Oxygen Sensor Mod for Single Wire Sensors - Mercedes-Benz Forum
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post #1 of 43 (permalink) Old 11-03-2014, 10:25 AM Thread Starter
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Oxygen Sensor Mod for Single Wire Sensors

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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post #2 of 43 (permalink) Old 11-03-2014, 07:37 PM
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Great idea and very helpful pictorial. Time to crank up the ohm meter.
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post #3 of 43 (permalink) Old 11-03-2014, 09:52 PM
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My '83 is running rich as hell...this sounds like something that needs doing before I tackle the mixture knob.
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post #4 of 43 (permalink) Old 11-04-2014, 04:59 AM Thread Starter
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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.


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Originally Posted by jaymaynard View Post
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:

https://www.benzworld.org/forums/r-c1...rect-duty.html
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post #5 of 43 (permalink) Old 11-17-2014, 07:29 AM
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Outstanding. Thanks for the quick how-to. Looks like a good winter project.

Kevin
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post #6 of 43 (permalink) Old 11-17-2014, 08:14 AM
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Outstanding. Thanks for the quick how-to. Looks like a good winter project.
Yours should not need this. The 560 has a three wire heated O2 sensor.
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post #7 of 43 (permalink) Old 11-17-2014, 08:47 AM
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Well then....you just saved me $9 and a whole lot of time.

Kevin
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post #8 of 43 (permalink) Old 11-17-2014, 09:32 AM
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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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post #9 of 43 (permalink) Old 11-17-2014, 10:13 AM Thread Starter
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Originally Posted by rowdie View Post
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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post #10 of 43 (permalink) Old 11-17-2014, 10:18 AM Thread Starter
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Originally Posted by N1ckB View Post
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.

Last edited by Myndex; 11-28-2014 at 01:32 AM. Reason: Spelling
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