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>emissions.  The problem is that the sensor has a finite life, but cannot be 

>tested except by replacement.  The idea is that for a particular vehicle, 
>the sensor is supposed to last at least some number of miles.  This is the 
>mileage that the warning light comes on at.  The new sensor is not supposed 

>to improve the way the bus runs, but rather prevent the bus from running 
>badly at some future time.

Not entirely true.  Oxygen sensors can be tested, using a scope or even a 
good high impedence voltmeter.  The sensor itself "puts out" from 0 to 
1.1volt signal when sufficiently warm; the computer looks for "cross-overs", 
or the number of times a signal crosses through the .45v point.  A healthy 
ox sensor will not spend any time at .45 volts.  Since this voltage will 
change directly with the amount of oxygen in the exhaust, the sensor implies 
the mixture, the computer is then able to make corrections in the dwell time 
of the injectors, or whatever to make the mixture come out just so.

Determine the output (sensed) wire; the other two are heater leads to get 
the sensor on-line more quickly.  Warm the engine, and watch for voltages 
above and below .45v, and rapid changes.  This would indicate a good sensor. 
 If the voltage parks at a high (.7v) or low (.2v) level, try richening or 
leaning the mixture and watching for a change.

low volts = lean: 
high volts = rich.

Testing the sensor on the workbench is also possible using a propane torch 
to warm the sensor and keeping track of the recovery time once the flame is 
removed.  The sensor should appear to be an "open" circuit when cool when 
tested for resistance.

I got this from a posting by Rick Kirchoff via Cary McAlister, from the 
Z-Car mailing list.   

Some lights respond to a signal from the computer telling us, that from 
actual sensor output, the sensor has failed.  On the VW, however, the light 
is just a timer, hooked up to go off every xx,xxx miles.  No brains 
involved.  This little dodge allows VW to meet the EPA 50,000 mile 
requirement on the original emissions package; just require the customer to 
replace the key sensor less often then 50,000 miles.....(at customer 
expense??)

bowmanr%em@mhs.elan.af.mil
Live Smart.  Think for yourself.  Transform the future.
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