Hello

Is there a quick and easy way to check the function of an oxygen sensor on a 1987 300E.

I have a digital volt meter and was hoping there is a location where I can measure a voltage to see the O2 sensor output swing.

Anyone know how this can be done?

Thanks!!

A scope is the best way. With the engine idling, fully warmed up and in closed loop the voltage should swing back and forth between about zero and one volt at about 1-3 Hertz. Under the passenger side carpet there should be a connector for the O2 sensor. Disconnect and attach your meter to the O2 sensor pigtail. If the meter has low response time you should see the voltage flip back and forth.

If voltage zero or a non-zero constant either your meter response time isn't fast enough or the O2 sensor is bad. If you have a "check engine" light on the dash it should illuminate when you disconnect the O2 sensor pigtail from the harness.

Duke

thanks for the post.

I was hoping I could find a place to measure without unplugging anything as I will lose the CIS control system functionality with the O2 sensor unplugged.

Yes my dash has a O2 sensor light, when I first got the car in 2000 it flickered a few times while driving on the freeway but has come back on since.

A generic O2 sensor is only $35 at pelicanparts. If the check engine light is on you should replace it or check for loose or crushed wires/connection.

__________________
1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

thanks for the reply

The O2 light is not on. 20 years ago when I purchased the car the) 2 light flickered for a few seconds once while driving on the freeway, it never came back on.

I just drove the car home from work and took some measurements based on this example.. note I got much different readings see at the bottom:

Voltmeter:
The voltage is converted to duty cycle according to the following formula:
duty cycle [%] = [1 - (Vp3 / Vp6)] * 100
Vp3 = voltage between X11 port 3 & port 2 (or ground)
Vp6 = (battery) voltage between X11 port 6 & port 2 (or ground) during the respective rev !
Example for a measurement at idle:
Vp3 (at idle): 7.1 - 7.6 V
Vp6 (at idle): 13.9 V
duty cycle at 7.1 V = [1 - (7.1 / 13.9)] * 100 = 48.9%
duty cycle at 7.6 V = [1 - (7.6 / 13.9)] * 100 = 45.3%
duty cycle mean value: (48.9% + 45.3%) / 2 = 47.1% (fluctuating with +/- 1.8%)


my vp6 was 13.5V

and my vp3 varied from first 3.8V - 4.4V then from 4V - 4.5V

I used a fluke digital voltmeter

this would give a range of pf 67% to 72%

this is not right, I will try again with a analog voltmeter. I actually have a old Sears Craftsman analog Volt Dwell meter from the 80s that I will try and will report back.

or maybe the above is correct readings and indicate an issue

ok

I just connected my sears craftsman volt/ Tach / Dwell meter to the system

according to the 6cyl scale the dwell read 33-35 degrees. Not sure if this is the same as duty cycle, but I think it is.

at idle there was very little fluctuation. as I brought the rpm up if move a bit more but not much.

I guess I should check it in the morning when it is cold.

stay tuned.

I jsut read

"If you have terminal 3 in your diagnositic connector then check your lambda value first. 50% is normal for most engines but the 103 engine (which I presume you have) prefers to run at 30-40% (I put it on 35% during services), they run quite well at this mixture, 50% is too lean on acceleration from experience. You can check you EHA (if fitted) for operation by watching the lambda value fluctuate +-5%."

so I guess my values of 33-35 are ok?

ok I just went out to the vehicle and did a static dwell measurement of pins 2 & 3. I turned the key to the second position (didn't start the car) and the meter read 25 on the 8 cyl scan and about 34 on the 6 cyl scale.

bear with me here I might seem to be all over the map but am converging.

On https://www.landiss.com/mixture.htm I read:
With the ignition on but the engine not running, the duty cycle tells which version of controller is installed. A 70% duty cycle indicates the standard controller with the ability to read some faults by monitoring the duty cycle. 85% indicates the California controller, which ordinarily displays the faults as a blinking LED on connector X92, although it can be programmed to display the faults as an X11-pin3 duty cycle. 100% indicates the controller does not display fault codes in this manner, or is faulty.

interesting that if you multiply my above values by 2 you get : 25 (8 cyl) x 2 = 50 and 34 (6cyl ) x 2 = 68 (close to 70)?

is my meter off by a factor of 2? the meter was designed to measure the dwell of ignition points. I used it on my 4 cyl VW rabbits from the 70s to set the point I recall using the 8 cyl scale and then doubling it.

here is a link to the manual of my sears meter
http://kentcomputer.com/77VW/sears161.216500.pdf


I feel like I am getting close but need some clarity and direction.

