[V8] O2 sensor

Kneale Brownson knotnook at traverse.com
Mon Oct 4 14:46:46 EDT 2004


The OEM O2 wiring has a couple feet of wire on the sensor that goes to a
connector under the air cleaner box, near where the engine speed and
ignition reference sensor connectors are.  So the wiring harness from there
takes the O2 sensor's ground to the exhaust system somewhere?  If the
ground is so sensitive, why wouldn't they run the ground to the ECU ground?

At 12:40 PM 10/4/2004 -0500, Hoffman Anthony  J A1C 552 CMS/MXMVC wrote:
>The rusty exhaust system, if my understanding is correct.
>
>Tony Hoffman 
>
>-----Original Message-----
>From: v8-bounces at audifans.com [mailto:v8-bounces at audifans.com] On Behalf Of
>Kneale Brownson
>Sent: 04-Oct-2004 11:57 AM
>To: dsaad at icehouse.net; v8 at audifans.com
>Subject: Re: [V8] O2 sensor
>
>So the V8 OEM sensor is a three-wire, isn't it?  Where does the original
>wiring ground the O2 sensor?
>
>At 08:57 AM 10/4/2004 -0600, dsaad at icehouse.net wrote:
>>The O2 sensor is "analog". It generates a very small voltage from zero 
>>to around
>>1 volt based on oxygen content. Normal operating voltage for a proper 
>>mixture is about 0.7 volts.
>>If there is a difference in the ground reference voltage between the 
>>ECU and the engine block then this voltage adds algebraically to the 
>>actual generated voltage - thus offsetting it. This offset voltage can 
>>really cause problems because the sensor output voltage is not linear - 
>>it is exponential - so a small voltage change can mean a big mixture 
>>change by the ECU. If the ground reference changes - as it could by 
>>normal engine movement and vibration on a one or
>three
>>wire setup (think rusty exhaust pipes/bolts, etc) then the ECU will 
>>never be able to stabilize mixture.
>>The software in many ECUs can detect this and make allowances but our 
>>vintage of Bosch software (I think version 1.2) does not do this. Any 
>>little thing out of whack causes the ECU to overcorrect too quickly and 
>>that shows up as "the surge".
>>The O2 sensor also "ages" meaning that over time it responds more 
>>slowly to mixture changes. This can also be compensated for by more clever
>software.
>>
>>The O2 is not the only sensor that can do this either. The MAF, engine 
>>temp, and air temp are all also susceptible.
>>
>>On the V8, the engine speed and crank reference are analog as well (I 
>>think they come under the magnetic reluctance catagory) however they 
>>are converted from a sine wave into digital pulses before being 
>>processed by the ECU. A voltage offset here can cause very wierd 
>>problems - like engines that will not run
>over
>>2000 RPM.
>>btw, the brake wheel sensors are the same type as engine speed.
>>
>>The cam position sensor on the right head is a digital type sensor. It 
>>outputs a
>>5 volt digital (square wave) pulse so no conversion needs to be done by 
>>the ECU.
>>
>>
>>Guess I should get to work now... but this is more fun :-)
>>
>>
>>Dave
>>
>>
>>Quoting Tony and Lillie <tonyandlillie1 at earthlink.net>:
>>
>>> I'm not saying the fourt was disconnected ever, rather, I switched 
>>> mine and Zay's cars to 4-wire using the auxilary fan ground on the 
>>> drivers strut tower as the ground for the fourth wire (gray). This 
>>> cured the "surging" the cars both did (mine was worse). My question 
>>> is why the different grounds would give diffierent signals. Also, 
>>> where the ECU sees the reference to be, and how an O2 sensor works.
>>>
>>> Tony
>>> ----- Original Message -----
>>> From: "Buchholz, Steven" <Steven.Buchholz at kla-tencor.com>
>>> To: <v8 at audifans.com>
>>> Sent: Friday, October 01, 2004 7:18 PM
>>> Subject: RE: [V8] O2 sensor
>>>
>>>
>>> ... well, if you don't have the fourth wire connected to anything it 
>>> is as if there is an open circuit ... I'd expect the ECU would 
>>> generate codes to this effect ... but yes, I'd expect your car to run 
>>> like crap this way!
>>>
>>> Connecting the fourth wire to *any* ground is better than *no* ground 
>>> ... as was pointed out, one of the OXS heater leads is in fact 
>>> grounded ... this would not be a terrible place to attach the ground 
>>> wire ... be extra careful to determine which pin is grounded because 
>>> the other gets power when the engine is running ... this would 
>>> probably be a bad thing for the ECU to see as a reference and might 
>>> well damage the ECU.  This also fits with the esthetics of building 
>>> an adapter for the Ford sensor ... the 4-3 wire adapter would then be
>self contained.
>>>
>>> Steve B
>>> San Jose, CA (USA)
>>> >
>>> > It is an interesting theory, where to ground the fourth wire.
>>> > I'm going to
>>> > get out the MM and check the voltage drop from the block to the fan 
>>> > ground (in my free time, of course :-) ). However, the car runs 
>>> > about 100% better since I put the 4 wire in, and Zay has reported 
>>> > the same for his car. Both use the fan ground for the fourth wire.
>>> >
>>> > Can anyone explain the theory of why the engine would be better if 
>>> > there is a voltage drop. The O2 sensor is not connected to the 
>>> > engine, but rather the ECU, so I don't see the difference anyhow. 
>>> > BUT, if one of you can educate me, I'd be happy to hear how it 
>>> > works.
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>>
>>
>>
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