Discovery II
ENGINE MANAGEMENT SYSTEM - V8
DESCRIPTION AND OPERATION 18-2-31
The heater is driven by the ECM providing an earth path for the circuit as follows:
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Upstream LH bank via pin 19 of connector C0635 of the ECM.
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Upstream RH bank via pin 13 of connector C0635 of the ECM.
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Downstream LH bank via pin 7 of connector C0635 of the ECM.
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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:
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Upstream LH bank via pin 15 of connector C0635 of the ECM.
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Upstream RH bank via pin 16 of connector C0635 of the ECM.
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Downstream LH bank via pin 17 of connector C0635 of the ECM.
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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:
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Upstream LH bank via pin 9 of connector C0635 of the ECM.
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Upstream RH bank via pin 10 of connector C0635 of the ECM.
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Downstream LH bank via pin 11 of connector C0635 of the ECM.
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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.
The HO
2
S can fail the following ways or supply incorrect signal:
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Sensor open circuit.
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Short circuit to vehicle supply.
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Short circuit to vehicle earth.
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Sensor disconnected.
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Stoichiometric ratio outside the correct operating band.
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Contamination from leaded fuel.
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Air leak into the exhaust system.
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Wiring loom damage.
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Sensors fitted incorrectly or cross wired.
In the event of a HO
2
S signal failure any of the following symptoms may be observed:
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Default to open loop fuelling on defective bank.
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If the sensors are crossed over (LH bank to RH bank), the engine will run normally after initial start up, but
performance will become progressively worse as the sensors go towards maximum rich for one bank of cylinders
and maximum lean for the other. The ECM will eventually default into open loop fuelling.
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High CO reading.
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Excess emissions.
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Strong hydrogen sulphide (H
2
S) smell until the ECM defaults to open loop fuelling. .
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MIL illuminated (NAS market only).
A number of diagnostic tests are performed by the ECM with regards to the HO
2
sensors:
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HO
2
sensor and system diagnostics
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HO
2
sensor heater diagnostics
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HO
2
sensor switching period (ageing) diagnostics
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Rear HO
2
sensor adaption diagnostic (NAS only)
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Catalyst monitoring diagnostic
For further details of the heated oxygen sensors and exhaust emission control, refer to the V8 Emission Control
section of this manual.
EMISSION CONTROL - V8, DESCRIPTION AND OPERATION, Exhaust Emission Control System.
