Function Of Tcm; Control Of Shift Timing; Control Of Overdrive; Control Of Lock-Up System - Toyota LAND CRUISER Service Manual Supplement

AT-10
AUTOMATIC TRANSMISSION (STATION WAGON) - OPERATION
4.

FUNCTION OF TCM

• Control of Shift Timing

The ECU has programmed into its memory the optimum shift pattern for each shift lever position (D, 2,
L ranges) and driving mode (Normal or Power).
Based on the appropriate shift pattern, the ECU turns No.1, No.2 and timing solenoid valves on or off
in accordance with the vehicle speed signal from the speed sensor and the throttle opening signal from
the throttle position sensor. In this manner, the ECU operates each shift valve, opening or closing the
fluid passages to the clutches and brakes to permit up-shift or down-shift of the transmission.
HINT: The electronic control system provides shift timing and lock-up control only while the vehicle
is traveling forward. In REVERSE, and NEUTRAL, the transmission is mechanically, not electronically
controlled.

• Control of Overdrive

Driving in overdrive is possible if the O/D main switch is on and the shift lever is in the D range. How-
ever, when the vehicle is being driven using the cruise control system (CCS), if the actual vehicle
speed drops to about 4 km/h (2 mph) below the set speed while the vehicle is running in overdrive, the
CCS ECU sends a signal to the ECT ECU to release the overdrive and prevent the transmission from
shifting back into overdrive until the actual vehicle speed reaches the speed set in the CCS memory.
On this model, if the coolant temperature falls below 55°C (131 °F), the engine ECU sends a signal to
the ECT ECU, preventing the transmission from up-shifting into overdrive.

• Control of Lock-Up System

The ECT ECU has programmed in its memory a lock-up clutch operation pattern for each driving mode
(Normal or Power). Based on this lock-up pattern, the ECU turns lock-up solenoid valve on or off in ac-
cordance with the vehicle speed signals received from the speed sensor and the throttle opening sig-
nals from the the throttle position sensor.
Depending on whether lock-up solenoid valve is on or off, the lock-up relay valve performs changeover
of the fluid passages for the converter pressure acting on the torque converter to engage or disengage
the lock-up clutch.
(Mandatory Cancellation of Lock-Up System)
If any of the following conditions exist, the ECU turns off lock-up solenoid valve to disengage the
lock-up clutch.
(1) The brake light switch comes on (during braking).
(2) The IDL points of the throttle position sensor close (throttle valve fully closed.).
(3) The vehicle speed drops 4 km/h (2 mph) or more below the set speed while the cruise control sys-
tem is operating.
(4) The coolant temperature falls below 70°C (1 58°F).
The purpose of (1) and (2) above is to prevent the engine from stalling if the rear wheels lock up.
The purpose of (3) is to cause the torque converter operate to obtain torque multiplication.
The purpose of (4) is both to improve general driveability, and to speed up transmission warm-up.
Also, while the lock-up system is in operation, the ECU will temporarily turn it off during up-shift or
down-shift in order to decrease shifting shock.