O2 Sensor Bias

Wen Yen Chan chanwe at ecf.utoronto.ca
Thu Aug 27 16:49:00 GMT 1998


Hello,

The circuit consisted of a voltage follower charging a small cap. through
an adjustable resistor in parallel with a diode. The output (and feedback
for the follower) was taken across the cap. The diode was oriented to
allow slow charging but quick discharging of the cap. The circuit's
negative supply (needed to allow the output to swing to zero) was provided
by a charge pump. 

I'll see if I can still find the schematics.

l8tr
Wen 

On Thu, 27 Aug 1998, Bruce Plecan wrote:

> 
> -----Original Message-----
> From: Wen Yen Chan <chanwe at ecf.utoronto.ca>
> To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
> Date: Thursday, August 27, 1998 9:17 AM
> Subject: RE: O2 Sensor Bias
> 
> 
> Could this be as simple as a comparator, and a 555 for on after delay?.
> Or could you share some more details?.
> Cheers
> Bruce
> 
> 
> >Hello,
> >
> >A while back I built a circuit to change the O2 sensor readings so that I
> >could make my engine run richer at idle. The circuit delayed the rising
> >edge of O2 sensor's signal and left the falling edge alone. The circuit
> >worked okay. To get around the computer's biasing, the output was coupled
> >through a large resistor (200k worked) to simulate the output resistance
> >of the sensor.
> >
> >l8tr
> >Wen
> >
> >On Thu, 27 Aug 1998, Chris Conlon wrote:
> >
> >> Larry wrote:
> >>
> >> > A lot of guys play with fuel pressure to alter their WOT O2 readings. I
> >> > understand that GM runs the stock motors a bit rich, and a lot of guys
> >> > lean them out to get more power.
> >> > ...
> >> > shouldn't it be possible to just place a resistor inline with the O2
> >> > sensor to "trick" the PCM that the O2 sensor is reading a higher
> voltage
> >> > than it really is, and thereby lean the mixture accordingly? Maybe I'm
> >> > barking up the wrong tree here...but is this possible? How? Any
> >> > problems?
> >>
> >> I don't know if any of this will help lean out WOT operation, but the
> >> question about the resistor got me scratching my head some.
> >>
> >> You probably could use a resistor tied to a 1v bias to make the sensor
> >> read a wee bit higher. (I'm pretty sure I've even seen flaky-looking
> >> commercial products designed to do just this.) Picking the resistance
> >> would probably be a pain. Also does an O2 sensor's output impedance
> >> vary much, within the warmed-up temp range? This mod would probably
> >> also mess up the 0.450v bias used to tell if the sensor is warmed up
> >> or not, maybe leading to problems during warm up?
> >>
> >> The clever way requires you to have faith that the O2 sensor reading
> >> is meaningful (not necessarily linear but repeatable) away from stoich.
> >> Ok this isn't really true of an O2 sensor, but just pretend for a minute.
> >> What you want, roughly, is an O2 sensor who's output crosses through the
> >> 450mv range at say 16:1 rather than 14.7:1. Suppose your O2 sensor
> >> reads 380mv at 16:1 (wild guess). Essentially you generate a 380mv
> >> reference, and take the O2 sensor signal, and feed the 2 into a
> >> comparator, or even an op-amp set up for maybe 20:1 gain. Divide and
> >> buffer the output as needed for a 0-1v range, and feed that into the
> >> ECM's input. Congrats, your ECM will now see cross counts at 16:1
> >> instead of 14.7:1, and adjust the AF accordingly. (If it doesn't freak
> >> out anyway.)
> >>
> >> Problems:
> >> This also defeats the sensor-warmup detection via the 450mv bias.
> >> You could add some more circuitry to get around that.
> >>
> >> The sensor voltage corresponding to 16:1 probably varies a good bit
> >> with operating conditions, and the curve is steep to begin with. If
> >> you had a wide-range O2 unit, well, that would be nice. :)
> >>
> >> How lean do you want to run? Detonation could be a real problem
> >> depending on the engine.
> >>
> >> I realize this isn't a very useful solution to Larry's original
> >> problem... but once the question of tricking the O2 sensor got
> >> into my head I just had to think about it until I was satisfied.
> >> Hopefully someone will find it useful or interesting.
> >>
> >>    Chris C.
> >>
> >
> >
> 
> 




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