PeachParts Mercedes-Benz Forum - View Single Post

stevebfl · #2 04-27-2002, 05:15 PM

Current is not the reference on Oxygen sensors, voltage is. They are capable of producing about 1v (at least MB sensors do - some BMW and Jeep use 5v sensors). The voltage is created something like a battery by placing a special catalyst at high temp between the atmosphere and the exhaust. Voltage is greatest when the greatest difference in O2 concentration occurs.

The sensors only ability is to measure the O2 in the exhaust. This is used by the fuel control computer to decide whether you are lean or rich. If lean the controller adds fuel (to its capabilities). If lean it subtracts fuel from the equation. The sensor doesn't create the mixture and your job in diagnosis is only to determine whether it still acts as its supposed to.

If the car is too rich the sensor will indicate it and the computer leans the mixture. If the fault is too great the system bottoms out and because there is only a rich signal all the time the O2 sensor gets blamed. Maybe the all time leading auto misdiagnosis. A properly running system in closed loop will have the O2 sensor voltage oscillating about the baseline voltage of .45v. Good sensors will react quick and will get to around .8v before the controller can turn it around and the voltage drops fast enough to get close to .1v before it is again turned. At road speed this probably happens about 2-3 times a second. If one watches O2 sensor voltage from start on an idling motor, the O2 voltage will stay constant at .45v for quite some time as the sensor heats to about 500 deg, At about that temp it starts generating. It is slow at first with weak attempts to gain voltage. The controller recognizes an active sensor when a certain small voltage fluctuation occurs. The first cycles are slow and as such the difference above and below .45v is small. As the sensor heats the speed of the control cycle speeds up. A sensor which stops oscillating at idle speed after cycling under load is probably worn out.