Yamaha F150B Service Manual - page 59

2-16

Electronic control system

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1

2

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9

10

A

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

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.

A

Intake air flow

B

Movement of ISC valve

a

ISC valve

b

Intake manifold

ISC valve opening

Engine condition

ISC valve opening at engine start

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.

ISC valve opening immediately 

after engine start

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.

Dashpot ISC valve opening

When the throttle lever is pulled back, ISC valve opening is controlled 

to prevent the engine speed from dropping off sharply.

ISC valve opening with engine 

speed compensation

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.

#1

#2

#3

#4

A

B

a

b