In a message dated 98-08-07 02:09:42 EDT, midwesty@taos.midwest.net writes:

<< ---------------------------
 Hi Michael,
     I have been getting into ECUs pretty heavy lately and it seems to me
 that a CO meter is the only way to test an oxygen sensor.  Most  digifant
 books say the digifant should have a CO level of 0.3 to 1.1. If it has the
 correct level the oxygen sensor is good. If it is out of this range it is
 bad or the ECU / wiring is bad.  The  vw Vanagon protraining manual suggests
 to test the ECU one should start the engine with the oxygen sensor
 disconnected. Hold the disconnected wire from the ECU to ground and the CO
 will rise.  If the CO does  rise the ECU is said to be ok.
     I like most of us have no CO meter. I can change the oxygen sensor many
 times for the price of the CO meter. It seems to me if we start the engine
 with the oxygen sensor disconnected then hold the wire from the ECU to
 ground we should expect the digifant to cause a very rich situation (black
 smoke). If however we touch the wire to the plus side of a 1.5 volt battery
 with the minus side of the battery grounded the ECU  should generate a very
 lean situation.  This may be a good way to test the ECU to see if it is
 reading the oxygen sensor correctly.
     However I feel  the only sure way to test the sensor is to use a CO
 meter or replace it.  Even if the ECU is able to control the mixture so the
 oxygen sensor reads 0.5v we don't know for sure without a CO meter the
 mixture is correct.  The sensor can have a wrong out put and the ecu will do
 its best to keep the voltage at 0.5v, when  a proper oxygen sensor may read
 0.7 volts.  Sensors fail in different ways with my limited experience I have
 seen no output, wrong output, complete negative output and slow to respond
 to changes.
      l plan to set up and post on the web a trouble shooting guide for many
 of the ECU components and thought you may have some thoughts on or shoot
 down my  observations / ideas on oxygen sensors.

 Darrell
  >>
Darrel, a CO reading between .3 to 1.1 with the O2 sensor connected still wont
tell you the condition of the actual O2 sensor. The only true way to find out
the condition of the O2 sensor, is to monitor  how quickly the voltage
fluctuates, high to low, the swing rate of the voltage and how tight.  New
VW's since 1996 with OBD2, the engine ECU is constantly monitoring the voltage
fluctuations, response time and how tight the swing is. If at any time the ECU
doesn't see this, the ECU will store a fault and turn the check engine light
on. It can happen with a new car with only 10K on it, I see it all the time.
OBD2 is so wonderfull because it monitors everything, and I can watch it all
on a VW computer.  Now that your vanagon doesn't have OBD2, you need to use
your wonderfull volt meter. With a hot motor, dissconnect the O2 sensor and
get a voltage reading, watch it carefully, analog voltmeter would be best, so
you can see how quickly and how tight the swing is of the voltage. The swing
rate should be  within a second, from a high to low voltage. Turn your mixture
screw counterclockwise, LEAN, your voltage will be low, around .1 (100
millivolts). Now turn it clockwise, your voltage will go up to .9 (900
millivolts).     .5 is perfect air fuel mixture, you'll see on your digital
voltmeter a high of .7 and a low of .2, fluctuating rapidly up and down,
sometimes you'll see .5 in there. What matters is, that it is an average of .5
volts. An analog volt meter would be better ( unless you digital meter has an
anlalog graph).   If you were to check the CO now it would say probably .6 CO,
that is a little too lean for a vanagon. That setting is what it will be on a
cold start  up, wich is too lean for a vanagon.  Keep in mind that the ECU
isn't looking at your O2 sensor voltage until the engine reaches a certain
tempurature. Per temp sensor, the ECU will then take O2 sensor voltage and
then adjust accordingly.  The purpose of setting the voltage higher is for
cold run characteristics ONLY. The ECU will always keep adjusting the amount
of fuel to the injectors so it will see an AVERAGE of .5 volts.   Swing by a
dealer or independent VW shop and hook it to the CO machine, along with a volt
meter and see for yourself. Have fun. I'd like to know how it  turns out.
Michael Modl.
 87 syncro
 87 quattro