Does the PCM (engine computer) “read” the current through the sensors, or the voltage? I assumed that it read voltages as sensor outputs. If you look at the wiring diagram for the 1998 Ford Taurus, you’ll see that the circuits leaving the sensors and the oxygen sensors, must be neutral because the O2’s don’t have a supply voltage (They make their own voltage).
The PCM can’t measure its output voltages to the sensors because it would be reference voltage (5 volts); but, it could read the currents (since current is of fixed value throughout any circuit). Is the PCM reading currents? (Click on Fig. 5).
The computer sends a reference voltage of 5 VDC to a sensor. As the resistence in that sensor changes the voltage signal back to the computer changes. Oxygen sensors produce their own voltage to the computer. But the heated portion of the oxygen sensor recieves 12 VDC. Titania O2 sensors do not produce their own voltage. Instead they act like a variable resistor. In these O2 sensors, the computer sends a 5 VDC reference voltage thru the O2 sensor.
Tester
Good try, Tester; but…
The 1998 Ford Taurus uses a four-wire planar zirconia oxygen sensor. Two wires are for the O2 heater. One is 12 volts to the heater; and, the other wire grounds the heater through the PCM (engine computer). The third wire is 5 volts from the PCM. The fourth wire is the O2’s signal to the PCM.
A zirconia O2 sensor generates its own voltage, usually from 0.1 volt to 1.00 volts, doesn’t it? Then, why does this zirconia O2 sensor have a 5 volt supply?
Scouring the Web hasn’t reveiled an answer. Here’s a sample piece of information about the different types of O2s: http://www.walkerproducts.com/faqs.html (scroll down to applicable part).