Idle Speed Control - Yamaha F150B Service Manual

Idle speed control

Engine ECM controls the amount of intake air into the engine at idle by driving the ISC stepper
motor based on the signals from each sensor to regulate the area of the passage bypassing the
main pipe where the throttle valve is installed.
Determination of the ISC valve opening
Based on the signals from each sensor, engine ECM determines the extent to which the ISC valve
must be opened, and controls the ISC stepper motor operation to obtain the target ISC valve open-
ing.
ISC valve opening
ISC valve opening at engine start
ISC valve opening immediately
after engine start
Dashpot ISC valve opening
ISC valve opening with engine
speed compensation
ISC valve
Engine ECM controls the amount of intake air at idle by driving the stepper motor, and changing the
area of the passage bypassing the main pipe where the throttle valve is installed. The stepper motor
moves the ISC valve in 120 steps from fully open position to fully closed position. The ISC valve
opens when the stepper motor is driven in reverse direction, and closes when the stepper motor is
driven in forward direction.
#4
A
Intake air flow
B
Movement of ISC valve
At engine start, ISC valve is opened relatively wider depending on the
engine temperature. This increases the amount intake air and
improves the engine startability.
After engine start, the initial ISC valve opening determined by the
engine temperature is reduced in steps and at constant interval until
the idle speed is obtained.
When the throttle lever is pulled back, ISC valve opening is controlled
to prevent the engine speed from dropping off sharply.
The difference between the actual engine idle speed and the target
engine speed is calculated, and ISC valve opening is altered so that
the target engine speed is obtained.
b
#3
Electronic control system
Engine condition
#2
a
ISC valve
b
Intake manifold
#1
a
A
B
2-16
0
1
2
3
4
5
6
7
8
9
10
A