Engine Controls - Heated O2 Sensor Description
84BMW02GROUP 13 Fuel Supply and Adjustments
Bulletin Number 13 02 84 (2032) Page 1 of 3
Montvale, NJ October 1984 Technical Dept.
This bulletin supercedes T.R.I. 13 01 84 (2028) dated September 1984. It announces that the heated oxygen sensor will not be introduced on the 1985 528e.
Please remove and discard T.R.I. (2028) dated September 1984 from your T.R.I. binder.
SUBJECT: Electrically pre-heated oxygen sensor ("lambda" probe)
MODEL: All so equipped
More and more stringent standards are being issued for the reduction of exhaust emissions from motor-vehicle engines, and are compelling manufacturers to control the fuel-air mixture composition with greater accuracy.
The oxygen sensor measures the oxygen content of the exhaust. The control signal it transmits is used by the Digital Motor Electronics system to regulate the proportions of fuel and air in the mixture. Together with a suitably designed exhaust catalyst, the pollutant content of the exhaust can be reduced in this way.
In contrast to the unheated oxygen sensor, which continues in use, the new generation of sensors contains a heating element controlled by a PTC (positive temperature coefficient) device. The advantage of the new version is the reduced time needed for the sensor to respond.
The heated oxygen sensor is installed in all 325e models and will be gradually phased into all 1985 models during the model year (except the 528e).
The new oxygen sensor has a life span of 50,000 miles. It has its own relay to supply heating current. As a result of the additional heating, it has also proved possible to reposition the sensor.
The new sensor is not installed in the downpipe from the exhaust manifold but further down.
stream directly ahead of the catalyst. This reduces the ambient temperature at which the sensor must operate and greatly prolongs its reliable working life. In addition, measuring accuracy is improved, since the exhaust gas from the various cylinders is more thoroughly mixed together.
Construction
The assembly housing retains the ceramic element of the sensor (3) and the two protective tubes (2). The front of the ceramic element is protected by a slotted tube and projects into the front tube of the catalyst, where it is surrounded by the exhaust gas flow. The rear, inner section of the ceramic element is in contact with the surrounding air.
The ceramic element is made from zirconium dioxide, with a surface electrode layer of gaspermeable platinum. To protect this thin layer against combustion residues, yet maintain consistent catalyst performance, a porous ceramic layer is superimposed on the platinum.
The contact spring transmits the electrical signal from the electrically conductive layer inside the ceramic element to the electrical connection.
Operating principle
The oxygen sensor acts as a measuring link in the control circuit needed to ensure efficient exhaust catalyst operation and maintain the correct fuel-air mixture (lambda = 1 = approximately equal to the stoichiometric mixture).
Even if combustion takes place with excess fuel, a proportion of residual oxygen is present, and permits information on mixture composition to be compiled.
The proportion of oxygen in the exhaust is measured and used as a control value to determine the fuel-air ratio.
The sensor works according to the following principle at a temperature in the region of
300~C (572~F), the ceramic material begins to conduct oxygen ions. If the proportions of oxygen on the inside and outside surfaces of the ceramic element are different, an electrical voltage develops between the boundary layers and is used as a control signal. At lambda = 1, it rises from 200 to 800 mV. On receiving this signal, the Digital Motor Electronics control unit reduces the fuel injection period (and thus the volume of fuel injected) without any significant time delay.
Maintenance instructions
Apply anti-seize compound and corrosion inhibitor, Part No. 81 22 9 400 088, to the sensor thread when inserting.
Note: Remember to shield the sensor when applying undercoat sealant to the car.