[V8] O2 Sensor Readings
cobram at juno.com
cobram at juno.com
Thu Jul 5 22:27:51 EDT 2007
I don't know if there is a discernable difference in the signal with or
sans the meter. The O2 sensor output will tell you there is a problem
somewhere in the fuel system long before the car throws a code, or the
problem kills the O2 sensor. Helpful on pre-OBD cars. No BTDT on a V8Q,
but I know a few guys who've been able to narrow down fuelish problems on
XJS's that were driving them crazy...but then again, they were already
half way down that road since they own a Jag. ;-)
We come into the world naked, screaming and covered in blood. Why should
the fun end there?
"urq" <urq at pacbell.net> writes:
> I don't see any mention of the input impedance of the unit ... so I'd
> caution. As was correctly pointed out earlier, the output impedance
> of the
> OXS is very high, and connecting a low impedance meter will change
> Over the years I've realized that there is very little value to be
> from monitoring the voltage output of the OXS. If you suspect it
> malfunctioning, make sure you've eliminated all other possible
> culprits, and
> then simply swap the OXS out for a new one. The ECU varies the
> mixture all
> the time to validate stoichiometric, so any sort of messing with the
> FPR or
> adding HC to the intake just becomes a variable the ECU deals with
> as it
> drives the mixture to stoich as best it can.
> The best tool for figuring out what is wrong with the engine is to
> VAGCOM or equivalent. Unfortunately it appears that ProDiag has
> died ... so
> unless you already have one it is no longer an option. With ProDiag
> I can
> read out and clear the ECU codes, and I can take a test drive and
> log the 10
> parameters the ECU provides. I suspect VAGCOM has the same
> capability. You
> can tell right away if the ECU has decided that it cannot control
> mixture effectively (generates a code) or if it is responding to an
> erroneous input (like an incorrect coolant temp).
> Steve B
> San José, CA (USA)
> -----Original Message-----
> This is exactly the tool you need - short of an oscilloscope.
> are also volt meters with similar functions (bar graphs) built in,
> they will set you back a good $300 for a nice Fluke or equivalent.
> cheapo meters are not worth much in this application.
> > Here's one that you can mount inside the car.
> It fits with 3 wires
> > (+12v, signal wire on O2 sensor, and
> > http://tinyurl.com/39bfnd
> > MikeL's V8Q was going through O2 sensors, the temp sensor was
> > culprit.
> > He wrote to the u.washington
> > "It's easy to test. let car sit
> overnight, then measure the resistance
> > between the 2 pins on the
> sensor. Should be between 1.5 to 3 ohms.
> > Mine was at
> 4.5 and it made the car run so rich that you got a huge black
> cloud when you stepped on it. As well as a pile of soot under the
> > tailpipe
> > if you let it idle for 5 minutes.....
> > BTW, This is what ruined my 02 sensor :-("
> > "d saad"
> <dsaad at icehouse.net> writes:
> >> I am not sure you can deduce much at all from this Scott.
> >> The
> >> O2 sensor is not easy to read.
> To do it right, you really need an
> >> oscilloscope.
> >> A nice digital volt meter with a calibrated bar graph
> >> would also work OK - as long as the response time was fast
> >> The
> >> problem is that the sensor is a
> very high impedance device - so
> >> just
> >> hooking
> up your meter affects the output. And the output is not a
> >> state voltage - unless you are at extreme rich or
> lean condition. It
> >> is a
> >> non-linear device with
> the usable range being a very narrow band
> >> around a
> >> air/fuel mixture of 14.7. For most practical purposes, the
> >> can only
> >> tell you if you are above or
> below this number.
> >> If you look at the
> >> signal
> with a o-scope, you see something like a square wave - with
> >> duty cycle of the waveform indicating the mixture. A
> volt meter can
> >> give
> >> you a rough idea of the
> duty cycle - because it averages the
> >> readings, but
> >> it is a very rough indication.
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