Ford Workshop Service and Repair Manuals

Transmission control unit

The transmission control unit processes the input signals and uses them to control the solenoid valves to regulate the pressure.

The following sensors are also integrated in the transmission control unit:

• Main pressure sensor
• Pressing force sensor
• TSS sensor
• Speed sensor, variable speed input gear
• TFT sensor (integrated in the electronics)

The signals from the OSS sensor and the TR sensor are hard-wired as further input signals into the transmission control unit within the transmission housing.

The vehicle wiring harness connector provides the current and ground connection for the transmission control unit as well as the connection to the CAN databus.

In addition, the signals from the following sensors and actuators are transmitted via the electrical connector to the transmission or the vehicle:

• Select-Shift switch
• Brake pressure switch
• Starter inhibit relay
• Backup lamp relay

The transmission control unit can be diagnosed with WDS via the DLC.

Communications with WDS take place on the CAN databus.

Gearshift strategies

Calculation of the transmission ratio

The main task of the transmission control is to select the transmission ratio in such a way that the guide variable "engine speed" always follows a fixed strategy.

The current engine speed is the variable which is controlled in a closed loop control system.

The deviation between target value and actual value provides the input variable for the current transmission ratio.

The current transmission ratio is adjusted by the transmission control unit via the variable speed gear adjustment solenoid valve.

The closed loop control is adaptive. The variable speed transmission ratio is determined according to the current driving style and the current driving situation.

The following input variables play a key role in the calculation of the transmission ratio:

• Accelerator pedal position
• Accelerator pedal actuation
• Accelerator pedal actuation speed
• Current vehicle speed
• Current vehicle acceleration or deceleration
• Operation of the brakes

The current driving style is determined from these factors. The system then determines the so-called sports factor.

The closed loop transmission ratio control is adapted according to this sports factor and the current driving resistance.

Cold start mode

Cold start mode is employed when it is important to get the transmission fluid up to operating temperature as quickly as possible.

To do this, the Torque Converter Clutch (TCC) is not engaged at temperatures below approx. 0 °C.

In addition, the engine speed is also raised.

Overheating protection mode

NOTE:The overheating protection mode cannot be implemented when the transmission is in hydraulic emergency running mode. Overheating of the transmission can lead to serious transmission damage. Avoid towing a trailer.

The overheating protection mode serves to protect the transmission against overheating and the serious damage that this can cause.

Above a transmission fluid temperature of approximately 130 °C, the engine speed and the engine torque are continuously reduced according to the transmission fluid temperature by the PCM in response to a request from the transmission control unit.

In the most extreme case this could lead to the vehicle stopping in order to protect the automatic transmission.

Fast-off detection

The fast-off detection is designed to give the driver a more direct driving feel.

A 'fast-off' situation is recognised when the accelerator pedal is suddenly released.

The system then prevents an "upshift" or rapid change of the transmission ratio.

Depending on the driving style and the driving resistance, the engine speed is reduced more or less quickly.

The following rule applies: when the driving style is sporty the engine speed remains higher for longer.

Hill descent detection

Hill descent detection is designed to allow engine braking to be exploited.

A 'hill descent' situation is recognised if the vehicle is accelerating without operation of the accelerator pedal.

The current variable speed transmission ratio is held if hill descent is detected.

If this is not sufficient (further acceleration without actuation of the accelerator pedal), then the transmission ratio is adjusted accordingly ("downshift").

If the vehicle still continues to accelerate then the driver should switch to Select-Shift mode and manually select the transmission range which offers the right amount of engine braking.

Cornering detection

The transmission control system detects cornering by comparing the wheel speed sensor signals from the two non-driven wheels.

Depending on the detected difference in speed the current variable speed transmission ratio is maintained to give the driver a more direct driving feel.

Automatic engagement of neutral when the vehicle is stationary

In order to reduce fuel consumption and hence lower emissions, the transmission is shifted into quasi-neutral when the vehicle is stationary, a gear is engaged and the engine is running at idle speed.

The transmission control is provided with the information whether or not the vehicle is actually stationary from a separate brake pressure switch in the brake system or, on vehicles with ESP, via the CAN databus.

The information about the engine idling is provided to the transmission control unit by the PCM via the CAN databus.

If both requirements are satisfied then the transmission control unit releases the pressure at the forward clutch and thus interrupts the power transfer.

The automatic engagement of neutral when the vehicle is stationary and the engagement of the forward clutch are monitored by a comparison of the speed signals from the transmission input shaft and the variable speed input gear.

Select-Shift mode

The driver can select the Select-Shift mode directly by moving the selector lever into the Select-Shift gate.

Seven fixed transmission ratios are stored in the transmission control unit, which convert the continuously variable automatic transmission into an automatic transmission with fixed gear ratios.

When the driver changes from automatic driving mode into manual driving mode, the transmission will not automatically be in one of these fixed programmed transmission ratios. It is far more likely that the transmission is somewhere between two of these transmission ratios.

Initially, the current transmission ratio is maintained when the driver changes into Select-Shift mode, and it is then taken into account in the calculations for the next gear change.

