OX sensor confusion

["Peter Orban" <peter@amtsg6.ime.nrc.ca>]

I have noticed that quite a bit of misconception is going around how to measure
the output from the ox sensor and how to find the a/f ratio from it.

It is correct that you should have a high impedance meter, at least 10 MOhm,
but a DMM would not give any meaningful reading.
When the engine is working in the closed loop, the output of the ox sensor is
oscillating between 0 and about 1 Volt. The time period is about 100 msec.
Digital multimeters usually use an integrating type of A/D converter, with a
conversion time way longer than 100 msec. Just connect your DMM to the battery,
and see how long it takes to settle the reading! Mine takes from about half to
a second. What a DMM would show when connected a pulsating signal depends on
the shape of the signal, the amplitude of the signal, and on the conversion
method. The point is that you really do not know what you are measuring!

Using a digital scope certainly solves the problem, as someone suggested. You
can also build a simple circuit from from a National LM 3914 (if memory serves
me well) display diver and LEDs, like the voltmeter on the '87 4kq, except a
bit faster. That circuit is floating somewhere on the net.
Also, Test and Measurement World magazine had a one page article some time ago
on diagnosing ox sensors and systems with the new LCD type digital scopes.

Hope it helps, Peter

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Peter E. Orban
National Research Council of Canada
e-mail: peter.orban@nrc.ca