ACtronics takes an in-depth look at Mercedes’ 7G-Tronic transmission control unit andoutlines what you need to know before working on one.
Most experts will thinkimmediately of Mercedes-Benz when they hear the term‘7G-Tronic’. And rightly so,because the 7G-Tronic transmission controlunit is part of the Mercedes-Benz 722.9gearbox: the first 7-speed automatic everavailable for passenger cars. The gearbox wasintroduced in 2003 on all Mercedes modelswith 8-cylinder engines. Among them are theW203 and W204 C-Class, W211 and W212E-Class, W220 and W221 S-Class and carslike the famous G-Class. However, the 722.9has one drawback: it has a limit of ‘only’ 735Nm. So cars with V12 engines had to carryon using the older 5G-Tronic, which couldhandle a torque up to 1,079 Nm.
General operation
The gear system of the 722.9 consists ofthree planetary gear systems and oneRavigneaux gear system: simply put, aplanetary gear system on a planetary gearsystem. This dual function makes many moredifferent gearbox ratios possible, whichmeans that nine gears can still fit inside arelatively compact gearbox (seven forward,two reverse). See Fig.1.
In order to save weight, Mercedes-Benz has made the gearbox bodyout of magnesium. They were thefirst manufacturer ever to do this.
How the mechatronic works
Of course the gearbox won’t change gears byitself so it needs some kind of mechatronicdevice. The mechatronic of the 722.9 gearboxconsists of a TCU, a hydraulic block(valvebody) and several control valves(solenoid valves). The hydraulic block acts asa labyrinth, allowing the fluid pressure toflow directly to the components to becontrolled. The control valves control thesefluid flows. Normal open solenoids andnormally closed solenoids are used tomaintain limited functionality in the event ofa malfunction. Compare the table below withthe annotations on Fig.2.
Part no./Function
11 Connector
21 Hydraulic block/valve body,consisting of two parts and apartition panel
31 Oil level float 1
32 Oil level float 2
Y3/8y1 Control valve for generaloperating pressure
Y3/8y2 Clutch control valve K1
Y3/8y3 Clutch control valve K2
Y3/8y4 Clutch control valve K3
Y3/8y5 Brake control valve B1
Y3/8y6 Brake control valve B2
Y3/8y7 Brake control valve B3
Y3/8y8 Lock-up clutch control valve
It’s up to the TCU to decide when and how toact. It relies on several inputs like gearselector position and current speeds. The 7GTronicTCU depends on three speed sensors:input shaft speed, central speed and outputshaft speed. Compare the below table withthe annotations on Fig.3.
Sensor/Function
Y3/8n1 Input shaft speed sensor
Y3/8n2 Central speed sensor
Y3/8n3 Output shaft speed sensor
Y3/8n4 Control module
Y3/8s1 Position sensor switching position
Known complaints and faultcodes
There’s a lot of knowledge at ACtronicsabout complaints and fault codes regardingthe 7G-Tronic TCU. Long story short: mostof the time it’s the speed sensors that willcause a lot of problems. Fortunately, there’sa perfect remanufacturing solution for thesekinds of problems.
Speed sensor-related fault codes: P0717,P0718, P0720, P0721, P0722, P0723,P2200, P2201, P2204, P2205, P2206,P2207, P2550, P2767, P2768.
Check the sensor functionyourself
If the car from which the TCU comes is stillavailable, the sensors of a removed TCU arerelatively easy to check yourself. The onlycondition: a diagnostic tool is needed to makelive data visible. Proceed as follows:
- Connect the connector from the car to the separate TCU
- Turn on the ignition (power supply)
- Find the required live data in the reader
- Use a (magnetic) screwdriver to move along the sensors several times
The frequency at which the screwdriverpasses the sensors now becomes visible asthe speed.
Note: With sensor Y3/8s1, it issufficient to hold the magneticscrewdriver in a certain position nearthe sensor. The sensor has four“fields”. Each field displays its ownshift position (“P”, “R”, “N” or “D”).Speed sensors Y3/8N1 and Y3/8N2respond to a magnet. Speed sensorY3/8N3 reacts to metal.
Instant replacement not possible
A 7G-Tronic TCU from a donor carunfortunately cannot just be placed inanother car. The device will have to be‘virginised’ first (i.e. removed of all coding,just like a new, unused part). You need tothen carry out “Commissioning after repairN15/3” with a diagnostic tool. Part of this“Commissioning after repair N15/3” is theSCN coding. This coding can be obtained byreading out the original TCU. If this is notpossible, it can be requested via theMercedes-Benz online platform. The SCNcoding for the car is only issued onceMercedes-Benz has approved the conditions.
Virginising is something that ACtronicscan also do. That is why we also have thepossibility to offer a replacement part, ifnecessary. “Commissioning after repairN15/3” still needs to be carried out, but theTCU is usable in any case.
Non TCU-related complaints
Unfortunately there are also complaints thatcan’t be solved that easy. For example, thelock-up clutch is known for slipping. Thisoccurs because of extreme wear. Checkwhether the magnets in the sump pan are fullof (iron) swarf. If this is indeed the case, thenit can be assumed that the lock-up clutch hasextensive damage. It is advisable to replacethe entire gearbox including the torqueconverter, because the iron swarf has nowspread throughout the entire transmission.
All modern electronics and mechatronicshave their share of problems. However, the722.9 gearbox really is a clever piece ofengineering. It’s fairly reliable, performssmoothly and is packed with some greattechnology. But in case something(electronic) goes wrong, just call ACtronics.We’ll know what to do.
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