O2 Sensor Info

[DH5481@aol.com]

Sorry this post is so late.  My AOL organize file died - for those of you
that don't have AOL, it means every E-mail you ever had turns into vapor.
 Serious bummer.  I had just written this when it disappeared into the ether
of my hard drive - I found it in my corrupted organize file (page 147), which
was 647K, converted it to ASCII, then WP, then copied and pasted.  I also
retrieved most of these mails I've been saving (at a great length of
time...).  Then 2 keys on my keyboard stuck, giving me ********* and a
headache...  So I worked hard for this one...

How to bench test an oxygen sensor:
  All you need to test an oxygen sensor is a propane torch, vise grips, and a
10 Meg ohm input impedance meter capable of measuring low voltage (1 volt and
less).  If you are going to buy one, ask for a high impedance (10,000 ohms)
digital type.  
Connect the voltmeter (ground to one lead, single wire to the other lead.  If
3 wire, determine the heating element by measuring resistance - about 15
ohms).  Heat the tip of the sensor, holding the tip completely in the flame.
 Move the torch flame over the sensor tip.  The tip shound be hot enough to
turn cherry red, and the flame must enter the opening into the sensor tip.
Meanwhile, keep an eye on your meter.  If you get a reading above 600
millivolts, and the reading changes quickly with the movement of the flame
over the tip, the sensor is ok.  Otherwise, replace the sensor.  Note:
 Testing a sensor with a meter other than the specified type will destroy the
oxygen sensor and ANY RELATED COMPUTER CIRCUITS, if connected. (Don't try 'in
place' readings unless you know what you're doing - Dave)

Oxygen sensor info:
  As sensor milage builds up, a natural aging process takes place.  With age,
the oxygen sensor slows down.  As the time needed to switch from 150 to 850
mV and back again increases, there is a gradual shift to richer conditions.
  Lets look at how this typically occurs.  If we have a sensor that takes 150
milliseconds (mS) to switch from 150 mV to 850 mV new, whereas it took 50 mS
when new, the computer will take 3 times longer to increase injector
pulse-width or reduce carburetor dwell readings.  Now instead of seeing a
peak injector pulse-width of 3 mS, we might find a peak pulse-width of 3.5 or
4 mS.  This is before the sensor reaches the computer's 850 mV threshold and
starts a reduction in pulse-width to drive the sensor back to 150 mV.  This
gradual change may cause a vehicle to run richer.  The result may be an
overworked catalyst, air fuel ratios may be 14.3 to 1 instead of 14.7 to 1
(ideal combustion ratio), and fuel consumption will increase.  This aging of
the oxygen sensor starts to occur the day it is installed and may be
significant enough to affect fuel mileage in as little as 15K to 25K miles.
 The aging process may be due to a number of factors:
   -  Natural aging of the zirconia material which requires the sensor to get
warmer (say 800 vice 600 degrees) before it will produce enough voltage to
allow the computer to switch.
   -  A buildup of resistance at the interface of the zirconia platinum bond.
 Think of this as corrosion on a conntor.  This causes false readings in the
circuit and is usually due to thermal cycling.
   -  Contamination on the outside of the cell reduces the size and number of
openings where exhaust gas can permeate the spinnel coating and react with
the zirconia-platinum junction.

Service tips:
  If you have a sensor that is hard to remove from the manifold, STOP!  Warm
up the engine first.  The thermal expansion can help free the frozen sensor
(manifold temps can exceed 1000 degrees, so be careful...)

  Another important tip to keep in mind when replacing O2 sensors is to erase
the computer memory (don't know if this applies to Audis...).  Otherwise
drivability problems can occur for the first few starts and stops.
 Disconnect the battery or ECM fuse for a few seconds.

Ordinary table salt can destroy an oxygen sensor - Make sure your hands are
clean!

Oxygen sensors are very delicate and easilt contaminated by impurities.  Lead
fouling and carbon deposits are the two most common causes of failure.  In
extreem cases, when the sensor has stopped functioning completely, the
catalytic converter may overheat and stop working.  Engine performance and/or
gas economy may be poor.

Hey - don't thank me - thank Tomco.  I don't have a clue how to get this to
the archives - be my guest if you know how.  Plus, the description of
operation may not be a perfect fit for Audi electronics - but the concepts
are the same.  I, for example, have lean conditions, with poor performance
(except when cold - then it sings that great Audi turbo song, and the grin is
from ear to...).  But as I posted earlier, this place has Bosch universal O2s
for $50.00, and both types of Pentosin (11 @ $14.95 and 7.1 @ $12.95) and
great ref material - the guys are real helpful - saved me $40.00 on a tie rod
end that nobody else could get, with next day service fm out of town.  Don't
know if the ship, but if anyone is interested, its K&K Foreign Auto in
Orlando (407 area code) I'm too lazy to drag out my yellar pages, and the
wife has been given me dirty looks for 2+ hours...

Dave Head
87 5KCSTQ