Thanks!!

To recap the vehicle is a 1987 300E the car was purchases from a MB dealership in New Jersey.

I came here as I just failed the smog / emissions test in California. HC was high at both 15 & 25 mph and NO was high only at 15 mph. I thought this test might help to indicate if my O2 sensor is working properly.
This morning after sleeping on it I decided to revisit the measurements. I used a Fluke DMM for all the DC Voltage measurements (should I have measured with the VAC setting? I used a Sears Tach / Volt/ RPM meter for the dwell.

Static (not running)
VDC DMM 6 = 11.77VDC
VDC DMM 3 = 3.60V
Dwell meter 52

The dwell of 52 does not agree with the following statement: With ignition switched on (engine not running) the duty cycle should be about 70% (California: 85%).
However does agree with the following statement:
If you’re using a voltmeter it should read 0.3 * Vp6 (California: 0.15 * Vp6.
In my case it would be .3 * 11.77 = 3.5V which is close to what I measured (3.6VDC) Actually I am measuring DC voltage the equation uses Vp (which I believe is V peak?)

Running (cold – open loop)
VDC DMM 6 = 14
VDCVDC DMM 3 = 7 VDC this would indicate a dwell of 50 = (1-(7V/14V)) X 100 = .5 X 100 = 50.
Dwell meter 44

Running (cold – closed loop)
VDC DMM 6 = 14
VDCVDC DMM 3 = 4.5-5 VDC this would indicate a dwell of 64 – 68.
Dwell meter 47-50 at idle, 47-52 slightly above idle.

I did all of this in about 3 minutes, idling from a cold start, so I didn’t drive the car or let it get up to full operating temp. I decided to come back inside and compose this.

So it seems like the system has some functionally. Perhaps the variations in values is based on VDC measurements with a Fluke DMM?

Any other ideas or should I move on and assume my CIS control system is moving properly.

Thanks You!

It's not clear to me what you measured, but he Landiss writeup is the way to measure duty cycle, so if you ran that procedure properly and found it close to 50 percent, the KE system should be working properly. It's probably been 15 years since I ran it.

Disconnecting the O2 sensor briefly to check its behavior will do no harm. It's a direct test of the O2 sensor behavior. As soon as you reconnect it the KE system should be fully functional, but you might have to cycle the ignition switch to get the light to go out.

You said you have an "O2 sensor" light. My '88 CA version M103 has a "check engine" light, and I thought '87 CA versions are the same. Maybe you have a 49-state version. I know the '88 versions have a more extensive diagnostic system, and the Landiss procedure can be used to verify which version you have.

Also, the emission certification label on the upper radiator support states whether it's 49-state only or CA.

Duke

yes I followed the writeup (however I did not disconnect the evap and plug the vacuum line). I recorded and posted using both a older analog dwell meet and also captured the DC voltage with a Fluke DMM.

The car was first sold in the USA new from a dealer in New Jersey. The sticker under the hood says it meets EPA, California and Canada emissions.

It is a 1987 300E 3.0L M103.980 engine.

what corner or edge of carpet do I pull up to access the O2 sensor in the passenger floor area? A link to a write-up with some pictures would be nice if anyone knows if one exists.

Thanks for all your help!

Remove the carpet on the floor of the pass. side... it's a single piece that easily comes out...just like a floormat. The connector should be under it assuming it's the same as my '88 190E 2.6. You can trace the wire from the O2 sensor through the grommet in the floor pan to the connector under the carpet.

There might be other wires like if your car has power seats. Mine are manual, and I don't recall any other wires/connectors nearby.

I guess '87 emission configuration is different than '88. I bought mine new in CA and my understanding is that CA has more diagnostics, and, of course, it meets 49-state and I assume Canadian standards because the CA standards are tighter.

Duke

Hi

I was away out of town for a while.

I just measured the O2 sensor voltage. Started the car it was already warmed up. Voltage measured around 440mV, let it run for a while and reved it a bit and it reading on the digital meter jumped around between 100mV and 700mV

According to what I have read this is the typical behavior.

I was surprised that it jumped around that much at idle and also when I would hold to rpm up around 2k rpm.


I also confirmed that 13.5VDC was going to the heater part of the O2 sensor.

I guess my O2 sensor is fine?

See my response in your other thread.

Duke