5A-18 AUTOMATIC TRANSMISSIONINTRODUCTION
The BTR Automotive Model 74 Four Speed Automatic Transmission is an electronically controlled overdrive four
speed unit with a lock-up torque converter. The lock-up torque converter results in lower engine speeds at cruise and
eliminates unnecessary slippage. These features benefit the customer through improved fuel economy and noise
reduction. Refer to table 1.1 for details of power, torque and configuration.
Of primary significance is the transmission control unit (TCU) which is a microprocessor based control system. The
TCU utilizes throttle position, rate of throttle opening, engine speed, transmission output speed, transmission sump
temperature, gear selector position and mode selector inputs, and in some applications a ‘kickdown’ switch to control
all shift feel and shift schedule aspects.
The TCU drives a single proportional solenoid multiplexed to three regulator valves to control all shift feel aspects.
The output pressure of this solenoid is controlled as a function of transmission sump temperature to maintain consistent
shift feel throughout the operating range.
Shift scheduling is highly flexible, and several independent schedules are programmed depending on the vehicle.
Typically the ‘NORMAL’ schedule is used to maximise fuel economy and driveability, and a ‘POWER’ schedule is used
to maximise performance. ‘WINTER’ schedule is used to facilitate starting at second gear.
Figure 1.1 details the differences between conventional and electronic transmission control systems.
Max Torque (Nm)
320 Configuration
260 mm Torque Converter Wide Ratio Gear Set
Splined Output for Transfer CaseMin Torque (Nm) 160
Model
M74 4WD
Transmission
Table 1.1 - M74 Torque, Power and Configuration
5A-20 AUTOMATIC TRANSMISSIONOPERATOR INTERFACES
There are three operator interfaces associated with the four speed transmission. They are: The gear select lever
The driving mode selector
The indicator light
These operator interfaces are described below.
GEAR SELECT LEVER OPERATION
The transmission uses a conventional selector lever. The selector lever can be moved from one position to another
within the staggered configuration of the selector lever gate to positively indicate the gear - selection as shown on
figure 2.1. For information about the gear selections available refer to table 2.1.
Figure 2.1 - Typical Gear Selector and Mode Switch
AUTOMATIC TRANSMISSION 5A-21
Downshift Type
RANGE ‘1’ (MANUAL ‘1’):
RANGE ‘2’ (MANUAL ‘2’):
RANGE ‘3’ (MANUAL ‘3’):
RANGE ‘D’ (DRIVE):
RANGE ‘N’ (NEUTRAL):
RANGE ‘R’ (REVERSE):
RANGE ‘P’ (P ARK):Inhibited Above
First gear operation only with inhibited engagement as a function of vehicle speed. Engine braking is applied with reduced throttle.
First and second gear operation with inhibited engagement of second gear, as
a function of vehicle speed. Engine braking is applied with reduced throttle.
First, second and third gear operation with an inhibited third gear engagement
at high vehicle speed. Refer to the vehicle owner’ s manual.Engine braking is applied with reduced throttle.
First, second, third and fourth gear operation. First to second (1-2), first to third
(1-3), second to third (2-3), second to fourth (2-4), third to fourth (3-4), fourth
to third (4-3), fourth to second (4-2), third to second (3-2), third to first (3-1)
and second to first (2-1), shifts are all available as a function of vehicle speed,throttle position and the time rate of change of the throttle position (forced
downshift). Lockup clutch may be enabled in 3rd and 4th gears depending on
vehicle type. Refer to the owner’ s manual.
Rear band applied only, with inhibited engagement as a function of vehicle
speed, engine speed and throttle position. The inhibitor switch allows the en-gine to start.
Reverse gear operation, with inhibitor engagement as a function of vehicle
speed, engine speed and throttle position. The inhibitor switch enables reverse lamp operation.
Rear band applied only, with inhibited engagement as a function of vehicle
speed, engine speed and throttle position. The transmission output shaft is
locked. The inhibitor switch allows the engine to start.
Table 2.1 - Gear Selections DRIVING MODE SELECTOR
The driving mode selector consists of a mode selection switch and indicator light. The driving mode selector is
located on the centre console. See figure 2,1.
The schedules available to be selected vary with vehicle types. Typically the driver should have the option to select
between ‘NORMAL’ , ‘POWER’ or ‘WINTER’ modes.
When ‘NORMAL’ mode is selected upshifts will occur to maximise fuel economy and the indicator lights remain
extinguished. When ‘POWER’ mode is selected upshifts will occur to give maximum performance and the ‘POWER’
mode indicator light is swi tched on. When ‘WINTER’ mode is selected, starting at second gear is facilitated, the
‘WINTER’ mode indicator light is switched on and the ‘POWER’ mode indicator light is switched off.
Refer to the vehicle owner ’ s manual for specific modes for each vehicle type.
