XJ-6 L6-4.0L (1991)
Idle Speed Control Valve: Description and Operation
IDLE SPEED CONTROL
Idle speed is regulated by the motorized idle speed control valve that controls throttle bypass air. The control valve is driven by the ECM. The
ECM uses inputs received from ignition ON, the crankshaft sensor, coolant temperature sensor and throttle position sensor as well as inputs for
gear position, air conditioning compressor operation and road speed to control idle speed.
ECM idle speed control occurs at closed throttle when road speed is less than 3 mph. The programmed idle speed accounts for engine temperature
and the loads placed on the engine by the transmission (gear position N, D, etc.), and air conditioning compressor clutch operation.
Engine Speeds
An ECM software function allows for a correction to the idle speed "base line" as the engine base idle changes with age. The automatic adjustment
values are held in RAM within the ECM and will be retained or updated as long as the ECM is connected to battery voltage. If battery voltage is
removed for any reason, the stored correction values will be lost. The values will be relearned only after the battery is reconnected, the engine
operated from cold to normal operating coolant temperature at idle, and the vehicle driven for approximately 50 yards above 3 mph.
NOTE: At road speeds above 3 mph, the idle speed control valve is opened to limit overrun intake manifold pressure. The amount that the valve
is opened is based on engine speed and throttle opening.
The ECM monitors its output signal to the idle speed control valve for on-board diagnostics.
Idle Speed Control
Engine Start-up
ECM idle speed control begins shortly after the engine is started, provided the throttle is closed (throttle position sensor at idle) and the road speed
is less than 3 mph. The stepper motor in the control valve is closed in stages until the target idle speed is reached.
Gear Position
When the gear selector is moved to Park or Neutral from drive, the engine management ECM receives a ground signal from the transmission
rotary switch (XJS) or transmission decoder (XJ6 Sedan). The ECM then closes the idle speed control valve a predetermined number of steps in
anticipation of the reduced engine load. When the engine is at normal operating temperature, the ECM maintains idle speed at 700 rpm in P or N
and at 580 rpm in R, D, 2 or 3.
Air Conditioning Compressor Clutch Operation
When the air conditioning compressor clutch is energized, a parallel circuit inputs battery voltage to the engine management ECM. The ECM
