I'm checking the O2 sensor output from the connector under the right front carpet on my '88 300TE. It only shows around .05 volts after I start it up. Is this normal? I didn't wait until it warmed up.

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

Tried pushing down on the airflap until just before stalling and still below .05 volt from the O2 sensor. I'll change it tomorrow. The main problem was a low idle speed plus my scan tool shows a code set for a problem in the Electro-Hydraulic Actuator(EHA) circuit. I checked that out and found it O.K.

I just read through my post and the original post and made some correction in my reply. Its very hard for me to keep the voltage, mixture, and direction of response clear and I had writen lean where rich was the right word.

I wrote all this to give a feeling of the necessary thought that has to be given to testing O2 sensors. Then I reread the original post and realized I had wasted my time. I can't possibly explain how this works to someone who doesn't even know that none of this takes place till the sensor is HOT, HOT, HOT.

Can I say this again HOOOOOOOOOOOOOOOT. Is that how you spell it?

BTW are you disconnecting the sensor and reading 0.05v on the wire to the sensor or are you reading it connected?

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I was disconnecting the sensor and reading under 0.05v on the wire from the sensor. Even while pressing down on airflap to just before it would die. Anyway, I got it out and I guess its the original one so I might as well change it with over 100,000 miles on it.

Seeing that you have the sensor out, you may want to try this. It is not as good as testing the system in the car, but will tell you if your sensor is generating a signal and cycling...
I can't see if you have a 3 or 4 wire sens in your pic,,, but if it is 4 the ground is the gray wire . If 3 , the case is ground [- neg..]
The 2 white are heater and you may want to see if you have continuity there while you are at it.

copy:
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Any O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, and pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always no benefit in replacing an oxygen sensor that is good.

>>

Damn good idea. It got to about .6V after 20 seconds. Put the heat on for 2 minutes and it goes to about .80-.85V with some drops to .4-.5V. It drops to under .1V in half a second after removing flame.

Put 12 v. to the heater and see if it heats up..
Remind you, this does not [ IMO] compare to scoping out an 02 sens and computer reaction to the sig. for cycle time and reactions , but it does verify that the sens is generating a sig.
[ which your post seemed to indicate it did not.]

Atikovi,
I am interested in results after replacement of o2 sensor. Any problems before replacement?
Thanks!

__________________
J.H.
'86 300E

Didn't notice any significant change. Problem was rough idle, low idle speed and bogging down when first accelerating from a stop. What helped the most was tweeking the idle mix adjustment. That helped greatly reduce the bogging down. Otherwise I've pretty much narrowed it down clogged fuel injector(s).