Background: automotive application, '79 oldtimer with a Cylinder Head Temperature (CHT) sensor (a 2-wire NTC thermistor) that is fed into an analogue electronic control unit (ECU). The sensor output is only visible to the ECU and used for fuel injection management. Also, I don't have visibility of the ECU circuit that conditions the thermistor signal (the ECU has 3 proprietary hybrid ICs with no datasheet available).
Some more notes:
- One end of the thermistor is grounded, the other feeds into the ECU.
- I've got +12 V and enough current available for additional circuitry, though.
I would like to monitor and display the engine temperature with a vintage analogue gauge. The obvious option is to install a second CHT sensor. Yet mounting an additional sensor on the engine case is non-trivial, and given that there is already a sensor, why not reuse its output?
I'm looking for a circuit that would allow me to tap into or split the thermistor output so that I can read the resulting additional output with the gauge. I would worry about matching the sensor output to the gauge later, but is this something that could be achieved without upsetting the ECU temperature reading?
Update: just for the background's sake, while the ECU schematics are not available, there is circuit diagram of the next generation of that particular ECU online: 
In that picture, I've marked the engine temperature sensing thermistor in red. The schematics, without being exact are probably close enough to get an idea of the circuitry behind the sensor that I'd like to tap into.
The BS10, BS20 and BS30 are the more modern DIP versions of the proprietary hybrid ICs in my ECU, while the rest of ICs are LM2902 quad op-amps.
Best Answer
Typically a thermistor is biased with a low constant current supply and the resulting voltage across it is measured to determine the temperature. If you select an op amp or instrumentation amp with sufficiently high input impedance (e.g. > 1 megohm) you can use this to measure the voltage across the thermistor without disturbing the incumbent ECU electronics. Due to the high input impedance, you should consider incorporating a low pass filter to reduce noise pick up.
The new op amp circuit can then drive an analog voltmeter that can be calibrated with a temperature scale. You can determine the op amp gain, offset, and scaling factor empirically if you do not have access to the NTC specs and knowledge of its bias current.