Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 15

OUTLINE OF TCCS - System Descriptio n
ISC (IDLE SPEED CONTROL )
The Engine ECU is programmed with target
engine speed values to respond to different
engine conditions such as :
• Coolant temperature (THW)
• Air conditioner on/off (A/C)
etc .
Sensors transmit signals to the Engine ECU,
which, by means of the ISC valve, controls the
flow of air through the throttle valve bypass and
adjusts the idle speed to the target value .
ISC valve
DIAGNOSTIC FUNCTIO N
The Engine ECU is constantly monitoring the
signals that are input to it from the various
sensors . If it detects any malfunctions in the
input signals, the Engine ECU stores data on the
malfunction in its memory and lights the
"CHECK ENGINE" lamp . When necessary, it
displays the malfunction by lighting the "CHECK
ENGINE" lamp, displaying on a tester* or output-
ting a voltage signal .
* OBD-II scan tool or TOYOTA hand-held teste r
"CHECK ENGINE" lamp
OHP 4
Sensors
OHP 4
FAIL-SAFE FUNCTIO N
If the signals input to the Engine ECU are
abnormal, the Engine ECU switches to standard
values stored in its internal memory to control
the engine . This makes it possible to control the
engine so as to continue more-or-less normal
vehicle operation .
BACK-UP FUNCTIO N
Even if the Engine ECU itself becomes partially
inoperative, the back-up function can continue to
execute fuel injection and ignition timing
control . This makes it possible to control the
engine so as to continue more-or-less normal
vehicle operation .
OTHER CONTROL SYSTEM S
In some engines, the OD cut-off control system,
intake air control system, and some other aux-
iliary systems are also controlled by the Engine
ECU .
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