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Home >> Ford >> 2012 >> Mustang Base, 2D Coupe, Automatic Trans >> Repair and Diagnosis >> External Pages >> Different car >> Section 714 (Four-Wheel Drive Systems) >> Diagnosis And Testing >> Four-Wheel Drive Systems - (Vehicles With: 2-Speed Torque On Demand Transfer Case) >> Principles of Operation

Principles of Operation

WARNING: This page is about a different car, the 2015 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

The 2-speed torque-on-demand system allows the operator to choose between 2H (4x2) and 3 different 4WD modes of operation: 4A (4X4 AUTO), 4H (4X4 HIGH) and 4L (4x4 LOW). When 4WD modes are selected the TCCM sends a duty cycle command to the transfer case coil which activates the clutch within the transfer case and transfers torque to the front driveline. When the 2H mode is selected, the TCCM duty cycle command is turned OFF and the IWEs located at each front wheel are disengaged to reduce additional drag from the front driveline.

Engagement and disengagement of the front wheels is done by the IWEs. The IWEs are normally engaged (no vacuum). Vacuum is supplied to the IWEs by actuation of the IWE solenoid commanded by the TCCM. When actuated, the IWE solenoid allows vacuum to pass through to each IWE. The IWEs disengage and allow the front wheel hubs to be turned freely from the front axle. Note: When shifting to 2H mode in order for the IWEs to completely disengage the vehicle steering wheel must be less than 90 deg and 5 seconds have elapsed since the shift to 2H has been initiated.

The electric shift motor is mounted externally to the transfer case. It drives a rotary cam which moves the range fork within the transfer case between the HIGH range (2H, 4A and 4H) and LOW range (4L) positions. The electric shift motor only moves during a shift to and from 4L and Neutral, it does not move during any other mode shift. TCCM directly controls the electric shift motor and can reverse motor polarity to reverse rotary cam/shift fork direction. The shift motor sense plate, an integral part of the shift motor assembly, informs the TCCM of the transfer case position.

This transfer case is equipped with an electronically controlled clutch which is located inside the case. This clutch is used to synchronize the speed of the front driveline with the rear driveline as well as transfer torque in either of the 3 different 4WD modes. When the MSS is switched between 2H and 4A or 4H modes, the TCCM first energizes the clutch. When the transfer case front and rear output shafts are synchronized, the front axle IWEs are engaged.

When the front and rear driveline are synchronized the TCCM send a duty cycle command to the transfer case clutch coil as a torque request based on combination of preemptive and wheel slip response algorithm. Preemptive response is based on steering wheel angle, vehicle speed, throttle positions and available powertrain torque. Wheel slip response is based on monitoring the average front and rear wheel speeds.

The TCCM has a cold strategy which is used to warm up the front axle in cold temperatures to improve driveline synchronization. When ambient temperature is below 32 F (0 C). IWEs engages after initial key cycle and a driven gear is selected. The hubs stay engaged regardless of 4x4 mode change for proximately 2 miles (3km). Once distance traveled has been achieved IWEs disengages if vehicle is in 2H. Distance traveled resets only if the temperature is below calibrated threshold and another key cycle has occurred or customer shifted to park and back to a driven gear within the same key cycle.

In 4A, the TCCM continuously monitors conditions and driver input to send torque automatically to the front driveline by controlling the transfer case clutch, providing 4WD capability. When 4WD is no longer needed (during cruising or steady state driving) system defaults back to RWD mode by setting the duty cycle output to 0. In situation where it is necessary to negotiating tight turns, clutch duty cycle output is reduced allowing slight difference between the front and rear driveshafts. However, the system is always on alert and when necessary, torque is sent automatically.

In 4H and 4L, the TCCM output behaves similarly to 4A mode but with increased duty cycle for greater 4WD performance.

TCCM increases the duty cycle to prevent or control slip under any of the following conditions:

Shifts from 2H mode to 4A or 4H mode can be made at any speed. When performing this shift, release the accelerator pedal prior to the shift and wait until the Shift in Progress message disappears in the IPC before accelerating, this improves the shift performance. The following are the inputs and outputs needed by the TCCM to execute a change between these modes and deliver the needed function:

Feature inputs 

Feature outputs: 

Shifts between 4X4 AUTO (4A) and 4X4 (4H) modes can be made at any speed. Listed below are the inputs and outputs needed by the TCCM to execute a change between any of these modes.

Feature inputs: 

Feature outputs: 

When shifting into or out of 4L (4x4 LOW) mode, the vehicle speed must be less than 5 km/h (3 mph) and the transmission must be in neutral. The following are the inputs and outputs needed by the TCCM to execute a change to this mode and deliver the needed function.

Feature inputs: 

Feature outputs: 

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