O2 Sensors...A little MORE....

["Scott M." <scottmo@teleport.com>]

A little more O2 sensor information....

> waves@epix.net wrote:
> > A comment was made that it is not wise to use nothing
other than a DMM to
> > seek voltage at the O2 sensor. This is not entirely
true. non digital
> > multi-meters will not fry or otherwise impair standard
O2 sensors, according
> > the instructor I had when I went to school at Bosch. As
a matter of fact,
> > they are actually preffered over DMM as you can see the
swing in voltages
> > much better.

Igor commented.
> They do, for a second or two, then the car stumbles and
the reading drops to 0. The 
> analogue metres have inherently less input impedance than
the digital ones, 
> therefore they shunt the Oxs brain, at least on my car.
Unable to track the Duty 
> Cycle fluctuations with the jumping digits of the digital
multimetre, I built my own 
> colour LED meter. It has a very high input impedance (the
input cascade is based on 
> LM2904 for all the EE typs here on the list. An LM358
works well also) and a very 
> intuitive analogue LED display.

Most of the Zirconina type O2 sensors have a higher input
impedance when cold but if I remember correctly, this drops
down in the 5-20K ohm range when the sensor gets warmed up.
I can't find the specs on the Titania resistive type
sensors. As Igor mentioned, it is important to know what
the input impedance is of the meter you are connecting
across the O2 sensor or for that matter across any of the
ECU inputs/outputs. Most modern DMM's have a 10Mega-ohm
input impedance. Many of the older analog meters have a
input impedance down below 100K ohms but they "may" work
once your O2 sensor is warmed up. If your DMM has a
"analog" type bar graph feature, this can be used to
monitor the O2 sensor voltage as it swings high and low
when the ECU/fuel system is in closed loop operation.

 waves@epix.net wrote 
> > After all, the typical O2 sensor is putting out simple
analog signals.

> Igor responded.
> Well, the O2 sensor is a passive device (it's essentially
a variable resistor), and 
> therefore does not produce any signals. It does, however,
change it's conductance, 
> and, therefore, alters the current flowing through it
from the brain to the ground.

Well, on some Jeep, Chryslers, Toyotas, and Nissans there
is a Titania type O2 sensor that is of the variable
resistor type that works in conjunction with the ECU
reference voltage and a resistor divider. Some of these
vehicles ECU's work with a 0-5V output and others use a
0-1V output.

Most of the VW/Audis I have come across use the Zirconium
dioxide (ZrO2) type O2 sensor (heated and unheated type)
that does indeed produce a voltage ~0-1V. Disconnecting the
O2 sensor and connecting up an Oscilloscope you will still
measure a voltage depending on the exhaust gas composition.

Here is an excerpt from the 1976 SAE article #760287
"Closed Loop Control of Lean Fuel-Air Ratios using a
temperature compensated Zirconia Oxygen Sensor"

"The sensor is based on the electro-chemical potential
developed across a zirconium dioxide solid electrolyte when
its two electrodes are exposed to differing oxygen
concentrations. One electrode is exposed to the constant
oxygen pressure of ambient air and the other to the oxygen
pressure of the exhaust gas which varies with equivalence
ratio. A voltage is produced which is a function of the
equivalence ratio."

Regarding Rolf's comment on the warm up time required
before the ECU/fuel system goes into closed loop. As I
commented before, the heated O2 sensors can allow closed
loop operation very quickly, even before the coolant temp
gets very warm. 

I again tested my 89 200TQ (10V turbo I5) with a portable
oscilloscope connected to the O2 sensor connection, after
starting the engine for the first time yesterday (ambient
air temp 75 F) the O2 sensor began producing a voltage that
began cycling (closed loop)  above and below the 0.45 V mid
range voltage  within 50 seconds of starting the car and
driving it one block. The dash mounted coolant temperature
gauge needle did not move off its rest position. Of course
a colder ambient air temp may delay this somewhat. I don't
know how the late model VW FI systems operate during
warm-up nor have I tested the Motronic system O2 sensor
operation during warm-up.

Happy O2 sensing and sniffing......

Cheers
Scott M.
89 200TQ
CHIPDIPPINDUDE