The ISC (Idle Speed Control) system is provided with a circuit that bypasses the throttle valve, and the air volume drawn in from the bypass circuit is controlled by the ISCV (Idle Speed Control Valve). The ISCV uses the signal from the engine ECU to control the engine at the optimum idling speed at all times. The ISC system consists of the ISCV, engine ECU, and various sensors and switches.
1. When starting
The bypass circuit is opened to improve startability.
2. When warming-up the engine
When the coolant temperature is low, the idling speed is increased so that the engine runs smoothly (fast idle). As the coolant temperature rises, the idling speed is decreased.
3. Feedback control and estimate
control
When A/C is used
When headlights are used
When the shift lever is shifted from N to D or from D to N while the vehicle is stopped. In the above cases, if the load increases or changes, the idle speed is increased or prevented from changing.
ISCV (Idle Speed Control Valve)
Types of ISCV
The ISCV is a device that controls the amount of the intake air during idling using the signal from the engine ECU and controls the idling speed. There are two types of ISCV as follows. 1. Type that bypasses the throttle valve and controls the amount of the intake air:
Because the throttle valve is completely closed during idling, the ISCV bypasses the required air volume during idling.
2. Type that controls the amount of the intake air using the throttle valve:
With this type, the throttle valve proper controls the amount of the intake air during idling. This system is called ETCS-i (Electronic Throttle Control System-intelligent), and conducts other control functions in addition to amount of the intake air control during idling. Refer to “ETCS-i” of the group in the “Other Control System” of the chapter for details.
Rotary Solenoid Type
The rotary solenoid type ISCV consists of a coil, IC, permanent magnet, valve, and is attached to the throttle body. The IC uses the duty signal from the engine ECU to control the direction and amount of current that flows in the coil and control the amount of air that bypasses the throttle valve, rotating the valve.
1. Operation
When the duty ratio is high, the IC moves the valve in the opening direction, and when the duty ratio is low, the IC moves the valve in the closing direction. The ISCV performs opening and closing in this way.
HINT:
If there is trouble, such as an open circuit, that causes the electricity to stop flowing to the ISCV, the valve is made to open at a set position by the force of the permanent magnet. This will maintain an idling speed of approx. 1,000 to 1,200 rpm.
REFERENCE:
Old Type Rotary Solenoid ISCV
The old type rotary solenoid ISCV received the duty signals from the engine ECU apply the current to two coils to change the amount the valve opens and control the amount of the intake air. The bimetal strip in the ISCV responds to the engine coolant temperature to maintain the appropriate valve opening for the engine warm-up state. A guard is also installed to prevent the valve from being stuck completely open or closed if some kind of electrical trouble occurs.
1. Operation
Valve open
When electricity is sent to coil A (RSO) for a long time, the valve is caused to move in the opening direction.
Valve close
When electricity is sent to coil B for a long time, the valve is caused to move in the closing direction.
Other Types of Bypass ISCV
Duty-control ACV type
The duty-control ACV type ISCV controls the amount of the intake air flowing in the bypass circuit by the duty signal from the engine ECU cause current to flow to the solenoid coil to open the valve. The larger the duty ratio of the electricity flows to the solenoid coil, the farther the valve opens.
ON-OFF control VSV type
The ON-OFF control VSV type ISCV controls the amount of the intake air flowing in the bypass circuit by the ON/ OFF signal from the engine ECU cause current to flow to the solenoid coil to open the valve. When current flows to the solenoid coil, the idling speed is increased in increments of approx. 100 rpm.
Stepper Motor Type
The stepper motor-type ISCV is attached to the intake chamber. The valve installed at the end of the rotor is in or out by the rotation of the rotor to control the air volume flowing in the bypass circuit.
1. Operation
The step motor utilizes the principle of pulling and rebounding of the permanent magnet (rotor) when a magnetic field is generated by electricity flowing in the coil. As shown in the illustration at bottom left, current flows at C1 causing the magnet to be pulled. When the current to C1 is cut off at the same time, the current is made to flow to C2, and the magnet is pulled to C2. The subsequent switching the current to in order of C3 and C4 in the same manner is used to rotate the magnet. The magnet can also rotate in the opposite direction by switching the current in the direction from C4 to C3, C2, and C1. This arrangement is used to move the magnet to predetermined positions. An actual step motor uses 4 coils to create 32 steps for 1 rotation of the magnet (rotor). (Some motors have 24 steps per rotation.)
Valve open
When electricity is sent to coil A (RSO) for a long time, the valve is caused to move in the opening direction.
Valve close
When electricity is sent to coil B for a long time, the valve is caused to move in the closing direction.