On the 2008-09 CTS and 2009 CTS-V, there may be confusion about the outside air temperature sensor and display operation and diagnosis.
When diagnosing a customer concern related to the outside (ambient) temperature sensor and display, review the information below.
General Information and Function
The outside air temperature (shown in the radio or navigation display) is read by a sensor that measures the air temperature just behind the grille at the hood latch bracket. (Fig. 5)
The sensor measures temperatures between -30°C and 51°C (-22°F and 123.8°F). The device is an NTC thermistor (a ceramic material that decreases in resistance with increasing temperature). The specifications of the sensor are very good; less than +/- 1°C (1.8°F) in very cold conditions and less than +/- 0.5°C (1°F) in warm to hot conditions. The sensors have a proven reliability record and almost never show issues when returned in service.
The main source of variation between actual outside temperature and the reading inside the vehicle is the variable airflow across the sensor at low vehicle speed and stopped conditions. The front end of a vehicle (at low speeds) is generally a very warm place when the vehicle is or has recently been running. This warmth would normally cause big differences between the real ambient temperature and the reading in the display. At higher speeds, when more fresh airflow is moving through the front end of the vehicle, a much more accurate indication of actual air temperature is provided.
To prevent large indicated temperature increases at low vehicle speeds, very slow mathematical averaging keeps the temperature from creeping up in stop and go traffic. When the vehicle is stopped (idling) or moving at very low speeds (less than 10mph/16kph) the sensor is ignored as it rises and the indicated display does not change (decreases in actual temperature are allowed). So in city traffic with stop-and-go conditions at lower speeds, the displayed temperature may never increase.
When a vehicle is shut down after running, the front end of the car tends to soak-up in temperature. The last good temperature indication at shut down is held for 3 hours rather than taking the raw value of the sensor on start up, if it is higher. If the raw temperature is lower, the raw value is used immediately (the sensor readings are almost never artificially low -- see the exception below). This 3 hour hold is an approximation. The actual air temperature can increase in 3 hours. After 3 hours, it is assumed the vehicle has cooled off enough to read the raw sensor value as the actual ambient temperature. When temperatures are dropping (driving into cold weather or as the sun sets), a much quicker mathematical averaging is used even at low vehicle speeds because artificially low readings are not common.
EXCEPTION: When the vehicle is parked in a cool environment for several hours (such as an underground parking facility), the initial temperature reading of the sensor is displayed on start up. As the vehicle is driven outside, the temperature may be significantly higher. The sensor has no way to know this and uses slow mathematical averaging at low vehicle speeds to bring the indicated temperature up. At higher speeds, the averaging is faster but not instantaneous.
Tech 2 Update for Proper Diagnostics
For the Cadillac CTS, the Tech 2 shows two indications of outside air temperature -- a filtered value and a raw value. Initially on the Tech 2, the raw value was shown incorrect when the sensor was open or shorted. It incorrectly showed the last known or default value as used in the HVAC automatic climate control calibration instead of the actual raw reading. This was corrected with Tech 2 software update version 29.003 in March 2009 and the raw value now shows the sensor output from 0.0 to 5.0 volts. An unplugged sensor reads 5.0 volts and a shorted sensor reads 0.0 volts. This should help with diagnosing wiring or sensor issues.
In service, using the vehicle display to diagnose problems with the outside air temperature sensor is not advised. The display is not updated when the vehicle is stopped, so holding or warming the sensor will not affect the vehicle display at idle. Unplugging the sensor will blank the display (even at idle) and set DTC B0158. If a sensor is then plugged back in, the display will update to the current sensor actual temperature. This has lead to several misdiagnosed sensor replacements, assuming that the new sensor gives a better reading than the old sensor. Use the Tech 2 with the latest software update to diagnose concerns with the sensor reading.
Instantaneous Update Feature for Diagnostics or Customer Usage
Since its launch in July 2007, the Cadillac CTS HVAC software has a provision for an instantaneous update of the outside air temperature display to the actual sensor reading. Pressing and holding the A/C (snowflake) button and the adjacent Recirculation button simultaneously for about 3 seconds will update the display to the current unfiltered sensor reading. (Fig. 6)
This can be used for diagnostics as well as by a customer who parks in an underground garage on a regular basis before heading out into warmer weather. After the display is updated, the same filters are used (at the new temperature) to help make the display accurate and stable.
- Thanks to Michael Ciarkowski
Leave a comment