Oxygen sensors...

[Brian Dessent]

EFI Dude wrote:

> I am trying to find information on the various types of oxygen sensors
> that are available, but haven't found much (so far) in the archives, though
> I'm sure that this discussion must've been beaten to death on this list.  Is
> there a quick link to a site that can give me some good info?  I'm still
> searching the archives though.

Take a look at this image:

ftp://ftp.diy-efi.org/incoming/nb_sensor.gif

Basically, all O2 sensors have a very sharp transition in their output
curve around a certain point.  For most applications, this is set by
design to 14.7 AFR.  So in the vast majority of cases, an O2 sensor is
really only good for indicating if you are a bit rich or lean of
stoich.  I would even say that aside from heated/non-heated and
dedicated ground/chassis ground, 99.9% of o2 sensors out there are
pretty much the same (someone please corrent me if you disagree.)

All O2 sensors have a continuous output, but the "pulsing" that you
mention comes about because of the closed loop nature of how ECMs work. 
The computer adds/subtracts a tiny amount of injector on-time to make
the mixture swing back and forth around stoich.  It knows this by
reading the output of the O2 sensor.  Note that closed loop mode is only
used under certain conditions: during idle and part throttle, and
assuming the O2 sensor has heated up.  Under acceleration enrichment or
engine braking (fuel cut) the O2 output is ignored, since in both cases
the desire is to run far from 14.7.  The O2 sensor must reach something
like 500 - 700dF before the electrochemistry works, and this happens by
the action of the exhaust gases.  But, some have a heating element (so
that it warms up faster and CL mode can be entered quickly.)

The wideband sensors use a trick of pumping ions to or away from the
junction.  The idea is to do whatever is necessary to maintain a ~450mV
stoich reading from the O2 sensor, and the amount of effort required to
do this indicates the actual AFR.

The easiest way to understand all this is just backprobe the O2 sensor
in your car and drive around.  Don't use a DMM, the output won't make
any sense, since it will be flipping back and forth from say 200mV -
800mV at around .5 to 4 Hz.  Either use a scope or build a simple LED
bargraph display with a lm3904.  Incidently, all the "Air Fuel Meters"
that you might see in the aftermarket catalog are just this, a glorified
voltage display.  If you have any electronics skills, just make one
yourself from a few dollars in parts.  And even then, realize that it's
only really useful for spotting trends and general troubleshooting,
forget any notion of getting a specific AFR number out of anything but a
WB sensor (and even then there are many variables like EGT that make
this less than straigtforward.)

Brian

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