No check engine light when O2 sensor Disconnected. Bulb OK
 

Continuation from here...I was making a mistake on my initial test method so I want to start a fresh thread so it does not get confusing.

1987 260E.

I was checking the o2 sensor with (+) and grounding the other side...Stupid. So I ran it INLINE this time (this IS the correct way to do it right?)

Car warmed up: 0.48v....stuck there. It does not change.
Since this was fishy...Next I unplugged the sensor completely. No O2-sensor light illuminates on cluster. The bulb is fine because it illuminates during ignition.
So why would I get no check engine/o2 sensor light when the sensor is disconnected? It is obvious this has nothing to do with the sensor I spliced in because regardless, if the sensor is disconnected I should get the CE light.

Now I am completely lost on where to go. :confused:

On a 1987 car, the check engine lamp is designed to come on when the computer sees NO O2 sensor activity for 5 minutes after the engine reaches 80C.

The computer cycles every 10 minutes or so. After replacing O2 sensor the manual calls out for light and heavy use with engine warm for 30 minutes to set the system. Light on my friends CE takes 30 minutes before he gets a CEL if its gonna happen.

M.B. Doc -- You are right on the money. After about 15minutes driving I saw the light come on.

OK -- so I have established the ECU recognizes the O2 sensor, now going back to the test procedure: Do you put the DMM inline between the two wires or do you need to measure it using an alligator clip and ground the other side of the DMM? Which is the correct way? I want to double check my basic tests before going further.

0.48v stuck is what it is reading. I want to make sure that is correct and then we can talk about why from there.

measuring the voltage when the wiring is connected is the correct method.

I don't understand this at all...

I removed the insulation and used an alligator clip attached to the black sensor wire for (+) side of DMM. I grounded the other and monitored it while driving.

I am getting 11.6mv while idling and 6.5mv while driving at ~40mph. I don't get it.
:confused:
EDIT: Just to double check I manually set to V and DC, but it still just reads 0.006v driving and 0.011 idling
Can anybody explain why I am getting these weird readings?

Lee over at Fastlane is ordering me the EHA Test Harness so I should have that soon, but in the interim I really want to get my head around having a reading that makes sense from the O2 sensor...

bump anyone?

one more bump....

Nobody here ever tested their O2 sensor?

Your voltage seems low, but you know that. Is it possible you are connected to a shield on the wire and not the center conductor?

Or the mixture is too lean....

Does your DMM have a duty cycle ability?
If so measure the duty cycle at pin #3 on the test connector. 50% on/off ratio

It does and I am getting 45-50% fluctuating...That's the reason why my overfueling is confusing the heck out of me. I have soot in the exhaust, o2 sensor coloration confirms and my fuel economy is ~12mpg. Filled up today. Got 178mi this tank before the light came on. Terrible....I was getting ~300mi to a tank before.

Since it is overfueling, I attempted to lean out the mixture, but the car idles worse and stumbles, a little more lean and it just flat out dies.

I ordered an EHA test harness from MB so its on the way to me from Chicago...Should get it early next week.

Hey ps2cho,

I don't know if you remember this, but I posted a while back about having low fuel economy on my car (300se). I just replaced the o2 sensor this past weekend, despite the readings being within spec. Duty cycle fluctuated at around 50% with the old sensor, and went to 60% afterwards before I adjusted it back to 50%. The exhaust doesn't smell as bad as before, and the car seems a little more responsive. I'm going to run a few tanks to see how many mpg I get, and hope that I don't have to swap the EHA :)

Figured it out....I was probing the shielding. The o2 sensor has ANOTHER wire inside. I only discovered this after cutting the wire completely in two. I knew something was wrong, just couldn't figure it out.

OK -- back on track.

Ignition ON, engine off = 0.50v
I did some driving and it fluctuates from 0.05v to 0.88v.

I can't really tell what it is showing me because the readings vary so much. I uploaded the video here so if you guys could watch it that would be great.

Watch all 1min 30seconds because at the end when I came to a stop it hangs at around 0.08v for about 10seconds before moving back up. I don't know if that indicates anything or not?
http://w124-zone.com/downloads/photos/260E/O2%20sensor/o2%20sensor.MPG
or youtube once it finishes processing:
http://www.youtube.com/watch?v=QAmRXaGyW0w

Is 0.0v lean or is 0.9v lean?

From what I've read, the sensor should give you values between 0.2 - 0.8v, with the latter end indicating rich.

so 0.9v is the richest end of the scale.

My o2 sensor is working that is for sure, but I cannot tell if it proves that I am running overly rich or not?