Dual probe ECT sensor on an Australian Ford Falcon?

I'm adding an aftermarket (non OBD) trip computer - called a "UTCOMP" to an Australian Ford Falcon. Specifically, it's a 2001 "AU" series Falcon with the Ford EEC-V computer.

I am tapping the feed from the ECT sensor back to the ECU and had calibrated my voltage-to-temperature curve in the UTCOMP software. However all of a sudden at about 68-70 degrees C, the ECT sensor in this car appears to switch to a bias voltage, causing the voltage to jump by about 3.1 volts.

I show a video here of the readings from the UTCOMP programming software showing the sensor cooling down (Key On, Engine Off), and as it drops down to about 68C (or about 4.2 volts) it suddenly drops down to 1.12 volts. At this point my sensor calibration goes from showing the incorrect temperature (about -1C) to the correct temperature (about 70C).

UPDATE: I've worked out what's happening here. See my final post.

[strike]It's almost like it uses a bias voltage, but only sometimes. The ECT sensor is only dual-wire so whatever it is would have to be an internal circuit in the sensor itself, as the 5v reference is shared with a bunch of other sensors including the TPS.
[/strike]


Is this a "thing" on Fords? I've never seen a dual-probe ECT sensor before.

Here are some screenshots from ForScan on my Android phone showing what the ECU sees:

I think I've worked out what's going on here. As this is a two-wire sensor it's using a voltage divider. For some reason, the voltage reference sent from the ECU through the permanent resistor inside the ECU is increased once the temperature (or more to the point the voltage reading) hits a certain value. Or maybe they have two different permanent resistors in circuit and the flip between them so that they can be monitoring integers of voltage rather than fractions?

Again I've never seen this before but I suppose this is what Ford does for some bizarre reason. It does mean I can't tap the signal in this manner though and if I want to separately monitor coolant temps on this vehicle I will need a separate sensor.

@1 min, 21 seconds