Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 124

W
NOTE
ISC - Functions of Engine EC U
Learned Contro l
Besides the previously mentioned control, learn-
ed control is also used for control of the ISC
valve .
Normally, the Engine ECU controls the idle
speed by changing the ISC valve position .
However, since the engine's running condi-
tions change over time, the idle speed also
changes (even though the valve positions re-
main the same) .
So through feedback control, the Engine ECU
outputs ISC signals to return the idle speed to
the target level . The ISC valve position when
the target speed is reached is stored in back-up
memory, and is used thereafter in idling . This is
known as learned control .
If all power to the Engine ECU is cut off due to
the EFI fuse or STOP fuse being removed or a
battery cable being disconnected, the learned
value stored in back-up memory will be erased .
Therefore, when the engine is restarted, the
ISC valve position is set at the initial value
stored in memory .
At this time, the idle speed may not be the
same as the target speed, but when the engine
warms up and feedback control starts, it will
gradually approach the target speed .
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