Toehead

Here is the problem:

WinALDL reports my MAT sensor reading 197.6 degrees. This is with the engine not running. All other sensors read correctly. I traced the wires back to the ECM. The black wire should be ground, correct?
The orange wire has continuity all the way back to the ECM.
what are the other options?

2_point8_boy

The black wire should be ground, correct. The tan wire should go back to the ECM. You should have near 0 OHMs on the tan wire and 0 ohms on the black wire.

What you should really do is a voltage drop on all the wiring for it. Measure from the MAT marness connector to the pin on the ECM, you'll need paperclips to backprobe it and you'll have to do it with the key on. just shove the paperclip into the back of the harness connector at the pin and measure VOLTAGE from those 2 points. You should have VERY NEAR 0 volts. Do the same on the ground wire, but this time from the MAT harness connector to a good clean ground. YOu should have near zero here too. This is a much more accurate way to see if you have an electrical problem in the circuit than reading the resistance in OHMS because of the way circuits work.

If all that's OK, make sure you have 5 volts coming down the line on the tan wire. Disconnect it at the MAT and measure the voltage between the tan pin and ground(or the black pin). Should be darn near 5 volts(+/- 0.75v).

If all that stuff is ok, then you'll have to replace the MAT sensor.

I bet the problem is actually in the MAT sensor itself, but I gave you all the proper instructions to diagnose it because it's something you should know generally because it applies to all the circuits on the vehicle. The only thing that changes are the "reference voltages." Some are 5 and others are 12.

Hope this helps you out some.

Toehead

Thank you very much. The reason i was thingking it was the wiring itself is because when I unplug the sensor, the temperature reading doesnt change. I also stuck a small resistor or arbitrary value between the orange wire and ground to see if the temperature reading changed, and it didnt.
Do you happen to know if the MAT resistance is higher or lower with increased temperature?

2_point8_boy

It's a Negative Temperature Coefficient Sensor, which means the resistance and signal voltage back to the ECM go down as the temperature goes up.

Toehead

Thanks man. Once again, 2.8 boy to the recue haha. I'll go check that right now. It could just be a corroded wire.

2_point8_boy

Yeah, there's got to be an open or very high resistance somewhere because that's the temp reading when the computer is reading close to a 5V drop across the sensor. if it was shoted, it'd read something like -42*F or something crazy like that.

Toehead

The MAT sensor is reading infinite resistance... Although sometimes i can get a reading out of it, which is what happened the first time i check it. I think that the contacts inside the sensor plug are corroded.
There is no voltage drop (very small) along either the ground or reference wire.
looks like we tracked it down. Tomorrow i will try cleaning the wire, and if that doesnt work, i will replace the sensor.

Toehead

Before I buy this sensor, is there any steps i can take to make sure it's not the ECM? For instance.. any specific resistors i can use to get a specific temperature. I already tried a short, but the temperature didnt change.

2_point8_boy

Check the voltage drop across the sensor. Backprobe the 2 pins with KOEO and measure the voltage. If it's 5V(or whatever your reference voltage is) it's a bad sensor.

That's why I don't recommend using live data for diagnostics unless you have an OBD-II system because once a fault is detected, the ECM doesn't display live data for that sensor anymore.