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1980 450SL named Freya. 202,000mi
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Discussion Starter #1
On the very early cars with O2 sensors, there was only a single wire for the signal voltage, and the ground went through the screw-in connection in the exhaust.

Wile this might have been a fine connection when the car was new, I recently measured the resistance from the connection point to chassis ground at the ECU, and it as 1100 ohms !!

Now, if the ECU had an extremely high input impedance, the 1100 ohms wouldn't matter much, but the ECU on the 1980 thru 1985 seems to be mostly passive components and resistive voltage dividers, as such I appears that 1100 ohms is enough to offset the calibration of the ECU.

The fix to correct this is simple and about $9. Attach some braided engine grounding strap from the chassis direct to the O2 sensor using a hose clamp.

The result is a good quality ground for the O2 sensor, enabling more stable and accurate operation.

THE MOD

This mod can be done without removing the sensor.

Purchase an engine grounding strap, and a small hose clamp.

Dorman Help! 60213 - Ground Strap | O'Reilly Auto Parts

There is a very convenient place to connect to chassis - the heat shield connection bolt is right over the O2 sensor. Clean the bolt area, and the heat shield, and the shield's bolt.



Reattach the heat shield, connecting the grounding strap to the bolt. open the hose clamp and put it over the sensor. Close it partially and slide it against the exhaust.




Now, wrap the grounding strap round the sensor once, tucking it into the hose clamp. Trim off the excess.




Tighten the hose clamp to about wrist tight - be careful and do NOT over tighten or bend the sensor body.




The completed installation:





Removing the excess resistance should result in the engine running leaner (that is, the ECU commanding a leaner condition). Part of the reason I failed smog recently was a too-rich condition, and i believe this is a contributing factor.
 

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Premium Member
2015 ML250 Bluetec and 1987 560SL
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3,603 Posts
My '83 is running rich as hell...this sounds like something that needs doing before I tackle the mixture knob.
 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #4
I'll add that the sensor shown above is the DENSO, which I am using instead of the Bosch. The Denso has an additional protection cage that is supposed to make it more resistant to contamination - as these older cars may have minor coolant leaks, this is an important consideration.

Also, the Denso has a stiff metal ring near the front that is ideal for wrapping the grounding strap on it.


My '83 is running rich as hell...this sounds like something that needs doing before I tackle the mixture knob.
And for that year, I might mention that you can't use a standard duty cycle meter on pin 3 of the X11 connector - you need to use an oscilliscope or do the mod I discuss in this thread:

http://www.benzworld.org/forums/r-c107-sl-slc-class/2189370-1980-1985-ecu-mod-correct-duty.html
 

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1999 Porsche 911 / 2013 ML350 / 2006 Mini Cooper S
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671 Posts
Outstanding. Thanks for the quick how-to. Looks like a good winter project. ;)
 

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'82 380SL; '95 Defender D90; '78 450SL; '14 328d; '02 540i Touring (sold); '83 280SE euro (sold)
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219 Posts
I had spliced the green coax type cable that runs to the O2 sensor, so would it work the same if I attached a ground cable to that?


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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #9
Yours should not need this. The 560 has a three wire heated O2 sensor.
Four wire sensor do not need this, where two of the wires are signal and signal ground.

THREE wire sensors that use one signal wire, case to ground, and two electrically separate heater wires (that are not case grounded) STILL need this mod.

This mid is about providing a solid, low resistance path to ground for the o2 signal. It is critical for older cars with corroded exhaust systems.

This is why newer cars use a FOUR wire system. Two for signal, two for heater.
 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #10 (Edited)
I had spliced the green coax type cable that runs to the O2 sensor, so would it work the same if I attached a ground cable to that?


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Depending on the specific application,maybe. Sometimes it is best to connect the shield ground only at the signal input. In some cases, connecting the shield ground on both ends and using it as signal ground can lead to ground loops.

Not sure a ground loop would form here, but I decided that instead of connecting the coax sheild and using it as signal ground, to simply strap to chassis as signal ground.

Another motivation was that the O2 sensor, being exhaust connected, gets very hot. I didn't was to transmit too much of that heat to the signal ground wire. By grounding case to chassis as demonstrated, we avoid transmitting more heat to the signal wire.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,461 Posts
Four wire sensor do not need this, where two of the wires are signal and signal ground.

THREE wire sensors that use one signal wire, case to ground, and two electrically separate heater wires (that are not case grounded) STILL need this mod.

This mid is about providing a solid, low resistance path to ground for the o2 signal. It is critical for older cars with corroded exhaust systems.

This is why newer cars use a FOUR wire system. Two for signal, two for heater.
In the end did you experience a noticeable improvement in performance?
 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #12 (Edited)
In the end did you experience a noticeable improvement in performance?
I have substantially less carbon on the plugs of the working cylinders.

I still have the missing cylinder, which I believe is the fuel distributor,which I am going to rebuild tomorrow or wednesday.

I don't have an exhaust gas analyzer, so until I go to the smog guy, it will be hard to determine. But I was impressed that since the o2 mod, the carbon on the plugs of the working cylinders is nearly gone, and the insulators on the old plugs I removed were white.

Because I am chasing down multiple problems, it's difficult to say "how much" a particular fix assisted, but the plugs are much cleaner. and I attribute this to a properly grounded O2 sensor that is making the car run with a more correct mixture.
 

