On Jan 25, 12:51pm, James A. TenCate wrote:
> Subject: Air/fuel mix meter update 1
>
> Just got through checking out the Cyberdyne air/fuel mix meter. It
> works just fine. The bar graph has 10 lights. 100 mV lights the
> first light, 200 mV lights the second light, etc. I put 300 mV
> on the meter and got three lights. The circuit drew less than
> 0.1 microamps so it probably won't load down the oxygen sensor.
I was crafting this when I got Jim's response to his own posting:
The National LM3914 is a "bar graph display driver," essentially 10
comparators that allow an input voltage level to light up a bar graph. If
done right, this little thingy could work quite well. It has a relatively
high input impedance (~100 Megohms) and can be made to respond nicely to a 0
to 1V input signal.
> Now I have to check my O2 sensors (which I suspect are bad). Wonder what
> caused that?
(I'm assuming that your TR8's O2 sensors are zirconia, not titania. Titania
changes _resistance_ with oxygen level changes, zirconia acts like a battery
with a variable voltage output with oxygen level changes.)
First of all be *sure* that the sensors are hot enough - a zirconia sensor's
output is meaningless for exhaust temperatures less than 600 degrees F.
(Realize that's hot enough to melt lead - I'm talkin' *really* hot here!)
Next, if you are getting no output, I'd suspect bad connections or other
mechanical damage. As a zirconia O2 sensor gets dirty, the minimum and
maximum output voltages get closer together, but the stoichoimetric point
remains at about 500 mV. Also, the sensor tends to respond slower as it gets
dirty.
Did you measure the output of the sensor with it unplugged from the ECM?
Remember, if there really was no output from your O2 sensor, your TR8 would be
running way rich as the ECM tried to raise the output of the sensor by
richening the mixture all the way.
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
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