mileage LAND ROVER DISCOVERY 2002 Workshop Manual

GENERAL INFORMATION
03-7
Ball and Roller Bearings
General
When removing and installing bearings, ensure that
the following practices are observed to ensure
component serviceability.
lRemove all traces of lubricant from bearing
under inspection by cleaning with a suitable
degreasant; maintain absolute cleanliness
throughout operations.
lConduct a visual inspection for markings on
rolling elements, raceways, outer surface of
outer rings or inner surface of inner rings. Reject
any bearings found to be marked, since marking
in these areas indicates onset of wear.
lHold inner race of bearing between finger and
thumb of one hand and spin outer race to check
that it revolves absolutely smoothly. Repeat,
holding outer race and spinning inner race.
lRotate outer ring gently with a reciprocating
motion, while holding inner ring; feel for any
check or obstruction to rotation. Reject bearing
if action is not perfectly smooth.
lLubricate bearing with generous amounts of
lubricant appropriate to installation.
lInspect shaft and bearing housing for
discoloration or other markings which indicate
movement between bearing and seatings.
lEnsure that shaft and housing are clean and
free from burrs before fitting bearing.
lIf one bearing of a pair shows an imperfection, it
is advisable to replace both with new bearings;
an exception could be if the faulty bearing had
covered a low mileage, and it can be
established that damage is confined to only one
bearing.
lNever refit a ball or roller bearing without first
ensuring that it is in a fully serviceable condition.
lWhen hub bearings are removed or displaced,
new bearings must be fitted; do not attempt to
refit the old hub bearings.lWhen fitting a bearing to a shaft, only apply
force to the inner ring of the bearing. When
fitting a bearing into a housing, only apply force
to the outer ring of the bearing.
lIn the case of grease lubricated bearings (e.g.
hub bearings) fill the space between bearing
and outer seal with the recommended grade of
grease before fitting seal.
lAlways mark components of separable bearings
(e.g. taper roller bearings) when dismantling, to
ensure correct reassembly. Never fit new rollers
in a used outer ring; always fit a complete new
bearing assembly.

EMISSION CONTROL - V8
17-2-34 DESCRIPTION AND OPERATION
Exhaust emission control operation
The oxygen content of the exhaust gas is monitored by heated oxygen sensors using either a four sensor (NAS only)
or two sensor setup, dependent on market destination and legislative requirements. Signals from the heated oxygen
sensors are input to the engine management ECM which correspond to the level of oxygen detected in the exhaust
gas. From ECM analysis of the data, necessary changes to the air:fuel mixture and ignition timing can be made to
bring the emission levels back within acceptable limits under all operating conditions.
Changes to the air:fuel ratio are needed when the engine is operating under particular conditions such as cold starting,
idle, cruise, full throttle or altitude. In order to maintain an optimum air:fuel ratio for differing conditions, the engine
management control system uses sensors to determine data which enable it to select the ideal ratio by increasing or
decreasing the air to fuel ratio. Improved fuel economy can be arranged by increasing the quantity of air to fuel to
create a lean mixture during part-throttle conditions, however lean running conditions are not employed on closed loop
systems where the maximum is
λ = 1. Improved performance can be established by supplying a higher proportion of
fuel to create a rich mixture during idle and full-throttle operation. Rich running at wide open throttle (WOT) for
performance and at high load conditions helps to keep the exhaust temperature down to protect the catalyst and
exhaust valves.
The voltage of the heated oxygen sensors at
λ = 1 is between 450 and 500 mV. The voltage decreases to 100 to 500
mV if there is an increase in oxygen content (
λ > 1) indicating a lean mixture. The voltage increases to 500 to 1000
mV if there is a decrease in oxygen content (
λ < 1), signifying a rich mixture.
The heated oxygen sensor needs to operate at high temperatures in order to function correctly (
≥ 350° C). To achieve
this the sensors are fitted with heater elements which are controlled by a pulse width modulated (PWM) signal from
the engine management ECM. The heater element warms the sensor's ceramic layer from the inside so that the
sensor is hot enough for operation. The heater elements are supplied with current immediately following engine start
and are ready for closed loop control within about 20 to 30 seconds (longer at cold ambient temperatures less than
0
°C (32°F)). Heating is also necessary during low load conditions when the temperature of the exhaust gases is
insufficient to maintain the required sensor temperatures. The maximum tip temperature is 930
° C.
A non-functioning heater element will delay the sensor's readiness for closed loop control and influences emissions.
A diagnostic routine is utilised to measure both sensor heater current and the heater supply voltage so its resistance
can be calculated. The function is active once per drive cycle, as long as the heater has been switched on for a pre-
defined period and the current has stabilised. The PWM duty cycle is carefully controlled to prevent thermal shock to
cold sensors.
The heated oxygen sensors age with mileage, causing an increase in the response time to switch from rich to lean
and lean to rich. This increase in response time influences the closed loop control and leads to progressively
increased emissions. The response time of the pre-catalytic converter sensors are monitored by measuring the period
of rich to lean and lean to rich switching. The ECM monitors the switching time, and if the threshold period is exceeded
(200 milliseconds), the fault will be detected and stored in the ECM as a fault code (the MIL light will be illuminated
on NAS vehicles). NAS vehicle engine calibration uses downstream sensors to compensate for aged upstream
sensors, thereby maintaining low emissions.
Diagnosis of electrical faults is continuously monitored for both the pre-catalytic converter sensors and the post-
catalytic converter sensors (NAS only). This is achieved by checking the signal against maximum and minimum
threshold for open and short circuit conditions. For NAS vehicles, should the pre- and post-catalytic converters be
inadvertently transposed, the lambda signals will go to maximum but opposite extremes and the system will
automatically revert to open loop fuelling. The additional sensors for NAS vehicles provide mandatory monitoring of
the catalyst conversion efficiency and long term fuelling adaptations.
Note that some markets do not legislate for closed loop fuelling control and in this instance no heated oxygen
sensors will be fitted to the exhaust system.