Discovery 2. Manual - part 487
ENGINE MANAGEMENT SYSTEM - V8
DESCRIPTION AND OPERATION 18-2-27
The HO
2
S uses zirconium contained in a galvanic cell surrounded by a gas permeable ceramic, this produces an
output voltage proportional to the ratio difference between the oxygen in the exhaust gases and to the ambient
oxygen.
The HO
2
S operates at approximately 350
°
C (662
°
F). To achieve this temperature the HO
2
S incorporate a heating
element which is controlled by a PWM signal from the ECM. The elements are activated immediately after engine
starts and also under low engine load conditions when the exhaust gas temperature is insufficient to maintain the
required HO
2
S temperature. If the heater fails, the ECM will not allow closed loop fuelling to be implemented until the
sensor has achieved the required temperature.
This value equates to an HO
2
S output of 450 to 500 mV. A richer mixture can be shown as
λ
= 0.97, this pushes the
HO
2
S output voltage towards 1000 mV. A leaner mixture can be shown as
λ
= 1.10, this pushes the HO
2
S output
voltage towards 100 mV.
From cold start, the ECM runs an open loop fuelling strategy. The ECM keeps this strategy in place until the HO
2
S is
at a working temperature of 350
°
C (662
°
F). At this point the ECM starts to receive HO
2
S information and it can then
switch into closed loop fuelling as part of its adaptive strategy. The maximum working temperature of the tip of the
HO
2
S is 930
°
C (1706
°
F), temperatures above this will damage the sensor.
HO
2
S age with use, this increases their response time to switch from rich to lean and from lean to rich. This can lead
to increased exhaust emissions over a period of time. The switching time of the upstream sensors are monitored by
the ECM. If a pre-determined threshold is exceeded, a failure is detected and the MIL illuminated.
EMISSION CONTROL - V8, DESCRIPTION AND OPERATION, Exhaust emission control system.
Input/Output
The upstream and downstream HO
2
S are colour coded to prevent incorrect fitting. The tips of the upstream sensors
are physically different to the tips of the downstream sensors.
The HO
2
S are colour coded as follows:
l
Upstream sensors (both banks) - orange.
l
Downstream sensors (both banks) - grey.
The four HO
2
S have a direct battery supply to the heater via fuse 2 located in the engine compartment fuse box.
The heater is driven by the ECM providing an earth path for the circuit as follows:
l
Upstream LH bank via pin 19 of connector C0635 of the ECM.
l
Upstream RH bank via pin 13 of connector C0635 of the ECM.
l
Downstream LH bank via pin 7 of connector C0635 of the ECM.
l
Downstream RH bank via pin 1 of connector C0635 of the ECM.
The HO
2
S output signal is measured by the ECM as follows:
l
Upstream LH bank via pin 15 of connector C0635 of the ECM.
l
Upstream RH bank via pin 16 of connector C0635 of the ECM.
l
Downstream LH bank via pin 17 of connector C0635 of the ECM.
l
Downstream RH bank via pin 14 of connector C0635 of the ECM.
The HO
2
S earth path for the signal is supplied by the ECM as follows:
l
Upstream LH bank via pin 9 of connector C0635 of the ECM.
l
Upstream RH bank via pin 10 of connector C0635 of the ECM.
l
Downstream LH bank via pin 11 of connector C0635 of the ECM.
l
Downstream RH bank via pin 8 of connector C0635 of the ECM.
The HO
2
S voltage is difficult to measure using a multimeter, the output can be monitored using TestBook. A rich
mixture would read 500 to 1000 mV, a weak mixture would read 100 mV to 500 mV, the reading should switch from
rich to weak. The open loop default voltage is 450 mV, this is used by the ECM to set the air/ fuel ratio until the tip of
the HO
2
S reaches operating temperature.