[V8] O2 sensor

[dsaad at icehouse.net]

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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