TCU Mercedes-Benz Siemens FTC (722.7)

Mercedes-Benz ran into quite a bit of trouble when it decided to add a small vehicle to its range. The car not only had to be compact and practical but also had to deliver a lot in terms of comfort and luxury. After all, it was a Mercedes-Benz. The 722.7 gearbox has the layout of a conventional manual gearbox, but uses a torque converter and several small hydraulic clutches to shift between gears.

Known complaints and error codes

Known complaints

  • An “F” appears on the display
  • The car goes into limp mode
  • The gearbox no longer shifts
  • The gearbox shifts randomly to “N” or an illogical gear
  • The car won’t start

Can be remanufactured

OBD II Description
P1840 (2120) PWM solenoid valve 1 / 4 shift
P1841 (2121) PWM solenoid valve 3 shift
P1842 (2122) PWM solenoid valve 2 / 5 / R shift
P1843 PWM solenoid valve torque convertor lockup clutch
P1844 PWM shift valve circuit
P1850 (2204) Transmission RPM sensor Y3/7n1
P1858 (2227) Starter lockout contact
P1884 (2123) PWM shift valve pressure
P1897 Control module N15/7 faulty
P1903 Control module N15/7 faulty

Remanufacture may be possible (additional diagnosis required)

OBD II Description
P1895 (200a) Internal fault in control unit

This error code will never actually appear independently. Therefore, look carefully at the rest of the error codes to make the correct diagnosis.

OBD II Description
P1709 Park / Neutral switch
P1756 Selector lever implausible
P1872 CAN signal from gear regognition module faulty
P1875 CAN communication ESP
P2031 No signal or error signal from control unit N15/5
P2210 Selector lever coding is invalid
P2211 The selector lever is in an intermediate position
P2212 The selector lever position is implausible
P2318 Fault in CAN communication with control unit N15/5
P2333 The CAN signal from control unit N15/5
P2338 The CAN signal from control unit N15/5
2310 CAN communication with TCS failed
2311 CAN communication with ECU failed
2312 CAN communication with ECU failed
2315 CAN communication with instruments failed
2316 CAN communication with A/C failed
P240C The CAN signal for the selector lever position from control unit N15/5

These error codes may indicate that the gear selector lever is defective. In many cases, this is also revisable.

icon-check-markCannot be remanufactured

OBD II Description
P1886 1-4 / -3 downshift PWM valve pressure too low OR 2-5 pressure too high
P1887 (2531) 1-4 or 2-5 shift slide valve jamming in basic position Shift valve pressure to low
P1888 1-4 or 2-5 shift slide valve jamming in basic position Shift valve pressure too high
P1889 2-5-R downshift PWM valve pressure too low Transmission slipping
P1893 Pressure too high at regulating valve or solenoid valve 1 / 4 or 3

These error codes often indicate mechanical defects. Check the gearbox carefully for wear and/or damage!

General operation FTC (722.7)


The 722.7 automatic gearbox is an electro-hydraulically controlled 5-speed gearbox, which also features a torque converter. “FTC” stands for “Front Transmission Control”. The next gear is selected via hydraulically operated mutli-plate clutches. Each gear has its own multi-plate clutch. These replace the conventional forks used in a manual gearbox. Mechanically, the 722.7 looks a lot like a manual 5-speed gearbox.

The Mechatronic is mounted on the underside of the gearbox. If required, control valves direct oil pressure towards the various clutches. The required oil pressure for the K3, K4 and lock-up clutches is fed through holes in the main shaft. The oil pressure for the K2 and KR clutches runs through the opposite shaft. In addition to transferring oil pressure, the shafts in the gearbox are also used for the distribution of lubricating oil. In this way, various bearings as well as all the multi-plate clutches are supplied with lubricating oil.

Opted for the torque converter

The addition of a torque converter is not necessary from a technical point of view because this could also have been done with a (slightly cheaper) automated plate clutch. However, a torque converter completes the total “Mercedes feeling”. A torque converter creeps in: something that the automatic transmissions in the more expensive Mercedes-Benz models also have. In addition, a torque converter multiplies the engine torque until it stops slipping. A feature that especially low-torque engines can benefit greatly from when pulling away.

The mechatronic in detail


Actuator Function
Y3/7y1 Control valve “for 1st and 4th gear”
Y3/7y2 Control valve “for 3rd gear”
Y3/7y3 Control valve "for 2nd, 5th and “R” gears”
Y3/7y4 Control valve for lock-up
Y3/7y5 Shift valve

Clutch Control

The labyrinth mounted underneath the TCU (=Transmission Control Unit) is equipped with several control valves and shift valves. By controlling the control valves (also known as solenoid valves), the fluid pressure can be directed towards these valves. This ensures that the valves are adjusted to the correct position. This allows fluid pressure to flow to the various multi-plate clutches. As soon as the clutch is engaged, the gear is engaged. In this regard, the 722.7 differs substantially from a manual gearbox.

A total of 5 control valves are used. As can be seen in the following diagram, control valve Y3/7y4 controls the lock-up clutch. The other valves work together to operate the other clutches (e.g. K1 = the clutch of gear 1):

Shift diagram

Valve name K1 K2 K3 K4 K5 KR
Y3/7y1 via valves RS14 and SS14 0% 0% 0% 0%
Y3/7y2 via valve RS3 0% 0% 0% 0% 0%
Y3/7y3 via valves RS25R and SS25 100% 100% 100%
Y3/7y4 Lock-up clutch 0%
Y3/7y5 Shift valve 100% 100% 100% 0% 0% 100%

Similar control valves


4 of the 5 control valves in this Mechatronic are of the normally closed solenoid type. This means that the valves are closed when de-energised and do not allow any liquid to pass through. This means that control valves Y3/7y1, Y3/7y2,Y3/7y3 and Y3/7y4 can be exchanged without penalty.

Valve Y3/7y5 is another case. This is not a control valve, but a shift valve: the valve can only be opened to the maximum or closed to the maximum. In addition, this valve works with a different voltage.

The TCU in detail


Sensor Function
Y3/7n1 Speed sensor
Y3/7n2 Control module
Y3/7s1 Starter lockout

This type of TCU is relatively simple: 1 plug, 1 speed sensor, 1 switch for the starter lockout and 1 central control module. Communication with the rest of the vehicle is completely via CAN. As a result, the plug only needs 5 connections. The speed sensor uses the Hall principle: a change in the magnetic field generates an electronic signal. This signal can also mimic itself.

Limp mode

After the TCU has detected an electrical fault or measures an unexpected coupling pressure, it will always activate the limp mode. The electronic part of the Mechatronic is completely switched off. This means that all control valves are in a Voltage-Free State. This will increase the overall working pressure to the maximum value, the lock-up clutch will disengage and the gearbox will shift to 5th gear and stay in it (the only gear that can operate with 0% pressure from all valves).

Check the sensor function yourself


If the car from which the TCU comes is still available, the speed sensor of the extended TCU is quite easy to check by yourself. The only condition is that a diagnostic tool is needed to make live data visible.

Proceed as follows:

  1. Connect the connector from the car to the separate TCU
  2. Turn on the ignition (power supply)
  3. Find the required live data in the reader
  4. Use a magnetic screwdriver to move along the sensor several times

The frequency at which the screwdriver passes the sensor now becomes visible as the speed.


If live data are not available, the operation can also be measured via the copper tracks at the rear of the speed sensor. Use a multimeter for this. Between track 1 and track 3, the voltage will change between 0 volts and 5 volts each time a magnetic screwdriver is moved past the sensor.