Heh, I was thinking of that - the multimeter doesn't affect it - I can go to the analog pin of the MCU being monitored, and the - (gnd) of the sensor, and get a reading - it doesn't appear to drop it at all.
It appears the temperature sensor on the circuit being monitored has a diode on the grnd (looks like a shottky so I think -- lightning protection?)
And the positive is a 10k resistor.
Is the analog inputs really in the megaohms of resistance though?
How can I make my connection high impedance ? Does putting a 1Mohm resistor at the connecting point accomplish this (i.e. stops it drawing current?)
If I connect, I'll be loading down the temperature circuit (if it's wrong and the impedance isn't greater in the duino), I think that'll just cause it to think the sensor is 'hotter' than it is, until the current draw stops (as current increases, voltage decreases, so the temperature reading to the controller would be 'hotter').