DMM and O2 sensor

[Tom Cloud]

>
>Ok, this may be a bonehead ? but here goes:
>
>When measuring the voltage from an O2 sensor with a digital multimeter, does
>it give the true reading or a slightly lower voltage.  The reason I'm asking
>is that the service manual says that the reading should be .45 V but "may
>read as low as .32 V on a digital voltmeter"
>
>Ed Hilker aka "Chief"

can't imagine what that means.  Typical DVM has 10 megohms
input impedance -- ten times more (or better) than is recommended,
so there should be no loading (i.e. the voltage should not read
lower).

If you're reading the O2 sensor while the system is under the
control of a modern efi, the voltage will oscillate back and
forth either side of stoich (which is about .5 volts).  That's
probably why they say the reading you get may be off.  But,
if you're not using an oem efi, I don't think the mixture
will be varied by the ecu (maybe some do, doubt it, that's
just to make the 3-way cat happy).

Tom Cloud <cloud at peaches.ph.utexas.edu>