During the next gear change the transmission ratio which is actually engaged by the transmission control is brought more closely to the programmed fixed transmission ratio, and by the next gear change after that at the latest the programmed fixed ratios are engaged and maintained.

A "kickdown" (automatic downshift to the next gear) is possible in Select-Shift mode.

To do this, the transmission control must receive an accelerator pedal position of at least 95% (WOT) via the CAN databus from the PCM.

If the engine reaches its maximum speed without the accelerator pedal position reaching 95% then the transmission control remains in the currently selected gear.

The driver can feel the kickdown point in the accelerator pedal as a slight increase in resistance to the operation of the pedal.

In order to prevent the engine from stalling, the control automatically shifts down to the next gear when the engine reaches a certain speed.

If the action of shifting down a gear would cause the engine to rev above its maximum speed limit then the transmission control unit does not allow the downshift to take place. The selected gear is displayed on the instrument cluster next to the engaged gear, and once the required engine speed has been reached the selected gear is then engaged.

1st or 2nd gear can be used to pull the vehicle away from standing in Select-Shift mode (winter operation).

It is possible to change back and forth in both directions between automatic driving mode and Select-Shift mode under any operating conditions.

Hydraulic emergency running mode

If the transmission control detects serious faults in the transmission then it switches into hydraulic emergency running mode.

This means that the solenoid valves are no longer actuated.

Continued driving in R and D is enabled via the mechanically operated manual selector lever with the following restrictions:

• increased main pressure,
• substantially reduced changes in transmission ratio,
• engagement of the clutch is not controlled (harsh gearshift judder).

If the transmission control unit is still connected to the CAN databus, then the communications to the other control units are kept alive.

However, the PCM then substantially restricts the engine torque output and the engine speed range in response to a request from the transmission control unit.

Hydraulic emergency running mode is displayed on the instrument cluster for the driver.

Signal transmission on the CAN databus

Engine speed

The PCM receives an engine speed signal from the Crankshaft Position (CKP) sensor.

This signal is transmitted via the CAN databus to the transmission control unit.

The transmission control unit uses this signal for the following functions:

• Calculation of the variable speed transmission ratio
• Controlling the TCC
• Automatic engagement of neutral when the vehicle is stationary
• Select-Shift mode

Accelerator pedal position and actuation speed

The PCM receives a signal about the accelerator pedal position and actuation speed from the Accelerator Pedal Position (APP) sensor.

This signal is transmitted via the CAN databus to the transmission control unit.

The transmission control unit uses this signal for the following functions:

• Determination of the sports factor in order to determine the current driving style.
• Calculation of the variable speed transmission ratio
• Kickdown function in Select-Shift mode

Load signal

The PCM determines a load signal which is transmitted via the CAN databus to the transmission control unit.

The transmission control unit uses this signal for the following functions:

• Determination of the gearshift strategy
• Calculation of the variable speed transmission ratio
• Calculation of the pressing force

Wheel speed signals

The wheel sensors supply a separate wheel speed signal for each wheel to the ABS or ESP control unit.

These wheel speed signals are transmitted via the CAN databus to the transmission control unit.

The transmission control unit uses this signal for the following functions:

• Vehicle speed input signal
• Vehicle acceleration input signal
• Cornering detection

Brake pressure signal

NOTE:Vehicles without ESP are equipped with a separate brake pressure switch which sends the brake pressure signal directly (not via the CAN databus) to the transmission control unit.

On vehicles with ESP a brake pressure signal is sent from the ESP control unit to the transmission control unit. This signal is used by the transmission control unit for automatic engagement of neutral when the vehicle is stationary.

Selector lever position and operation

The transmission control unit transmits the selector lever position (in automatic mode) or the engaged gear (in Select-Shift mode) via the CAN databus to the instrument cluster, which in turn displays the information for the driver.

After a repeat operation of the selector lever in Select-Shift mode, the gear which has been selected but has not yet been engaged by the transmission control unit is also displayed.

When the driver engages R or D, he/she is also warned with warning and information displays on the instrument cluster if engagement of the forward or reverse gear is delayed due to low ambient temperatures.

A corresponding warning message is also displayed if the transmission control goes into hydraulic emergency running mode or if serious transmission faults are present.

Actuation of the MIL

The MIL warning lamp is actuated by the instrument cluster if there are any faults in the transmission control which lead to a substantial increase in fuel consumption.

The information required to actuate the MIL warning lamp is sent by the transmission control unit to the instrument cluster.

Sensors

TSS sensor

The TSS sensor is part of the transmission control unit.

It is an inductive sensor which measures the rotational speed of the transmission input shaft.

The signal is used for the following functions:

• Engagement of the TCC
• Engagement of the forward clutch
• Engagement of the reverse gear brake
• Automatic engagement of neutral when the vehicle is stationary

Speed sensor, variable speed input gear

The speed sensor for the variable speed input gear is part of the transmission control unit.

It is an inductive sensor which measures the rotational speed of the variable speed input gear.

The signal is used for the following functions:

• Engagement of the forward clutch
• Engagement of the reverse gear brake
• Automatic engagement of neutral when the vehicle is stationary
• Calculation of the variable speed transmission ratio

OSS sensor