DIY-WB... It's alive...

[Richard Wakeling]

    As far as I am aware all the Aussie ECM/PCM have 450 mV output. A cold O2
sensor has an extremely high impedance so it does not pull this 450 mV down. As
the O2 gets hotter and begins to produce a voltage its impedance quickly falls
allowing the O2 voltage to overcome this 450mV bias.
    If any instrument (Some 10 led O2 displays) is connected to the O2 circuit
that does not have extremely high input impedance it will pull this 450 mV bias
down. If this was to occur when the engine is WARM but the O2 sensor is COLD the
integrator will think the engine is lean and unnecessary fuel will be added until
the O2 has reached its ready state.
    So If you have a vehicle that runs poorly when you first start it, make sure
you have nothing hanging on the O2 circuit pulling it down.

Cheers Richard

rr wrote:

> All opamps have an input bias current. The question has been
> "is the O2 sensor input biased to 450mV, on purpose?" In
> other words does the sensor have a 450mV bias output from
> the ECM to the O2 sensor?
>
> I have not found any evidence that it does. I'm thinking that
> what folks are seeing is the 'home' position being read by the
> ECMs ADC & reported as 450mV. This is with the sensor
> disconnected, hence a bias must be being applied to the sensor.
>
> BobR.
>
> Ludis Langens wrote:
>
> > rr wrote:
> > >
> > > The spec sheet for the LM9044
> > > makes no mention of a bias voltage output to the O2 sensor.
> >
> > The data sheet I have does mention a bias.  Check the spec called
> > "Non-Inverting Input Bias Current".  The typical value is +/- 380
> > nanoAmps.  (In other units, 0.00000038 amps.)
> >
> > > With a Fluke 83 measuring this input to ground the ECM then reports
> > > a 409mV O2 sensor voltage. The Fluke reports a 397 mV level
> > > on the O2 + input (open). This is quite a change for a high input
> > > impedence DVM.
> >
> > Measure with the meter connected between the + input and the ECMs 5 volt
> > supply.  Or measure between the + input and the + end of a D cell (or C,
> > or AA, or ...) with the negative end of the cell jumpered to ECM ground.
> >
> > --
> > Ludis Langens                               ludis (at) cruzers (dot) com
> > Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/
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