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1987 560SL
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2,370 Posts
My apologies if this was already covered here, but I am just learning oxygen sensors. Is there a way to test the oxygen sensor while on the car, without the use of an o-scope? Can this be done with a good voltmeter? I tried to remove the sensor and it is quite tight; so I would like to know if it is really defective before I put loads of force on it. Thanks.
 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #14 (Edited)
My apologies if this was already covered here, but I am just learning oxygen sensors. Is there a way to test the oxygen sensor while on the car, without the use of an o-scope? Can this be done with a good voltmeter? I tried to remove the sensor and it is quite tight; so I would like to know if it is really defective before I put loads of force on it. Thanks.
On an older car (I.e. Your 1980) I'f it is more that 30,000 miles old it should be changed.

If you want to test it, once it is up to operating temp, it should be giving a voltage output of 0.45 volts assuming the mixture is set correct. This voltage will be fluctuating up and down, but should be averaging about 0.45 if the car mixture is set correct.

Even so, the o2 sensor may be bad, in that it may be responding too slow to exhaust changes.

If it is more than 30k miles old, it should be replaced, regardless.

Really, the way to know is using a exhaust gas meter - are you failing smog?
 

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1987 560SL
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2,370 Posts
If it is more than 30k miles old, it should be replaced, regardless. Really,the way to know is using a exhaust gas meter - are you failing smog?
Thanks for this reply. I have no idea how many miles on it. I purchased the car this summer with no records. Somebody put a new stainless exhaust system on it that still looks pretty good. Somebody pulled the bulb from the O2 warning circuit on the instrument cluster. So the work was done but I don't know when.

I don't have an exhaust gas meter, and here in Ohio nobody tests for smog. The car runs great and the plugs came out with nice color. My objective is getting it as close as possible to running like new.
 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #16 (Edited)
If you don't have to pass smog, the. You can essentially ignore the O2 sensor, which (along with the ECU) are there to maintain a 14.7:1 air fuel ratio.

------- EDIT (Nov 27 2014): If your mixture departs from 14.7, you will tend to destroy and clog your catalytic converter. The converter needs the fluctuating and near stoch mixture to operate properly. Only ignore the O2 sensor if there is no catalytic converter. If you have a catalytic converter, then for this car replace the O2 sensor every 30k miles. It is a standard service item. -----------


If you disconnect the O2 sensor, the ECU will send a 50/50 signal to the frequency valve, and you are then just using the K-jetronic system for mixture. If you look at the 1979 manual, it shows to set mixture using a CO meter. There you could set a little lean for fuel economy, or a little rich for better power/performance.

If you want 14.7 for best emission, then the O2 should be replaced every 30k miles (it's only $16), and the O2 case should be grounded to the chassis. Modify the ECU to allow you to use a standard duty cycle meter to measure duty cycle, and set idle duty cycle to around 50% to 55%.

See these threads:

http://www.benzworld.org/forums/r-c107-sl-slc-class/2189370-1980-1985-ecu-mod-correct-duty.html


http://www.benzworld.org/forums/r-c107-sl-slc-class/2182425-k-jetronic-lambda-overview-adjustment.html


If you mod your ECU so that you can use a duty cycle meter (or you use an oscilloscope) you'll know the O2 is working by seeing its fluctuations up and down once it is at operating temp. See this video for an example:

 

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1980 450SL named Freya. 202,000mi
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574 Posts
Discussion Starter #17 (Edited)
To add to this thread, as I review my notes of the past couple months of various smog tests, I wanted to add:

Prior to grounding the O2 sensor, with the mixture set for 50/50 duty cycle (at idle) the CO was 2.0% on the Dino at 15 mph or so. NOx was about 1250.

After grounding the O2 sensor, and adjusting the mixture to 50/50 duty cycle at idle, the ℅ dropped to 0.95% on the Dino at 15 mph or so. NOx was 1800.

This indicates substantially leaner running under load, indicating the O2 sensor is now correctly commanding the ECU.

Both of these tests were in "manual" mode at the smog guy's place. Both tests were BEFORE I put on the new catalytic converter. The new converter brought NOx down to 3, and CO came down further with the new cat to 0.64% at 15 mph and 0.02% at 25 mph (official test result).


I believe grounding the O2 sensor body was instrumental in reducing CO caused by rich running. It is useful to point out that newer cars use grounding straps or 4 wire O2 sensors to eliminate the electrical resistance problem in aging exhaust systems.

Grounding the sensor body is thus useful for single wire and three wire sensors. (Three wire sensors still have only one signal wire, the other two wires are for the heater, and do not affect the ground electrical resistance, which is still through the exhaust pipe). Four wire sensors do NOT need this mod. The mod is particularly important foe original exhaust pipes and manifolds in older cars.
 

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'82 380SL; '95 Defender D90; '78 450SL; '14 328d; '02 540i Touring (sold); '83 280SE euro (sold)
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Was your old catalytic converter getting super hot before you got all the issues that was causing the rich condition??


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1980 450SL named Freya. 202,000mi
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Discussion Starter #19
Was your old catalytic converter getting super hot before you got all the issues that was causing the rich condition??


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Yes, if you look at it in this post, you'll see the metal shows discoloration from overheating due to rich conditions.

This was tall to me a couple months after installing the old converter 8 years ago by the muffler shop when they were fixing something else. But at the time, no one had a fix for "rich running Mercedes" and the converter was eventually clogged and destroyed.

My hope is my NEW converter will last longer, and I believe the rich-running problem is solved (at least partly) by the grounding of the O2 sensor.

I'm going to look at my new converter in a couple months, and see if it looks like it's also overheating or not.
 

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'82 380SL; '95 Defender D90; '78 450SL; '14 328d; '02 540i Touring (sold); '83 280SE euro (sold)
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I definitely need to try the O2 sensor grounding. I've done a lot on my 82 380SL but I still can't get the adjustment right. I may still have a problem with my WUR and still need to do the modification to the ECU that you had posted to be able to use my standard duty cycle meter.


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