Range Rover P38
LAND ROVER V8
25
DESCRIPTION AND OPERATION
Idle Air Control (IAC) valve - up to 99MY
The idle speed control stepper motor is located on the
side of the inlet manifold. Idle speed is controlled by
the stepper motor, which comprises two coils,
mounted to the throttle housing. When energised in
the correct sequence, the coils move a plunger which
opens or closes the throttle bypass valve controlling
the quantity of idle air. The stepper motor controls idle
speed by moving the plunger a set distance called a
step. Fully open is 200 steps (180 steps for vehicles
up to 97MY) and fully closed 0 steps. Failure of the
stepper motor will result in low or high idle speed,
poor idle, engine stall or non start. If the number of
recorded steps changes beyond a set threshold
(opening or closing) without a corresponding change
in airflow, then a fault code will be stored. The GEMS
diagnostics also check for short circuit conditions
during normal stepper operation and open circuit
during power down. Detected faults are indicated by
illumination of the malfunction indicator light (MIL) on
North American specification vehicles.
The stepper motor coil resistance is 53 ohms
±
2
ohms.
CAUTION: The pintle must not be moved
by force.
Fault codes:
•
P0506 - Low idle speed
•
P0507 - High idle speed
•
P1508 - IACV stepper motor open circuit
•
P1509 - IACV stepper motor short circuit
Heated Oxygen Sensor (HO
2
S) - up to 99MY
The heated oxygen sensors consist of a titanium
metal sensor surrounded by a gas permeable ceramic
coating. Oxygen in the exhaust gas diffuses through
the ceramic coating on the sensor, and reacts with the
titanium wire altering the resistance of the wire. From
this resistance change the ECM calculates the
amount of oxygen in the exhaust gas. The injected
fuel quantity is then adjusted to achieve the correct
air:fuel ratio, thus reducing the emissions of carbon
monoxide (CO), hydrocarbons (HC), and oxides of
nitrogen (NO
x
). Two HO
2
S sensors are fitted, one in
each exhaust front pipe and positioned in front of the
catalytic convertor. On North American specification
vehicles, an additional HO
2
S sensor is fitted behind
each catalytic converter. These additional sensors are
used to monitor the operating efficiency of the
catalysts. Note that if the wiring to these sensors is
crossed, the vehicle will start and idle correctly until
the sensors reach operating temperature. Then the
ECM will read the signals from them and send one
bank of cylinders very rich and the other very weak.
The engine will misfire, have a rough idle and emit
black smoke, with possible catalyst damage.
The oxygen sensors are heated to ensure rapid warm
up and continued operation when the exhaust
temperature may be below the working temperature of
the sensor. Both the upstream sensor heaters and the
downstream sensor heaters are connected in parallel.
The heaters are directly driven from the GEMS ECM
by a pulse width modulated (PWM) signal to enable
temperature control of the heater to be achieved.
When the sensor is powered up, the duty ratio of the
PWM signal to the heater is started low and then
increased over a period of approximately 30 seconds.
This is to ensure the sensor is not heated up too
quickly, which might cause the ceramic interior of the
sensor to crack. The duty ratio of the heater signal
may be altered during normal operation to maintain
sensor temperature.
In the event of sensor failure, the system will default to
’open loop’ operation. Fuelling will be calculated using
signals from the remaining ECM inputs.
On North American Specification vehicles, a fault with
any of the HO
2
S sensors is indicated by illumination of
the malfunction indicator light (MIL). ECM diagnostics
also use the Heated Oxygen Sensors to detect
catalyst damage, misfire and fuel system faults.
