NTC thermistor sensors

[Brian Dessent]

Does anyone know the usual way in which ECMs measure the resistance of
the NTC thermistors?  The most straightforward methods I can think of
would be to apply a constant current [or voltage] and measure the
resulting voltage [or current].  It seems that the constant-current
method would expand the high end of the range and compress the low end,
and the opposite for constant-voltage.  The usual thermistor varies from
about 50 ohms to 50k ohms exponentially, right?  I guess it depends on
whether you are interested in the low-end range of temps or the
high-end.

Anyway, does anyone have any experience on this issue from
reverse-engineering ECMs?  Also, is there a "standard thermistor" curve
such as with thermocouples?  What I would like to do is piggy-back some
of the sensors currently in use by the stock ECM -- make their signals
available for datalogging or dash-display while not affecting the
readings that the computer sees.  If the ECM supplies a test current and
measures resulting voltage, then this should be rather easy with an
op-amp buffer.  If the ECM applies a test voltage and measures current,
then a current mirror would seem appropriate.  However, if the test
current varies (to aid linearity or dynamic range) then it would seem
that a full "resistance mirror" op-amp circuit might be necessary.  Any
advice?

Brian
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