Transmission This is an electronic device responsible

Transmission Control System. (Based on BMW Transmission)

Transmission Control

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What is it?

This is an electronic device responsible
for the operation of the electronic automatic transmission system. It
contributes heavily to the system’s improved efficiency and reliability over
the manual transmission. Software can configure the TCM to perform in an array
of driving styles. Automatic transmissions present to the user advantages such
as improved fuel economy and reduced driver fatigue due to the lack of
compulsory manual shifting.

How it works

The TCM receives
input from signal sources such as engine sensors (RPM etc), throttle body
sensors(to determine throttle position) and processes these to produce output
which influence other gearbox elements(switches etc) ensure optimal engine
operation. Eg. Improved fuel economy


Electrical System of the
automatic gearbox             

ASC- Automatic Stability Control

DME- Digital Motor Electronics Aka Engine
Control Module(ECM)

EDS- Pressure regulator

MV- Magnetic Valves(shift solenoids)

Achieves gear shifts by manipulating hydraulic fluid


TCM Input

Turbine Shaft Speed Sensor(TSS)

As the name implies, this supplies TCM with TSS input shaft speed sensor data. This coupled with its output is used to generate gear range
and slip information required by the TCM for processing

Engine speed

Not only do TCM’s use TSS as source for gear range and slip
generation. Engine speed signals can also be manipulated by the TCM to
influence input shaft speed.

The nature of the TCM’s input signal(from TSS or Engine
Speed)  can vary between an AC analog
signal and a digital square wave. This is down to the sensor type(i.e. the
former signal is produced by an inductive type sensor where  as the latter is produced by a Hall Effect
Turbine Speed Sensor).

What TCM can do with this input?

Monitor faults

Torque Converter clutch slippage(it’ll engage
the relevant valves and hydraulic systems to reduce slippage if need be)


Output Shaft Speed Sensor

Provides the output shaft speed data used  by the TCM in (1). Unlike
the TSS, this generates only analog signals as the sensor is inductive.


Transmission Oil Temperature Sensor

This provides the TCM with oil
temperature information. The specific sensor type used for this role may be of
the NTC(Negative Thermal Coefficient) type.

What can TCM do with this input?

To display transmission fluid levels when the
gearbox is under diagnostic inspection

To start the gearbox warm up program

To prevent TCC from operating in less than ideal


Kickdown signal(Diagram in BMW pdf)

It can be
described as a direct ground input to the TCM. It notifies the TCM on
when to change down a gear. TCM kickdown request signals can be generated in
two ways:

Manually via a kickdown switch

Electronically from the Electronic
Control Module(ECM)

Manually via a kickdown switch:

Usually used on models without an
electronic throttle control system

Electronically from the ECM

simulate a manual kickdown request, the PWG(Pedal Position Sensor) uses a
kickdown detent. This is initiated when the voltage across the PWG reaches a
specified value.


Brake Switch

This is located on the brake pedal linkage.


What does the input do?

In the TCM, this
input is processed to deactivate:

Shift-lock solenoid

Torque Converter

When the brakes are applied. ???



Transmission Range Selector Switch (Diagram in
BMW pdf)

TCM uses this input
to determine the state of a manual valve. Using coded input signals, the range
switch can determine selector lever postion(i.e. if the selector lever
is in D or P for example).

EImage of the system described above

Explaining the image

When the gear is selected, a
combination of switches are activated or deactivated based on predetermined
patterns. The switch locations allow the TCM to control the flow of hydraulic
fluid through the transmission, activating the relevant gear set pattern.


Transmission Program
switch (BMW)

As the ‘box is electronic, its
characteristics can be varied through programming. Program modes may vary from
‘Normal’ where the gearshifts are optimised to improve fuel economy and drive
comfort to ‘Race’ mode where gearshifts are optimised to be more aggressive and
sports like. These shift patterns are stored in memory and the appropriate one
is applied when the user select a desired mode.


Steptronic Components (BMW diagram)

Image from Electronic Transmission PDF(BMW)

A Steptronic system allows the driver to
manually engage the gears of the automatic transmission. It is very similar in
operation to the normal automatic transmission with the main difference being
the addition of a manual switch and an Up/Down microswitch.

“automatic mode”, the TCM monitors the gear lever position from P to D just as
it does in a conventional system. However, when the “manual mode” is engaged,
the TCM is alerted via a ground signal from the manual gate switch. This is
provided to the TCM through the Transmission Position Indicator.


9.      Magnetic Valves

These are attached
to valves within the transmission. They allow the TCM to control the flow of
hydraulic fluid throughout the transmission system. The TCM engages and
disengages these valves in a predetermined way to achieve a gear selection.

Other uses include:

Pressure Regulation


TCM and the User

The TCM alerts the
frequently updates the user on the health of the transmission system. Examples
of sensor data shared with the user include,

Gear Position

mode(Normal, Sport, etc.)


Some terms:

Lock-Up torque converter

Kickdown detent

Shift-lock solenoid

Torque converter

Hydramatic transmission

Over-lap shift





B Electronic transmission control Accessed

C Abbreviation List

D How an automatic transmission works