Auto Service Professional

FEB 2016

Magazine for the auto service professional

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15 | February 2016 A titania sensor is very different. When heated, the electrical resistance of titania changes as the concentration of oxygen surrounding it changes. A titania sensor does not generate voltage... it changes the output voltage of the current fowing through it. The sensor element is a fat wafer with electrodes on either side, so it's sometimes called a "planar" sensor. The PCM supplies a constant reference voltage to one electrode and measures the voltage drop through the element at the other electrode. Since the reference voltage is typically 5 volts or higher, this sensor produces a nice fat signal that reacts much faster than a Nernst cell, and it doesn't need reference air either, so it's smaller and less vulnerable to contamination. But like the zirconia sensor, the output signal is not linear; it rises or falls sharply on either side of stoichiometry, so it's still only capable of indicating a rich or lean air/fuel ratio. Air/fuel ratio sensors have a Nernst cell and a second cell right next to it called an "oxygen pump" or pump cell. The two cells are built on a fat strip of zirconia, so it's sometimes called a "planar" oxygen sensor. Be careful not to confuse it with the titania sensor. Just like the basic O2 sensor, the Nernst (sample) cell generates a voltage when there's very little oxygen in the exhaust. However, instead of using that voltage signal for fuel control, it's sent to a control circuit that operates the pump cell, which is basically another Nernst cell operated Powerplant Courtesy of Robert Bosch Corp. In comparison to thimble and planar types, the wide-band sensor (bottom) produces a current signal that the PCM converts to a voltage signal for diagnostics.

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