Vanden Plas L6-4.0L (1990)
opens the idle speed control valve a predetermined number of steps to anticipate the change in engine load.
Ignition Switched OFF
When the ignition is switched OFF, the control valve indexes to a known parked position. On 1990 model year vehicles, the reference is from the
fully opened position. On later vehicles, the reference is from the fully closed position, 7 seconds after the ignition is switched OFF.
IDLE SPEED CONTROL VALVE
The idle speed control valve assembly located on the top of the intake manifold contains a manually adjusted base idle throttle bypass passage and
an additional variable bypass passage. The flow of bypass air through the variable passage is regulated by a stepper motor driven by the ECM. The
stepper motor has two coils that are pulsed by the ECM. The pulses are phased 90° apart. The order in which the coils are pulsed determines the
direction of stepper motor travel. The coil resistance is approximately 50 ohms.
The stepper motor has a total travel from fully opened to fully closed of approximately 230 steps.
During engine setup (using JDS or PDU), the stepper motor is positioned at 200 steps from fully opened to allow the base idle speed to be set,
using the manual air bypass screw. Once the base idle speed is set correctly, the stepper motor will hold the closed loop idle speed within a few
rpm of the nominal idle speed (700 rpm in "N" and 580 rpm in "D"; normal operating coolant temperature).
When the ignition is switched OFF, the control valve indexes to a known "parked" position.
On 1990 model year vehicles, the reference is from the fully opened position. On later vehicles, the reference is from the fully closed position.
On 1993 and later model years, there is a 7-second delay after the ignition is switched OFF before the control valve closes.
To accurately maintain a correct idle speed, the ECM "learns" the idle stepper position. If the ECM is disconnected from battery power, the idle
position must be relearned. The simple way to allow the ECM to relearn the position is to start the engine from cold and allow it to idle until fully
warm, then drive the vehicle for a short distance.
Idle Speed Control Valve
