The 25 ohms with the ignition switch turned on is not accurate because other sensors share that signal return line, and there will be about 0.2 volts on it. That's how the computer knows current is flowing in the circuit and it helps it determine which fault code to set. Otherwise the sensors could be grounded right where they're mounted.
It sounds like you've covered all the bases and logic has gone out the window. The ground wire appears to be ok. You found 5.0 volts supply, and the sensor measures a typical value. This is where the confusion sets in. Until I hear back from you, I'm going to guess you have a problem with the Engine Computer. Here's my logic. The only thing left is stretched pins in the sensor's electrical connector. That would cause an open circuit, you'd still have 5.0 volts on the signal wire, and the proper 0.2 volts on the ground wire. However, the sensor's resistance goes up as its temperature goes down. Normally an open circuit defaults to minus 40 degrees. A reading of 260 degrees suggests the signal voltage is dropping way too far. (Acceptable signal voltage range is 0.5 to 4.5 volts). You should see very low voltage on the signal wire if the sensor was shorted or the two pins in the connector were touching each other. Since you have 5.0 volts, the signal wire is not shorted to ground.
Ok, I'm having a brain burp. Something must be taking place in the computer I'm not aware of. What I mean is it is indicating 260 degrees as a default in recognition of a problem, not as an actual temperature. Back-probe the electrical connector to remeasure the sensor resistance through the pins. I think you're going to find an open circuit due to a stretched pin. There is another resistor in series with the sensor's signal wire. It lives in the computer and is what causes the voltage drop to occur on that line. The fact that 5.0 volts is found at the sensor when it is unplugged proves the entire circuit from the computer to the sensor is ok. The fact that no voltage drop occurs when the sensor is plugged in proves no current is flowing in the circuit due to a break. You proved the ground wire is ok and the sensor is ok. The pins have to be open or the sensor is opening up under load. That was real common in my years of fixing tvs, but it's highly unlikely for a simple temperature sensor.
I guess I'm getting tired. I'm starting to over-think this. If the circuit is not broken anywhere, current must flow through the sensor and voltage must be dropped across the resistor in the computer leaving you with less than 5.0 volts on the signal wire. If you still have 5.0 volts, unplug the connector and poke about a 4.7k resistor in that terminal and ground the other end. If the voltage still doesn't drop to a normal, ... Oh, ... Say 2 to 4 volts, there has to be a problem inside the computer. If the voltage does drop, there has to be something wrong with the sensor or the connector pins.
Saturday, May 29th, 2010 AT 3:14 AM