The 722.6 is a 5 speed automatic gearbox by Mercedes that consists of 3 pressure solenoids, 3 solenoids, 3 clutches and 2 brakes which can be controlled to engage different forward gears. There is an additonal brake that is used only when in reverse gears.
The 722.6 controls 2 PWM solenoids and opens a shift solenoid to conduct a gear shift. In each gear shift, one clutch pack is applied, and another is released. At any given time there must be only 3 clutches applied in the gearbox whilst it is in a forward gear. Below is a diagram showing how this all works:
In the event the TCU was to actuate a shift solenoid out of order (For example command the 1-2 solenoid then 3-4 solenoid), the gearbox will automatically go into Neutral. This is a hydralically commanded Neutral engagement and the TCU can only regain control of the gearbox once the car has been turned off to drain any pressure in the gearbox.
This section is about an overview of what the solenoids in the 722.6 do
This solenoid controls pressure of the engaging clutch during gear shifts. It is off when the gearbox is in a gear, and only active when transitioning between gears. When the solenoid is off, the shift pressure rail is at maximum pressure, and when the solenoid is actuated to 100%, the pressure rail is at minimum pressure.
This solenoid controls the main line pressure of the gearbox and thus what pressures are working to hold the current engaged clutches of the gearbox. During gear shifts, it additionally helps with the smooth transition of clutches during gear changes by working to limit the release of pressure from the outgoing clutch. Like the shift pressure solenoid, when the solenoid is off, the line pressure is at maximum pressure, and when the solenoid is actuated to 100%, the pressure rail is at minimum pressure.
This solenoid directly controls the torque converter lockup clutch. The solenoid is controlled by a 1000Hz electrical PWM, which itself is pulsed at 100Hz to create 'buckets' of hydralic pressure. The torque converter solenoid pulsing can be visualized in the below osciloscope trace:
The TCC solenoid goes through 3 distinct control phases, marked on the above graph:
The duration of T1+T2+T3 will always be 10ms (100hz), and the Duty of (T1+T2):10 will be what is described in DAS as "TCC PWM Duty".
These 3 solenoids work as a switch to command a gear shift. When activated, they enable the shift pressure rail to send fluid to the various overlap and command valves in the hydralic valve body to transition clutch packs. See below on how this works.
The 722.6 has 3 planetary gear sets, which form 5 forward gears and 2 reverse gears. They are called Group 1,2 and 3, looking from the front to back of the gearbox.
In the following diagrams, RED coloured elements imply they are locked stationary, and the names of each element are as shown: