O2 Sensor.. Alert ! ..and new info

[Wil Ferch]

Guys:
I've had a lengthy exchange with a rep from Bosch-USA on
the various types of O2 sensors. My primary concern was trying to get
economic alternatives to the very expensive Porsche ( Bosch) units, as well
as understand the various differences that might exist..and also if there
are any caveats. Some interesting things came out of our discussions.
Summary follows:
- the various O2 sensors pretty much act the same way, in terms
of output signal vs input readings, however...
- there are various ceramic types, and for the three-wire
heated systems , various heater strengths ( 12W and 18W)
- for the mid 80's , 3.2L Carrera's, using DME control... you
may substitute:
Bosch 13913, 3 wire unit originally intended for
Ford applications. I believe this has an 8" lead, and you must splice -in
the original Porsche connectors ( more on splicing caveats later).
Bosch 13953, similar 3 wire unit for Fords, but
with ( I believe) 16" long connectors.
Bosch 15725, Bosch "universal" , with universal
connector kit that avoids certain problems compared to using generic
connectors ( again, more later).
Bosch 15726... an alternative to 15725....also OK
to use for 3.2L Carreras.
Bosch 15715, 4 wire .. NOT OK to use ( more on 4
wire later).
By the way , the "long" and "short" Bosch part numbers relate as
follows. The short number ( ex. 15725) is the last 4 digits of the long
number with a "1" in front of it ( long number .... 0258005725 ).

Anyway, here's what I found VERY interesting. The O2 sensor works on
the principle of comparison signals, and the reference signal is "local"
oxygen that is not in the exhaust stream. These sensors are located in
harsh environments, so the reference O2 signal is transmitted UNDER THE
WIRE INSULATION ...back to the computer....via the engine compartment
(sealed) connector. The CAVEAT is that if you use a normal crimp connector
( to re-use the Porsche plugs, for example) you might introduce engine
compartment "air" unknowingly as the reference signal. This can be bad.
Furthermore, I usually use the marine-grade connectors when making
electrical mods to my car. These have a glue inside the crimp, so when you
apply heat to seal the crimped-ends, the glue flows to make a water-tight
seal. Normally, this is good, but in THIS case, it would seal-off the
air-path back to the computer, and compromise the air ( oxygen) reference
signal. WOW ! ... who would have thought about this ??? Bosch's generic
connectors used with the generic units overcome this...by design.
Lastly, I inquired about the use of 4 -wire sensors...thinking the
additional ground would be a "good thing"...much like the upgrade we do on
the cylinder head temp sensors ( later 2 wire temp sensor versions have an
additional ground). The 3-wire O2 sensors use 2 leads for the heater (
non-critical as to hook-up... bi-directional) , and the third lead is the
actual connection / air source. The fourth wire would be the ground. My
thinking thatg since the O2 sensor works as a switch between 0 and 0.1
Volts...a rather low voltage.... an additional ground would be nice.
Instead, I was advised that this additional ground, ( for the same
reason...low voltage operation) may be unduly influenced for stray signal
noise. We were advised to stick with the 3 wire design.
Hope this helps someone...esprcially the new-found info in splicing
caveats !

Wil Ferch
85 Carrera

[Bill Verburg]

Will, I may be all wrong here but, the electric signal generated by the sensor certainly travels through a wire under the insulation but the difference signal is generated entirely inside the O₂ sensor body.

Here's a detail of the operational portion of a typical sensor


Here is a cross section of a typical O₂


Here is the simplified Bosch diagram


The 3^rd wire would just be for the heater +

Here is a link to an SAE paper devoted to the Bosch O₂ sensors SAE on Bosch

[jwill911]

This is great information Wil. And very timely for me. After having just been bitten by bad reference sensors on my `87 3.2L I'm very sensitive to anything else that may need attention. My 02 sensor is ~56K miles old and I've been searching for an alternative to the $200+ Porsche part. And at the same time a little bit leary of the all too often used statement generic Bosch 02 sensor. I'm still not sure why you can't get Bosch OEM 02 sensors with the Porsche correct connector on it, unless Porsche has some guy in the factory making them up.

Thanks,
John

[Wil Ferch]

Bill:
What can I say except:
-- don't shoot the messenger. That is emphatically what the Bosch Technical rep says happens. He went on to say that indeed there is air ( O2) in the sensor itself, just as you show in your references and illustrations, ...but that the replenishment supply is as I had indicated , (through the wires back to the computer, ..all of which I found amazing), ..and as he himself desribed. I know he frequents this list and the Rennlist board , and he may wish to chime-in himself...
--- Wil Ferch

[Bill Verburg]

I believe that the reference air enters the sensor thru the same opening as the 3 wires. The sheath for the wires stops a few mm shy of the sensor insulator providing an ideal point of entry. The newest sensors actually have some sort of pumping device.

By the way we use the 'Anchor' waterproof connectors w/ tinned copper & brass connectors, shrink wrap & glue everywhere on our boats. Never had a failure or problem. I also use and highly recommend them whenever I do wiring on my cars. I also recommend the use of tinned wire where ever possible.

[Wil Ferch]

Bill:
Just so you don't think I'm singling you out to pick on...it was an old (Rennlist) post of yours that I've always valued and archived that lead me to the Ancor connectors in the first place.

Fine stuff.

But...believe what you will. The O2 sensor seems to have an on-board capsule of air, but the BOSCH TECH REP ..says the air communication is maintained to this chamber by the air path under the wire insulation. At first, I didn't believe this either, and I asked about the plug connector in the engine compartment..surely it is not sealed and also can be a source of air ..contaminated air. He emphatically says, "NO"..it is sealed and the air path continues to the computer under your seat. Don't know how else to answer that you "feel" this is incorrect. I would welcome counter-views backed up in some way...like someone maybe taking these things totally apart and then saying the the Rep is wrong.... "full of it"...or somesuch...wouldn't you agree?? As it is, how can we dismiss what the manufacturer says is correct?
---Wil Ferch

[scottb]

Wil: Any thoughts on soldering the splice, and then using heat shrink tubing for insulation?

[Wil Ferch]

Yep:
Soldering the splice and then using heat shrink would work, also confirmed by the same discussion with the Bosch rep. The universal kit with its connectors supposedly works, too.

Even as this is debated...these two ways seem to be reasonble ways to go..and would satisfy everybody, those who believe the rep, and those who do not.

--Wil Ferch

[ChrisBennet]

I could see where the sheath that protects the wires from the sensor could feed the reference air to the sensor but that sheath ends at the rubber grommet that goes through the engine tin. The wires leave the sheath and go through the grommet as three individual wires.
-Chris

[Wil Ferch]

Chris:
Hello again...
The wire we're talking about is the main connector, not the two other leads for the heater nor the sheath that surrounds all these. The main connector apparantly has "communication" with the sensor itself...in the from of connecting to the air chamber within the sensor...as best as I can tell.

Don't understand all the push-back. A factory rep says its so.....and I post that summary here... to help avoid unwanted troubles if spliced wrong. Accordingly, two splicing methods were later given that work..if you believe this air-communication thing or not.....

With that, what more do we need? For all those that don't wish to believe this, and need more info, maybe they can contact Bosch -USA directly as I did..and continue the dialogue there.
---Wil Ferch

[Wayne 962]

Perhaps I don't completely understand, but it sounds like gibberish to me. If this were true, then an adapter to go from the generic units to the wire harness wouldn't work? I find that very difficult to believe. The connector is just that - an electrical connector. If you have an adapter, and the 02 sensor is outputing the correct values, then you should have correct results.

How much is the generic sensor?

-Wayne

[Wayne 962]

Hmm, I don't understand it. The universal one is about $20-$30 retail!

So, would a small *electrical* adapter work? I don't believe that the rep is correct - the air path doesn't continue back to the computer. The electrical path does, but the wire harness of the 911 contains nothing special like a sealed air chamber. That just sounds rediculous to me.

Of course, I have been known to be wrong before, but I'll bet $100 this time I'm not...

-Wayne

[Steve W]

Reference air passing through the sheath of the wires??!! The plug and boot being part of a controlled air path to the computer??!!! What next, am I gonna hear about a controlled oxygen tank enclosed with the Motronic box? ROTFLOL!! I wasn't gonna say anything in this thread, but I cannot resist. I gotta agree with Wayne on this one. Sounds like a Bosch sales rep stroking you with information he has no idea about. Let's see, I've got an an old Carrera 02 sensor in front of me and it looks nothing more special than three wires covered in a irradiated PVC jacket entering the body of the sensor through a simple rubber grommet with three holes in it. Doesn't look any more airtight than my doors are when I shut it. I'm sure you guys here are no dummies, I as a former engineer would like to grill this sales rep, ahem, I mean "tech", on his technical explanations of this just for laughs. He does frequent these boards right? Bad information has got to be worse than no information.

[widebody911]

I looked at mine today. The sheath is open right where the wires go into the sensor, and right before the connectors. It looks like it just keeps the wires bundled together - nothing fancy. There's certainly no way in Phoenix that air is gonna make it all the way back to the computer! I've seen vaccum guage setups, with a thin, hard plastic tube from the engine to the guage - that's what you'd need in this case. Otherwise the slightest kink would choke it off.

[scottb]

I bought a universal one last week from another supplier (sorry Wayne, but you didn't have the universal one on the site). The guy told me that they have a shop also, and they always use the universal one unless the customer specifies the specific Porsche model. The black wire goes to the black wire on the connector, and the two white wires are the heating circuit and are interchangeable. I was advised to solder and heat shrink the connections.

[Wil Ferch]

Gee:
Maybe being advised to solder and heat shrink has some merit, re: the O2 path thread ???
Starting from basics....the O2 sensor works like a battery, and it needs a reference signal ...air ( O2)...to compare to the value seen within the exhaust stream. Listen carefully to what the TECHNICAL rep says. The body has an encapsualted chamber that has the air. It has an open path NOT to the outside at the body, otherwise road slush and such , ..and water.. might find it's way into that chamber , rendering it useless. Maybe the seal IS imperfect, but if you think about it, that internal air chamber having an air "path" underneath the insulation ...( not the protective SHEATh...where did THAT come from ??)...means simply that any replenishment air can be re-supplied from the source at the DME. Weird...but not without some rationale.
At this point...as I said...I would hope this frequent contributor from Bosch...who has already corrected a number of misconceptions on O2 sensors for the 944 guys...chimes in. It would be nice to get to the bottom of this, on a factual basis...

-Wil Ferch

[Doug Zielke]

Originally posted by scottb
I bought a universal one last week ............. I was advised to solder and heat shrink the connections.

Same here. As a matter of fact, on the enclosed instruction sheet, it said to solder and heat shrink the new lead to the existing lead. A small piece of heat shrink was even included with the sensor. The sensor was a Bosch, intended for CIS cars.

It's beyond me how any significant amount of air is going to travel up a wire under tight insulation. Perhaps we are about to see the birth of a new urban myth.....the O2 sensor that breathes.

[island911]

What if the reference side does not need to breathe?
What if the reference side does not "use-up" any O2? . . .and yet some tech guy assumes it must ? ? ?

[bell]

i've always assumed that the "readings" would come from inside the sensor only. if the wire picked up oxygen from the wire itself whould it not also read it inside the computer from where this wire connects in the computer? i think you may have misunderstood what he was explaining, but this is my opinion.

also, ANY electrical connection creates resistance, this is most likely why they suggest "splice and solder", this method will create the least resistance as opposed to crimps and others.

great thread guys

[Wil Ferch]

Hi:
With all due respect..I did not misunderstand what the rep said, because it sounded so improbable at first to me too. Reference signal should be "local atmosphere"..if it were in the body of the sensor, ..(as it actaully is).....and if the "opening" of the chamber is "open" to the computer under the seat....what's the difference? It won't be contaminated by water or mud or other nasty efflua...it would be a "representative local oxygen sample". It should be the same under the seat as it is in an enclosed chamber that is perfectly sealed.

I guess whether we believe this or not...it can't be argued that it is good practice to either:
1.) use the universal connectors that come with the universal sensor, or...
2.) Solder, then heat shrink, when using the Ford pieces, and using our old connectors. Why risk a problem by using the marine connectors with glue?

Isn't this a reasonable compromise ??

--- Wil Ferch