My friend has a 2000 SSEI with the dual zone climate control and no matter what the outside temp is the climate control reads 140*F. He unplugged the sensor and it went down to 50*F and then climbed up to 140*F again in about 10 minutes and has stayed there ever since. ANY IDEAS???

Thanks,
Daniel

Please tell us the location of the sensor that was unplugged. When any sensor is unplugged, a default value or error should occur. This one showing 50 then back to 140 is very odd, unless it moved and shorted out.

I agree. If the sensor was disconnected, the reading should stay stable at the default value. If it still varies, that would indicate a problem in the circuitry instead of the sensor.

Unless the connector was able to ground out. That could account for it faulting again.

The sensor is a negative temperature coefficient thermistor. That means as the temperature goes up, the resistance goes down. The system can measure between -40 to 300*F. If it had a direct short, I'd expect to see the 300* temperature. If it'* a resistive short (corrosion, worn through wire to ground, etc.) those are generally not a stable resistance and the temp. should vary up and down.

Measure across the connector - it should read a steady 5 volts. Then measure the resistance across the sensor. It should also eventually reach a stable resistance. Then hold it in your hand, and the resistance should go down, again reaching a stable resistance.

The system also uses other sensors before a stable true reading will be displayed. The instrument panel module reads the resistance, and then change the reading to pulses that are then converted to temperature. If it senses an error, it will go to a default reading, which is likely somewhere in the middle of the temperature spread so that control over the temperature can still be maintained. A scanner that can read body control codes should be able to get any codes for improper operation.

The system also checks the coolant temperature, and vehicle speed. If the coolant temperature is less than about 20 degrees above the temperature that the ambient air temperature sensor sees, and the car has not been started for 2 hours, it displays real-time temperature readings. If it is above about 20 degrees, it displays the last temperature that was stored until you're going faster than 10 miles an hour, and after a 5 minute delay.

Using the information listed above, may be able to help lead you to the problem.

Sorry about taking sooooo long. I appreciate all of your help! The sensor that he unplugged was in the front by the grille. The longer he unplugs it the lower the temp (to 50*) then he plugs it back in an it slowly climbs to 140*.
Make sense to anybody??

Thanks,
Daniel

Daniel, please reread Archon'* post above.

What you describe doesn't make sense knowing the sensor should fault either high or low if it'* unplugged or fully grounded.

I know that'* what is making me think that it is the actual climate control unit rather than the sensor. I had a local mechanic look at it and he had no clue (Bonnevilles are a breed all to themselves LOL). I did reread the post and it makes sense but the problem that he is having really doesn't make sense with that in mind.

--Daniel

Did you do any of the measurements that I listed?

There'* more testing that can be done. Do you have a multimeter?