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Two point Lambda Probe 1 - Generally


In contrast to the broadband probe, the two point-Lambda probe, as a passive sensor, has the function of continually informing the control device by means of a voltage signal, about any deviations from the ideal mixture composition. This is where the, actually wrong expression "Lambda regulated catalyser", comes from, since only the combustion process in the engine, but not that in the catalyser was included in the regulation. There is equipment, e.g., the central injection, where this is the most important sensor of all.


The test shown above should explain the operation of the two point probe. One takes a Lambda probe and screws it into a section of the exhaust pipe, the possibly available heating is then connected with a 12V- power source. Using a customary camping burner, it can now be heated from below until approx. 300°C is reached. However, this only functions, if the top opening of the pipe is closed so much that an opening of only 10 - 12 mm diameter is free. One now only needs to connect a multimeter to earth and to the cable for the Lambda probe-voltage, depending on the air supply from the bottom, one can generate a lean mixture with less than 0,5 volts or a rich mixture with more than 0,5 volts.
The diagram in figure 3 shows the regulation characteristics of this type of Lambda probe. There is an extremely steep zero-throughflow. This causes the control device, even with a small deviation from Lambda = 1, to counter-regulate by altering the injection-time length. The relatively small area of deviation is also called the Lambda-window.
Of course, one can also measure the voltage of the Lambda probe with simple means, on the running engine. Because the pointer shows the deviations much more obviously, one should use an analog-multimeter. Using the measuring range of approx. 1 to 2 volts DC, the minus terminal is connected to the engine earth and the plus terminal to the probe voltage cable, without breaking the connection to the control device. Should the the engine be cold-started, the pointer remains static, until the Lambda regulation begins at between approx 0,2 and 0,8 V, then the pointer starts to swing. Although this is not a proper exhaust-emission test, it is an indication of a regulating system.


The pointer of an analog multimeter says nothing about the true changes of the measurement values per time unit. Even the digital multimeter cannot indicate the values as quickly as they change. In reality, the sampling rate of the Lambda probe is much higher. 05/11