ISC - Functions of Engine EC
U
ENGINE SPEED CHANGE ESTIMATE CONTRO
L
Immediately after the neutral start switch, tail
lamp relay, defogger relay or air conditioner
switch is operated, the engine load also changes
.
To prevent the engine speed from changing becau-
se of this, the ECU sends signals to the ISC valve
to open or close it by a fixed amount
before
changes in the engine speed can occur
.
RELEVANT SIGNALS
Neutral start
switch (NSW)
Electrical load
(LP, DFG, or ELS
)
• Engine speed (NE
)
• Air conditioner (A/C)
*
* Some models onl
y
OTHER CONTROL
S
Controls other than those described above
include dashpot control, which controls the ISC
valve so as to prevent a sudden drop due to
sudden changes in engine speed when the IDL
contact in the throttle position sensor closes
.
In some vehicle models equipped with EHPS
(electro-hydraulic power steering), the idle
speed is also increased whenever the electrical
load increases greatly due to the operation of
the EHPS
.
Another control, used in some turbocharged
engines, prevents the turbine from seizing up if
the hydraulic pressure should drop too low to
provide sufficient turbine lubrication when the
idle speed returns to normal following high-
speed or high-load operation
. It does this by
causing the idle speed to drop gradually so that
the oil pump will supply a sufficient amount of
oil to the turbocharger
.
®
~ 1 V V 1
C
1
. When terminal T or TE1 of the check con-
nector or TDCL is connected with terminal
E1, the Engine ECU gradually changes the
duty ratio of the ISC valve for several
seconds and eventually fixed the duty ratio
at a constant value
.
As a result, engine speed returns to the
original idle speed after increasing for
several seconds
.
2
. When the current flowing to the coil is inter-
rupted due to disconnection of the ISC
valve connector or other reason, the valve
stops at the position at which the S or N
pole of the permanent magnet is facing the
core of the coil
.
As a result, idle speed is slightly lower
when the engine is cold and slightly higher
after the engine has warmed up than during
normal operation
. (Example
: Idle speed
after warm-up is approximately 1000 -
1200 rpm
.
)
Core
Normal
position Valve leve
r
n
. W/
Guar
d
Permanent After
magnet
When cold warmed-up
109
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