Other Articles - September - 2019

Heavy Duty Shifts: Another Look at the Duonic 6 Speed DCT

Things that seemed impossible just a few short years ago are now common place in our industry. Eight, nine, and ten speed units as well as Hybrid and Dual Clutch applications are entering the market at a rapid rate. In a joint venture between Mercedes and Mitsubishi, the Fuso brand was born. Fuso is sold worldwide and has quickly become one of the leaders in the class 3, 4 and 5 small/medium duty truck markets.

As most of you know, Dual Clutch Transmission (DCT) applications are becoming much more common in our industry. With the fuel economy and weight savings offered by the DCT design, many companies are selling DCT equipped vehicles. The world’s first DCT for the medium duty truck market was introduced on the Fuso Cantor family of trucks starting in 2012.

The unit is available as a conventional or as a hybrid transmission application. Known as the Duonic (portions of which were covered in the August issue of GEARS), or the M308S6 transmission, it is a wet dual clutch design which uses the clutches to drive a gear box which resembles a manual transmission.

There are two generations of the Duonic transmissions, Gen 1 from 2012-2014 and Gen 2 2014 1/2 and later. NOTE: Vehicles built after August 1, 2013 (2014 and later models) use a “waffle” design clutch rather than radial design clutch plates and require different TCM software or clutch failure may occur.

Like other DTC applications, the Duonic unit utilizes two clutches and two input shafts one driving the odd gears within the transmission and one used to control the operation of the even gears within the unit.

The Duonic clutches are arranged slightly different when compared to most other DCT applications. Both clutches are hydraulically controlled. The inner clutch drives the odd gear input shaft (1, 3, 5, R) while the outer clutch operates the even gear shaft (2, 4, 6) (figure 1).

The clutches are controlled by a pair of solenoids mounted to the valve body. The valve body is not what one would think of when referring to the term, but instead it is actually what we would refer to as the pump assembly. The transmission has a “creep mode” and crawler function that partially engages the wet clutches to move the vehicle slowly for parking and other maneuvers.


Similar to most late model automatic transmission applications, the clutches may not be fully locked. Known as “micro slip control”, the TCM commands the clutches to slip slightly keeping the input shaft speed slightly lower than the engine speed. Like most automatic applications, this operational feature is used to isolate vibrations from the engine from reaching the drivers compartment.figure 1


The clutches on this application are released when the vehicle comes to a stop, is in reverse or a forward range.


To shift the unit, the 3 gear box shift rails are controlled by 3 hydraulically controlled plungers known as “magnetic valves” (MV1, MV2, MV3) (figure 1). Each plunger operates similar to a hydraulic piston transferring its motion to the shift rails. Each plunger is controlled by two solenoids mounted on the rear of the unit. The plungers and solenoids are mounted in a housing which Fuso refers to as a “solenoid body/ gear shift unit”. The position of each shift rail is monitored by the TCM through individual shift rail position sensors.figure 2

To make a shift, the Duonic TCM energizes the shift solenoid for the appropriate shift rail it wants to control. The solenoid pressurizes the plunger chamber moving the appropriate shift rail control plunger. The plunger moves the shift rail. To move the rail in the opposite direction, the TCM energizes the solenoid controlling the chamber on the opposite end of the shift rail control plunger. Multiple controllers are used to control the vehicles operation including:

Duonic ECU: Acts as the TCM to control the transmission. Referred to as a “Duonic Electronic Control Unit”.

ECU: “Engine electronic control unit”. This module provides various engine input signals to the TCM and other modules as well as it controls the engine functions.figure 3

SAM: “Signal Detection and Actuation Module” This module acts as the body control module for the CAN bus. It manages the power supply for various components and transfers the signals (inputs/outputs) between various vehicle modules including the ECU, TCM and instrument panel cluster. The SAM directly receives the input from the shift lever position sensor located in the shifter. The shifter position values are then communicated to the TCM via the CAN bus (figure 2).figure 4


Like other transmissions, the Duonic applications utilize multiple speed sensors. Two output speed sensors are used both producing 5 volt square waves (figures 3 and 4).


G Force input comes from 2 G Force sensors, G/Main and G/Sub. The sensors provide a voltage input to the TCM based on the G force present (figure 5). This input is used for:

*Determining if the vehicle is operating on and incline which is used to determine the gear the unit starts in.

*Determining if the vehicle is in at turn, which changes the unit’s shift points and may even inhibit the shifts while turning.


A brake pressure sensor is used to control the apply of the clutches at a stop. The sensor is an input to the TCM (figure 6).


3 shift rail sensors are used, one for each shift rail. The sensors are mounted on the top of the transmission and act as and input to the TCM. The sensors operate from a 5 volt source fed from the TCM.


Like other transmissions we deal with, the Duonic applications also require a relearn process when service has been performed. Known as “initialization”, it is very similar to resetting the adapts or learning the clutch (DSG) on other transmissions.

Initialization is performed with a scan tool or manually any time if the transmission is replaced, the fluid is serviced, shifter replaced, TCM replaced, TCM recalibration, internal repairs or shift complaints have occurred.

This process will relearn:

  • Gear Shifter Position Values
  • Clutch Fill Times
  • Learned Clutch Torque
  • G Force Sensor Values
  • Gear shift Rail Position

To manually perform the initialization process, make sure the transmission fluids have reached at least 120°F (50°C). then proceed as follows:

  1. Park the vehicle on a flat, level surface and chock the wheels.
  2. Fully engage the parking brake.
  3. Turn the ignition to the “ON” position (DO NOT START!).
  4. Fully depress the accelerator to the floor (100% throttle) and hold the pedal in this position (until step 11).
  5. Fully depress the brake pedal and hold it in this position.
  6. Move the gearshift selector lever down from the “P” position through “R” to “N” and then to the “D” position for one second.
  7. Move the selector lever to the “A/M” position and hold in this position (until step 10). The gear position indicator should display “N” on the cluster.
  8. Release and fully engage the parking brake lever for one second.
  9. Release and fully engage the parking brake a second time, then leave the parking brake fully engaged. The gear position indicator should display a flashing “1” on the cluster.
  10. Move the selector lever to the “P” position.
  11. Release the accelerator pedal.
  12. Start the engine and leave it running at an idle.
  13. Release the brake pedal. The gear position indicator should display a flashing “2”.
  14. Allow the system to continue initialization and running through its entire cycle. The gear position indicator should display a flashing “2”, then “3”, “4”, and “5” and the engine speed will increase automatically as initialization proceeds. IMPORTANT!: Do not touch any controls until the cycle completes. Immediately after the indicator starts flashing “5”, turn on the A/C to complete Inner Clutch Torque Learning.NOTE: The A/C system is used to add a load to the engine. If the ambient temperature is below 70°F, use the cab heater to fill the cabin with a heat load to utilize the A/C system.
  15. After approximately 10 to 13 seconds, the engine speed will decrease for a second time. Turn off the A/C to complete Outer Clutch Torque Learning 1.
  16.  When the gear position indicator displays a flashing “N”, initialization is complete.
  17. Turn the ignition key to the “OFF” position.

As you can see times are changing. Even markets that have been predominately dominated by manual transmissions are now seeing some form of automatic or DCT transmission in those applications.

I would like to give a special shout out to Rolly at Rainbow transmissions, Rolly at ATRA, and Fuso engineering for help with pictures and information on this unit.

Until next time remember “problems are only opportunities in work clothes.”