Advanced Automotive Fault Diagnosis

10.3.5 Electronic control of
transmission
The main aim of electronically controlled auto-
matic transmission (ECAT) is to improve on con-
ventional automatic transmission in the following
ways:
●
gear changes should be smoother and quieter;
●
improved performance;
●
reduced fuel consumption;
●
reduction of characteristic changes over sys-
tem life;
●
increased reliability.
The important points to remember are that
gear changes and lock-up of the torque converter
are controlled by hydraulic pressure. In an ECAT
system electrically controlled solenoid valves can
influence this hydraulic pressure. Most ECAT
systems now have a transmission ECU that is in
communication with the engine control ECU.
With an ECAT system the actual point of
gearshift is determined from pre-programmed
memory within the ECU. Data from other sen-
sors is also taken into consideration. Actual
gearshifts are initiated by changes in hydraulic
pressure, which is controlled by solenoid valves.
The two main control functions of this system
are hydraulic pressure and engine torque. A tem-
porary reduction in engine torque during gear
shifting allows smooth operation. This is because
the peak of gearbox output torque which causes
the characteristic surge during gear changes on
conventional automatics is suppressed. Because
of these control functions smooth gearshifts are
possible and, due to the learning ability of some
ECUs, the characteristics remain throughout the
life of the system.
The ability to lock-up the torque converter has
been used for some time even on vehicles with
more conventional automatic transmission. This
gives better fuel economy, quietness and improved
driveability. Lock-up is carried out using a
hydraulic valve, which can be operated gradually
to produce a smooth transition. The timing of lock-
up is determined from ECU memory in terms of
the vehicle speed and acceleration.

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