while the larger unit can handle up to 700Nm of torque input; hence, the last three digits of the name designation. The new 7-speed transmission has a host of technical modifications over the 5-speed gearbox it replaces. In this issue, we will explore some of the important changes.
Transmission Improvements
Adding two additional forward speeds and changing the reverse gear ratio was relatively easy, accomplished by adding a more complex planetary gear set known as a Ravigneaux gear set. While the previous 5-speed transmission (722.6) was a good unit, it was plagued with some minor issues addressed by design changes in the new transmissions.
For example, 722.6 transmissions commonly leaked fluid at the electrical connector. In more severe cases, fluid actually traveled through the wiring all the way to the remotely mounted transmission control unit. This problem was completely eliminated in two ways. First, what better way to deal with fluid entry into the control module than simply putting the control unit into the fluid? The control module is now mounted in the center of the electrohydraulic actuator and completely submersed in the fluid. This allows the control module to maintain a more constant temperature. As an added benefit, the smaller electrical connector is now above the fluid level, making leaks basically nonexistent.
Early 5-speeds used two one-way roller clutches, called Sprague rollers. These occasionally failed and caused a variety of gear-change issues. These one-way rollers have been completely eliminated on the 7-speed gearbox. The work once performed by the roller clutches is now performed by extra brake clutches also required to accommodate the two added gears.
In addition, the control unit on the new 7-speed has joined the popular trend of being flashable, that is, externally reprogrammable in place. This has several benefits, the biggest being cost to manufacture. By having only a small number of modules that can be programmed to many different applications, the need to stock and produce a variety of modules is no longer necessary. Another benefit is the ability for an M-B dealer to download patches and fixes that can improve overall performance, similar to updating your iPad.
- Torque converter
- Turbine wheel
- Stator
- Impeller
- Transmission housing breather
- Oil Pump
- B1 brake clutch pack
- K1 clutch pack
- Ravigneaux gear set
- B3 brake clutch pack
- K2 clutch pack
- Simple planetary gear system
- BR brake clutch pack
- K3 clutch pack
- B2 brake clutch pack
- Park pawl gear
- Exciter ring for rpm sensing nout
- Range Selector Lever
- Ring magnet for rpm sensing n2
- Shift valve housing
- Ring magnet for rpm sensing nturb
- Control solenoid valve
- Control module
- Torque converter lockup clutch
As mentioned, the main component to achieve the two extra gears in this transmission is the Ravigneaux gear set. This is a fancy name for a gear set that combines two simple planetary gear sets into one compact unit. By using this gear set, more gear ratios are available. Brake disks B-1 and B-2 are used to hold and release the sun gear and/or the planetary gear to achieve the desired ratio.
The traditional clutch stack consisted of alternating friction disks and steel disks. This has been replaced by a new style clutch pack with disks that have friction material on one side and steel on the other. There are several benefits, including a more compact clutch assembly, cheaper production costs, improved heat distribution within the clutch pack, and higher load capacity to handle ever-increasing horsepower demands.
Two clutch packs stacked on bell housing.
B1 clutch pack closest to bell housing, and B3 clutch pack on top of it. 
Ravigneaux gear set by itself with a clutch pack in it.
Ravigneaux gear set mounted on top of clutch packs.
Entire transmission; main housing is magnesium; bell housing is aluminum.
Single-use connecting bolts are aluminum.
Another noteworthy design modification is the main transmission housing. The 722.9 housing is now constructed from die-cast magnesium, which reduces the weight and increases the strength of the unit. While magnesium has great benefits, there are some important things to remember. Magnesium and other metals are not exactly best friends. Steel bolts used in a magnesium part tend to corrode and over time will destroy the threads. Steel bolts have a different expansion rate, too. For this reason, high-quality aluminum bolts are used to secure the bell housing, valve body, and the oil pan to the main housing. These bolts must be replaced each time the unit is disassembled, with torque specifications carefully followed. Because of the softer nature of these aluminum fasteners, the use of power tools is taboo.
This transmission adjusts shift quality by self-adaptation to compensate for normal wear. Each gear change has its own set of adaptation data for both upshifts and downshifts. Fill time and fill pressure are measured with each gear change, in three temperature ranges; when measurements vary beyond an acceptable range, it adjusts the pressure and speed of fill.
