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3A2-4
CONTROL SYSTEMS
BTRA M74 4WD automatic transmission consists of two
control systems. One is the electronic control system that
monitors vehicle parameters and adjusts the transmission
performance. Another is the hydraulic control system that
implements the commands of the electronic control sys-
tem commands.
ELECTRONIC CONTROL SYSTEM
The electronic control system comprises of sensors, a TCM
and seven solenoids. The TCM reads the inputs and acti-
vates the outputs according to values stored in Read Only
Memory (ROM).
The TCM controls the hydraulic control system. This con-
trol is via the hydraulic valve body, which contains seven
electromagnetic solenoids. Six of the seven solenoids are
used to control the line pressure, operate the shift valves
and the torque converter lock-up clutch, and to turn ON
and OFF the two regulator valves that control the shift feel.
The seventh solenoid is the proportional or Variable Pres-
sure Solenoid (VPS) which works with the two regu-lator
valves to control shift feel.
Transmission Control Module (TCM)
The TCM is an in-vehicle micro-processor based trans-mis-
sion management system. It is mounted under the driver's
side front seat in the vehicle cabin.
The TCM contains:
• Processing logic circuits which include a central mi-
croprocessor controller and a back-up memory system.
• Input circuits.
• Output circuits which control external devices such as
the Variable Pressure Solenoid (VPS) driver, On/Off
solenoid drivers, a diagnostics output and the driving
mode indicator light.
CHANGED BY
EFFECTIVE DATE
AFFECTED VIN
Processing logic
Shift schedule and calibration information is stored in an
Erasable Programmable Read Only Memory (EPROM).
Throttle input calibration constants and the diagnostics
information are stored in Electrically Erasable Program-
mable Read Only Memory (EEPROM) that retains the
memory even when power to the TCM is disconnected.
TCM continuously monitors the input values and uses
these, via the shift schedule, to determine the required
gear state. At the same time it monitors, via the solenoid
outputs, the current gear state, whenever the input condi-
tions change such that the required gear state is different
to the current gear state, the TCM initiates a gear shift to
bring the two states back into line.
Once the TCM has determined the type of gearshift re-
quired the TCM accesses the shift logic, estimates the
engine torque output, adjusts the variable pressure sole-
noid ramp pressure then executes the shift.
The TCM continuously monitors every input and output
circuit for short or open circuits and operating range.
When a failure or abnormal operation is detected the TCM
records the condition code in the diagnostics memory and
implements a Limp Home Mode (LHM).
The actual limp home mode used depends upon the fail-
ure detected with the object to maintain maximum drive-
ability without damaging the transmission. In general in-
put failures are handled by providing a default value. Out-
put failures, which are capable of damaging the
transmission, result in full limp mode giving only third or
fourth gear and reverse. For further details of limp modes
and memory retention refer to the Diagnostic Trouble Code
Diagnosis Section.
The TCM is designed to operate at ambient temperatures
between - 40 and 85°C (- 40 and 185°F). It is also pro-
tected against electrical noise and voltage spikes, how-
ever all the usual precautions should be observed, for ex-
ample when arc welding or jump starting.
BTRA 4 AUTO TRANSMISSION
REXTON SM - 2004.4
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