Using Arduino to monitor engine sensors

[Quinten]

I'd like to use an Arduino to monitor my coolant temperature sensor and from what I've read, the ECU in my car (MBE 956E) uses a 5V reference through the sensor and simply notes the rise/drop in voltage to determine the temperature.  Can I just add a wire to the sensor where it "exits" the sensor, so that one goes back to the ECU, and the other to an analog pin on the Arduino?  Or is it a little more complicated than that?

[Kit Car Electronics]

Yes, it's as easy as that (provided you use a 5V Arduino). If the MBE uses a common signal ground for its sensors, then it will have a fixed internal pull-up resistor (to 5V) for the coolant sensor, so that the sensor creates a potential divider circuit. So Open circuit will read 5V, short circuit 0V, but all other temperatures read somewhere between these values. Join your Arduino ground to the MBE signal ground circuit and you're good to go.

The Arduino input impedance is very high (like 100MOhm) so you shouldn't affect the voltage reading that the MBE gets by sharing the pin.

[Quinten]

Brilliant, that's exactly what I needed to hear