LR3/Disco 3
The HEGO sensor uses Zirconium technology that produces an output voltage dependant upon the ratio of exhaust gas
oxygen to the ambient oxygen. The device contains a Galvanic cell surrounded by a gas permeable ceramic, the voltage
of which depends upon the level of O2 defusing through. Nominal output voltage of the device for l =1 is 300 to 500m
volts. As the fuel mixture becomes richer (l<1) the voltage tends towards 900m volts and as it becomes leaner (l>1) the
voltage tends towards 0 volts. Maximum tip temperature is 1,000 Degrees Celsius for a maximum of 100 hours.
Sensors age with mileage, increasing their response time to switch from rich to lean and lean to rich. This increase in
response time influences the ECM closed loop control and leads to progressively increased emissions. Measuring the
period of rich to lean and lean to rich switching monitors the response rate of the upstream sensors.
Diagnosis of electrical faults is continually monitored in both the upstream and downstream sensors. This is achieved by
checking the signal against maximum and minimum threshold, for open and short circuit conditions.
Oxygen sensors must be treated with the utmost care before and during the fitting process. The sensors have ceramic
material within them that can easily crack if dropped / banged or over-torqued. The sensors must be torqued to the
required figure, (40-50Nm), with a calibrated torque wrench. Care should be taken not to contaminate the sensor tip when
anti-seize compound is used on the thread. Heated sensor signal pins are tinned and universal are gold plated. Mixing up
sensors could contaminate the connectors and affect system performance.
Failure Modes
Mechanical fitting & integrity of the sensor.
Sensor open circuit / disconnected.
Short circuit to vehicle supply or ground.
Lambda ratio outside operating band.
Crossed sensors bank A & B.
Contamination from leaded fuel or other sources.
Change in sensor characteristic.
Harness damage.
Air leak into exhaust system.
Failure Symptoms
Default to Open Loop fuelling for the particular cylinder bank
High CO reading.
Strong smell of H02S (rotten eggs) till default condition.
Excess Emissions.
It is possible to fit front and rear sensors in their opposite location. However the harness connections are of different
gender and colour to ensure that the sensors cannot be incorrectly connected. In addition to this the upstream sensors
have two holes in the shroud, whereas the down stream sensors have four holes in the shroud for the gas to pass
through.
GENERATOR
The Generator has a multifunction voltage regulator for use in a 14V charging system with 6÷12 zener diode bridge
rectifiers.
The ECM monitors the load on the electrical system via PWM signal and adjusts the generator output to match the
required load. The ECM also monitors the battery temperature to determine the generator regulator set point. This
characteristic is necessary to protect the battery; at low temperatures battery charge acceptance is very poor so the
voltage needs to be high to maximise any rechargeability, but at high temperatures the charge voltage must be restricted
to prevent excessive gassing of the battery with consequent water loss.
The Generator has a smart charge capability that will reduce the electrical load on the Generator reducing torque
requirements, this is implemented to utilise the engine torque for other purposes. This is achieved by monitoring three
signals to the ECM:
Generator sense (A sense), measures the battery voltage at the Central Junction Box(CJB).
Generator communication (Alt Com) communicates desired Generator voltage set point from ECM to Generator.
Generator monitor (Alt Mon) communicates the extent of Generator current draw to ECM. This signal also