Some early transmissions had an issue with snap rings popping out of the machined groove on clutches B-1 and/or B-3. This was most likely felt as a clunk when easing to a slow stop. Confirmation with the Star Diagnostic System is easy and usually appears as a clutch fill time of 20 for all three temperature ranges. The Star Diagnostic System can provide much-needed information to help the technician identify transmission problems. For example, determining if there is an internal hydraulic leak, lack of pressure in a particular circuit, or possibly a worn transmission, before beginning a teardown.
Another form of adaptation is driver adaptation. This is much more noticeable on AMG models. Ever notice when you’re cruising along and make a full-throttle pass, that after you roll off the throttle the transmission seems to stick in a lower gear and won’t shift right away? That’s driver adaptation! The control module senses your more aggressive driving and assumes you might be ready to get back on the power soon, so it holds that lower gear in preparation to leave in the dust that BMW beside you. If you don’t apply the accelerator hard within a certain timeframe, it assumes the police are watching and the race has been postponed, and returns to a standard shift mode. Okay – I got carried away, but you get the idea.
Electrohydraulic control modules (valve bodies) have become quite complex. Aside from opening and closing hydraulic circuits as in previous designs, a host of electrical solenoids and sensors have been added. The valve body contains valves, restrictors, a selector valve, eight solenoids, two speed sensors, control unit, and so forth, all built into one unit. Because flow volume from each solenoid tends to be slightly different, the flow values for each solenoid are measured and calculated into the equation to assist in more accurate adaptation.
Electrohydraulic control module
The main function of the electrohydraulic control module is to evaluate various input signals, calculate shift points, evaluate gearshifts, and adaptating and activating the eight solenoid valves when needed. In addition, the EGS control module can recognize and store fault codes for more than 100 problems. Fault codes are stored in priority; up to 16 fault codes can be stored at any given time. Due to this new integrated design, only five electrical pins are needed. If a replacement electrohydraulic controller is ever needed, it must be personalized to that particular vehicle. This personalized process is called SCN – Software Calibration Number – coding. This emissions-related coding mandated by European and American legislation is locked down to prevent any manipulation by aftermarket tuners. Models produced after 2005 will also need CVN – Calibration Verification Number – coding. This code string basically verifies that the software was installed correctly and is installed in the vehicle for which it was intended. If either of the codes is incorrect or incomplete, the vehicle will not start. For this reason, electrohydraulic control modules are not interchangeable between vehicles.
Changing Transmission Fluid
When this transmission was first developed in 2004, it was thought to be a lifetime filling. Mercedes has since changed its stance and now has intervals for transmission services based on the particular model and year. In my opinion, any gearbox that contains wet clutches, fluid, and friction needs regular service. How often is debatable, but I believe 60,000 miles is about right. Even though I discourage the average do-it-yourselfer from attempting this service because of the tools and equipment needed, here is the basic rundown.
The yellow overflow pipe in the transmission oil pan
Servicing this transmission now requires a completely different process than its predecessors. When the transmission fluid needs to be changed, the fluid is pumped into the transmission through the same drain plug used to drain the fluid – not a straightforward process. The transmission oil pan has an overflow pipe made of plastic. This tube attaches over the drain plug and must be dislodged with a drift through the drain plug hole in order to drain the fluid in preparation for the pan to be removed. Only after the drain plug has been removed and the overflow tube dislodged can all the fluid be drained.
New aluminum bolts are always used to secure the transmission and reinstall the pan.
After the transmission fluid is drained, the six aluminum transmission pan screws can be removed and the pan lowered. The filter sits loosely on the valve body and can be removed by pulling down with slight effort. Once removed, wipe the sealing surfaces and press in the new filter. The transmission oil pan will need to be cleaned and the overflow tube reinstalled on the pan. Most pans also have a thin magnet on the bottom that will need to be cleaned. This magnet is used to capture small fragments of ferrous metal particles that might be present in the fluid from normal or perhaps premature excessive wear. A new pan gasket is installed on the pan and new bolts are always used to reinstall the pan.
Most weekend warriors would find filling and adjusting the fluid level to be frustrating. Because the transmission has no dipstick and upper fill tube, the fluid must be pumped into the transmission with a pump or fluid transfer tank. This requires special equipment; most do-it-yourselfers won’t want to pony up the extra costs for tools seldom used. To make it even more difficult, the temperature of the transmission fluid is a critical element when adjusting the fluid level. Approximately six liters of special fluid are pumped into transmission. The engine is then started and transmission fluid temperature is monitored using the Star Diagnostic System equipment and increased to a specific temperature, depending on car model and its equipment. The filling device is removed and extra fluid drained until mere drops ooze from the drain plug. Finally, the plug is reinstalled with a new copper seal.