I've been cruising around the net trying to find some info. No solid leads, but here's the closest I've found.
Reviewing our thermistor, Alldata says:
68F = 2.3 to 2.7K ohms
175F = 280-360 ohms
Sample from a web site for an ACI/2252 NTC thermistor
68F=2.8K ohms
175F=283 ohms
200F=177 ohms
230F=115 ohms
260F=77 ohms
300F=42 ohms
So I think this is a close enough curve given the general non-linear nature of thermistors. On to my next step.
Several sites suggest a whetstone bridge can be modified to employ a fixed resistor where the normally variable unit is used. That way when the temperature rises to some point defined by the fixed resistors, the bridge will become sufficiently unbalanced to allow enough current to flow to trigger a relay. This site specifically discusses using the modified bridge as a temp alarm
http://www.atpsensor.com/ntc/ntc_app...ions_main.html
I'm assuming at lower temps the resistence is high enough to limit current flow, also inhibiting the relay although the bridge is still unbalanced. This is what I now think the TCU does. I'm not an engineer, anybody out there care to comment on if this makes sense so far?
I was originally thinking of simply putting a voltmeter across the lines into the TCU and sensing temp that way even though I knew it would be inverse and not linear. Now I wonder if I would have to buffer somehow to avoid screwing up the bridge.
If I'm right (??), a more adventurous soul than me could open the TCU and change the bridge resistor values to get the light to come on at a lower temp (something I've seen asked for in earlier threads) without screwing up any other parameters of system operations.
I'd still like to employ the built-in sensor for some unfathomable reason vice just buying the kit thing from Summit or ???? so now soliciting info AND circuit ideas.