The sensor is identical... the wire length and connector plug are different.

Raw data
 

I will post raw data first
The connector I used is behind the ABS and horn on the driver's side. It unscrews and pin 3 is clockwise from pin 2. The com part of the multimmeter was connected to pin 2 and volts to pin 3.
With key on but engine off I got 3.76 on the first measurement but then 4.06 on all subsequent ones.
With engine idling i got 11.35 steady. I measured first after the car cooled a little and would get 5 for a flash then 7. Repeat measurements were flash of 9 then 11. I drove the car 2 miles and the got 11.35 steady.
Battery voltage engine off was 12.08 and 14.06 engine on.
So Engine off test 1-v(pin)/v(max)= 1-4.06/14.06= 1-.29= .71= 71%. However this is engine off pin 3 and the battery voltage when engine on. Value for battery voltage engine off would be 1- 4.06/12.08=1-.34=.66=66%.
Values for engine idling would be 1-11.35/14.06= 1-.81=.19=19%.
I do not see fluctuation in digital multimeter other than for a split second when engine may have cooled some.
What does everyone think. Correct me as needed. Was car hot enough?-water temp was up off the stop.
I will add I thought the exhaust had a different odor for the last month or so.
Is this fun or what!

Was the water temp gage at 80+? that would be hot enough. Until then I'm not sure there arent other enrichment factors that would through off the signal. The fluxuation of the voltage usually starts between 40 and 60 but the test calls for 80c and ideling. The type of fluxuation your looking for is over a range of 1v or less.

I just ran the test on my 86 sans duty cycle
Battery = 13.2v (engine off)
KOEO = 3.91v (key on engine off)
Temp 80+ ideling = voltage hunts back and forth between 7.2 and 7.7
Temp 80+ ideling o2 sensor unplugged = 7.1v constant

Thanks for the info!
 

I don't have anything to add other than THANKS for the nice detailed info. I'm printing for future ref. I've done 02 sensors on every Benz I've owned but it was after a CEL indicated failure. I should check my '92 (I'm assuming it would work the same way even though it's an 8 not a 6) to see if there is anything I need to address there.

Again, THX!

My temp guage does not seem to go to 80 this winter. I will check after 15 min of driving. I thas been 35-45 here. Would a problematic thermostat keep the water temp cool and fool the O2 sensor?
I saw no fluctuations at all-perhaps 11.35 to 11.36 occ.
Thanks. I will measure again.

By Jove I think youve got it.
 

I believe it would!
The system enriches when cold to warm the engine.
What temp does it get to?

will drive hard tomorrow.

water temp
 

I drove about 7 miles today. 2 miles at 35 mph, 4 at 60 mph and 1 at 25 mph. The temp guage only got to the line below 80 which I assume to be 60. The guage has 40, 80, 120 C marked and lines between which I assume to be 60 and 100 C. If I remember correctly the guage got to about 60 when I drove it longer too. Should the temp get to 80 even in winter? It is about 40F here.
Perhaps my thermostat is bad.

That was the next thing that came to my mind. If the car is running too cold that could account for some of this sudden drop in milage. Running too cold is also considered bad juju for engines. I'd at least look into having it tested/replaced (I figured to test it you have to remove it so why not just replace it -- of course it could be a $1200 part. :D )

I agree with Holeshot's assesment, except about the price of the part ($23.00 or less plus shipping on this site): http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1OH0MY2181OT0KACVY&year=1987&make=MB&model=300-E-002&category=G&part=Thermostat+Kit
Depending somewhat on the length of your trips and the average temps, your car is spending too much of the time enriching (adding fuel) to warm up the engine.
When I get a chance I will try to look up the specific temp. at which enrichment stops. But it is not good for the engine to run cold you should change the thermostat at the earliest oppourtunity and hopefully that will fix all symptoms.
Running rich for too long has other deliterious effects on the catalytic converter (which is an expensive part) and can kill the o2 sensor as well.

Thermostat was broken in open position
 

Thermostat had structural failure. One of the copper struts broke and the diaphram was open 1 cm wide on that side.
The fact that the engine was heating up did not register with me at first.
I put the thermostat in so the diapham opens down (took old one out fast to avoid extra coolant leak) and put gasket between thermostat and top-hopefully correct.
Now car heats quickly to just above the 80 C line- makes sense with 87 C thermostat.

Now for the O2 sensor- pin 2 and pin 3 on the X11 still give 11.35 v. constant. So does this means the O2 sensor is bad? I will remeasure today after a longer drive though water temp was 80+ C.
I do not get a constant 7.1 v (with O2 sensor unplugged) as A264172 says.

Did running cold for a month or so bother the O2 sensor?(if it is bad)
Thanks everybody so far esp A264172!

Nothing like a farily easy fix. The O2 could be damaged or dirty. Running it for a while and checking it again is unlikely to do any more harm than already done.

The last time I replaced the O2 on my 300E I took the sensor into the big-box auto stored, had them cross-ref the Bosch number and picked the lest expensive one and soldered on the MBZ connector to the end. Done.

The first time I did it I went to the dealer and.. the only one they have for me had not plug on it whatsoever (and of course at more than twice the price) so when I did it the next time I just got the proper sensor and did not worry about the plug.

(now if only I can sort out the short in my 500.. :( )

My drive to work has been reduced from 15 miles one way to 1.5. The heater hardly gets warm.

my average mpg has dropped drastically. I would say from the very low 20's to maybe 15 mpg... however, in a week I am driving very few miles.

I'll take the lower mpg for now. I do have to consider having my wife drive this car to work once a week, however. She has a 30 mile commute which would be better for the car.

This is one of the few processes I am a little familiar with so... glad to lend a hand.
A reading of 20% duty cycle with no fluxuation (when the car is fully warmed up and ideling) indicates a problem with the 'throttle valve switch' (part of the throttle body) or it's wireing. The switch is located on the lower portion of the air intake manifold.
And only controls fuel delivery under full load and at idle so your gas mpg should not be affected by it.
This thread: http://www.peachparts.com/shopforum/tech-help/58398-86-190e-8v-throttle-valve-switch.html?highlight=throttle+valve+switch
...has a few thoughts on that switch... but you might want to triple check your duty cycle under 'proper' conditions before you go digging at it. ;) Are you sure there is no fluxuation of the voltage when the care is ideling and the temp is up to above 80?

When the needle first hits the contact there is occ a reading 2v lower for an instant. Once the pins are in 2 and 3 the voltage is steady.
I will check again this PM after driving 7 miles.

Just in case, if I buy the Ford O2 sensor do I replace the part in the exhaust or near the seat? In other words does the exhaust part break or the part near the seat(is this just a connector?)