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Old 05-05-2002, 12:36 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
That is entirely appropriate if it is reading a very lean mixture. There is no appropriate voltage for an oxygen sensor only a range of possible voltages. The type O2 sensor used in MB can produce one volt in a rich mixture (WHEN HOT).

Try pushing down on the airflap. This will enrichen the mixture (to the point of killing the motor if taken far enough). If the engine slows due to being too rich and you are still only reading below .5v then the sensor is dead.

If the sensor responds to the change in mixture then the next step is to see if the control circuit is working. The only way for it to be working with an O2 sensor voltage stuck at .05v is if the system is being ourwhelmed and control system is as rich as possible (one will not be able to fix these systems untill this is understood - system as rich as possible but still lean).

To verify try putting one hand on the positive battery terminal and the other on the disconnected lead to the controller from the O2 sensor. What this does is tell the controller that the O2 sensor is now reading voltage (about a volt or two will be seen through your body). This will cause the control system to lean out. If your sensor was right (that you really had a .05v mixture) then the car should die as it goes even leaner. What you have to realize is that a .05v signal will cause the controller to use all its power to go rich (it has definite limits to its ability to change mixture). Once it reaches its extent of compensation if the system is still rich the O2 sensor will still show it. If you are in the full lean compensation but are still rich then the car should die if one tricks the system (as described above) into going to the lean stop. By doing this test one can tell what the situation is.

With all the tools necessary one would set the CO to .5% and then hook up the O2 sensor. The system would start measuring that mixture and a closed loop will be established where a low voltage lean mixture is read and the system goes rich, as it passes the .5%CO point the O2 sensor will increase in voltage. A fast sensor will get to near .8v before the controller can turn it around. A car in such adjustment can have its mixture window viewed by first grounding the disconnected lead from the sensor to the controller. This is 0.0v and sends the system full rich as yours should be with that constant 0.05v O2 reading. Next do the trick with one hand on the pos battery terminal and the other on the O2 input lead (this also can be similated with a "D" cell battery but the hand on battery is easier to set up).
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Last edited by stevebfl; 05-05-2002 at 01:06 PM.
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