Thermistor circuits are designed to pull the source voltage to ground, that is the signal.
So on a 2 wire thermistor, one wire is ground, that's the one that will be measured as 0v connected or disconnected.
The other wire is the signal wire, it is a 5v feed from from the computer. With the thermistor disconnected, this wire will measure 5v and scan data generally will display -40°f.
As the thermistor heats up, it gradually "connects" the signal wire to ground, dropping that voltage to a level that corresponds to the temperature of the thermistor.
Since nothing is ever actually -40°f (on a car) you should never measure 5v on the signal wire of a thermistor that is connected and working.
If you do measure 5v on the signal wire then:
1) the connector is unplugged
2) the ground circuit is bad, so even a good thermistor cannot pull the signal voltage down below 5v
3) the thermistor is bad (internally open) and not pulling the signal voltage below 5v.
And remember, on any 5v signal circuit you will virtually never measure a full 5v or full 0v. This would indicate an open circuit (5v all the time) or a short to ground (0v all the time).
That's why it can be said that with the thermistor connected, a measurement 5v on the signal wire and ground on the ground wire is a bad thermistor every time.
Noah, sorry for replying so late, but I wanted to thank you.
It seems that I needed to see a written summary of thermistor circuit operation. Somehow, it just clicked once I read your post.
I now see why I was confused on SD saying that 0v after the thermistor would indicate an open.
I thought that you were supposed to see voltage after the thermistor, now I know it's supposed to be pulled to ground all the time on a working circuit.
Hence, why an open ground (no grounding) would cause anyone to see the 5v AFTER the sensor with the connector pluged in. There's nothing to pull the V down.
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