NTC thermistor sensors

[rr]

GM ECMs use a two resistor voltage divider that is read with
an ADC. One resistor to +5 v, the NTC resistor being the other
to ground. The lower the NTC resistance, the lower the voltage.

BobR.


Brian Dessent wrote:

> 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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