O2 sensor pullup resistor values? (longish)

[Peter Florance]

Yes as I tried to say below, I found that my Ljet ecu supplies .45 volts at
the input when the sensor is disconnected or cold. I guess I could reverse
engineer it by loading the ECU input with resistors to determine the value
(I'll start with 1.2 meg - thanks Ludis) to see what the resistance works
out to.  That seems pretty safe as the bias would be a max of .375 uA.  I
opened an extra box but the trace from the pin lead to a hybrid with a cover
riveted to the board. I didn't need to know that badly.....

AFA averaging, I'm fighting a poor mixture control at idle. I have a sensor
in each down pipe which connect to 3 of in-line cylinders. This a euro car I
converted to lambda and then added a Bosch heated sensor last weekend which
works no better than the un heated (either which can only sample 3 cylinders
currently). The US cars have 6 into 1 manifold (BMW 528i) with single
unheated sensor. My feeling after watching with a scope was there was some
problem with out fast the lambda worked at idle vs. the frequency of the
exhaust pulses of only 3 cylinders. The idle is rough and smells rich.
My thought was to average the sensor with simple linear circuit but I
realize I may be limiting the swing of the output.


Eventually it will be a Megasquirt but I'll still have to solve the problem
(or maybe the MS will work better as it may sample faster at idle if that's
the problem).

Peter Florance
> -----Original Message-----
> From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org]On
> Behalf Of Bernd Felsche
> Sent: Wednesday, December 05, 2001 10:39 PM
> To: diy_efi at diy-efi.org
> Subject: Re: O2 sensor pullup resistor values?
>
>
> Peter Florance tapped away at the keyboard with:
>
> > > -----Original Message-----
> > > Behalf Of Bernd Felsche
>
> > > Peter Florance tapped away at the keyboard with:
> > > > Anyone have any idea of typical ECU pullup resistor (to ~ .45V)
> > > > values for O2 sensors inputs? I'm trying to make a analog circuit
> > > > to average 2 sensors for my L-jet box and would like to
> > > > incorporate it in the front end of the buffers.
> > >
> > > The "pullup resistors" would be infinite. The sensor is a voltage
> > > cell; it provides a voltage. Attempting to bias the sensor can cause
> > > damage to the sensor. If you want to level-shift or add/average two
> > > sensor inputs, the easiest solution is to use op amps to perform the
> > > task. Their high input impedance ensures minimum current draw and
> > > sustained operation.
> > >
> > > > Also interested in any comments regarding averaging O2 sensors for
> > > > dual exhaust systems and idle performance (in line 6 engine)
> > >
> > > Before you go and build a simple adder circuit however, look at the
> > > response curves and the consequences of what will happen if one is
> > > receiving rich and the other a lean mixture. The ECU needs then to
> > > see a constant switching between the levels for correct operation.
>
> > On my L jet box there is some sort of 'bias' that provides .45
> > volts at the sensor input before the sensor warms up or if it
> > disconnected. I've read that's how they sense whether the sensor
> > is working or not.  How do they accomplish that?
>
> Probably on the output of the buffer amplifier, internal to the box.
> The sensor's output voltage rises from 0V when it heats up; that
> transition may be used to switch off the bias.
>
> You can check this by measuring the open-circuit voltage at the
> ECU connector, with the lambda disconnected.
>
> The sensor can only source a few micro amps. Trying to run a sizeable
> current through it, which is essential to bias the output directly,
> will destroy the sensor.
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