The ECU measures this resistance using a voltage divider,
with one "leg" being the IAT and the other "leg" being
a known resistance. Let the IAT sensor resistance be Rt
and the ECU resistor be Ru, with Vt being the voltage
output by the divider.
The TDS-1 uses an empirically-derived value for the value
of the ECU resistor (Ru) of 2340 Ohms. Given
this value, the TDS-1 can measure the voltage present on
the IAT sensor signal (Vt) and convert that to a temperature,
just as the ECU does. The TDS-1 does not interfere with
the ECU's processing of this signal (it uses a "high impedance"
connection).
Unfortunately, due to the nature of positive temperature coefficient thermistors, the voltage changes a very small amount per degree of temperature difference. Because of this, the temperature measurement is not highly accurate. However, for the purposes of monitoring intercooler heat soak, and turbocharger efficiency, this accuracy is sufficient.