NTC thermistor sensors

[Peter Gargano]

Brian Dessent wrote:
> 
> ... does anyone have any experience on this issue from
> reverse-engineering ECMs? 

Ludis has schematics for GM ECMs inputs - here's the 1227165's (or the
Australian 1227808's) input circuitry:

  http://www.cruzers.com/~ludis/1227165sheet2.gif

Look near the centre of the schematic - to the right and down a bit.

Note that for the coolant temp CTS# signal, the ECM controls CTSHI
so that either a 348 ohm resistor, or a 348 + 3650 ohm resistor (total
= 3998 ohms) is in series with the sensor. The ECM then uses two tables,
for each of the series resistors, to look up the calculated temperature
from the A/D value read at CTS (CTS goes to an A/D converter).

This scheme allows greater accuracy at each end of the sensor's resistance
range with the 8 bits of A/D accuracy use on that ECM. Note that more
A/D bits of accuracy may not solve the problem when using a single series
resistor, as noise then may become a factor at each end of the resistance
range.

Peter.
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