Zirconia Lambda Sensor TTL2.0
What it does
The Lambda sensor is a post-engine sensor fitted
into the exhaust system of a vehicle that measures
the amount of Oxygen in the exhaust stream. It signals
the ECU (Electronic Control Unit) with a voltage signal
relevant to this Oxygen level. The ECU interprets
this as an indication of the vehicle’s air/fuel
mixture strength.It is an essential part of the vehicle’s
engine management system and the longevity of the
catalytic converter is dependant upon its correct
operation.
All petrol engined vehicles made for the UK market
since 1993 have at least one Lambda sensor fitted.
They are normally fitted into the exhaust manifold
or down pipe of the vehicle in a position that is
as close to the engine as possible. The body shape
resembles that of a spark plug.
Vehicles with EOBD (European On Board Diagnostics,
year 2000 onwards, for the UK market) may have two
sensors fitted, one before and one after the catalytic
converter.
How it works:
The sensor is comprised of a ceramic element, protected
by a steel guard-tube that allows gasses to pass through,
housed in a stainless steel casing.
The sensing element in the Lambda sensor is a thimble
shaped ceramic tip made from a material known as Zirconium
Dioxide (hence the name Zirconia). Mounted on both
the outer and inner surface are permeable Platinum
electrodes. These electrodes carry the sensor’s
signal through the wires and on to the ECU. The outer
surface of the element (thimble) is open to the exhaust
gasses and the inside of the element (thimble) is
open to atmosphere.
The design of the sensor’s tip is such that
(when it’s up to operating temperature) a difference
in Oxygen level between the inner and outer surfaces
results in a small electrical voltage being generated.
It is this voltage that the ECU measures, the higher
the difference in Oxygen levels, the higher the voltage
generated. The sensor is “self generating”;
hence it acts as a stand alone part that doesn’t
need an input for it to operate. The voltage generated
is very small, from minimum to maximum the output
range is only one volt.
Zirconia sensors can have one to four wires. They
all have an output wire (normally black). The two
and four wire sensors have a separate earth (normally
grey). The three and four wire sensors have an internal
heater element which enables the sensor to get up
to operating temperature quicker (normally the two
white wires). Please note that wire colours can vary
between Lambda sensor manufacturers, the most common
ones have been referred to.
Reasons for failure:
There are two principles areas
of failure.
1. Heater element;
failure of which results in an elongated warm-up period,
increased emissions from cold and possible catalytic
converter damage.
2. Sensor element;
contamination of the porous ceramic tips by becoming
blocked/polluted which results in poor response to
mixture changes or in extreme cases total failure.
The pollution can be as a result of chemical pollution
from exhaust paste, antifreeze, gasket sealant or
from excessive carbon build up due to engine wear
or extended service life.
Testing:
The heater can be checked with an Ohmmeter, comparing
the reading to known data. A heater failure usually
results in an open circuit heater element, thus rendering
the heater completely inoperative. This type of failure
is very common with the Lean-Burn sensor fitted to
the Toyota Carina “E” and Avensis.
Check the output circuit response time by monitoring
the output voltage with varying engine load (blip
the throttle). There should be an almost instantaneous
response. Output voltage can be measured with either
a voltmeter or an oscilloscope and should rapidly
fluctuate in the region of 0.2 to 0.8 volts.
Output response time errors commonly cause MOT failures.
The Lambda sensor does not react quickly enough to
changing engine loads and speeds, hence at MOT when
the emissions are being checked at a fast idle level
the emission levels will rise above the legal limit
before settling back down to, possibly, pass the second
fast idle test. The cause of this fault is almost
always the Lambda sensor.
Off car testing:
Zirconia sensors can also be tested off the vehicle
using the gas torch method. For this you will need
an accurate digital voltmeter, a bench vice and a
gas torch.
Secure the Lambda sensor in the vice; attach the
voltmeter positive wire to the Lambda sensor’s
output wire (normally black), attach the volt meter
negative wire to either the lambdas ground wire (normally
grey) or the body of the sensor. Select a voltage
scale on the meter suitable to display a 0-1.5 volt
reading. Heat up the tip of the Lambda sensor (the
part that fits into the exhaust) with the torch and
monitor the voltage reading.
Once the sensor is up to operating temperature (usually
just as the sensor shield gets to red hot) the sensor
will generate an output signal. You should expect
the voltage to get close to 1.0 volt (i.e. 0.9+ volts)
and then react quickly when moving the torch away
from the tip. The test will display if the sensor
is operative and its reaction to changing Oxygen levels.
Further, resistance checks are necessary to check
the heater circuit operation if appropriate.
For further technical information
concerning Lambda Sensors please phone our technical
help line 01527 839307
