Ambient Temperature Sensor / Switch HVAC: Description and Operation

Fig.12 Ambient Temperature Sensor:

AMBIENT TEMPERATURE SENSOR
Ambient air temperature is monitored by the Compass/Mini-Trip Computer (CMTC) through ambient temperature sensor messages received from the Body Control Module (BCM) over the Programmable Communications Interface (PCI) data bus network. The BCM receives a hard wired input from the ambient temperature sensor. The ambient temperature sensor is a variable resistor mounted in front the radiator, behind the grille, near the center of the vehicle.

The Ambient Temperature Sensor (ATS) is a variable resistor that operates on a 5 volt reference signal sent to it by the BCM. The resistance in the sensor changes as temperature changes, changing the temperature sensor signal circuit voltage to the BCM. Based upon the resistance in the sensor, the BCM senses a specific voltage on the temperature sensor signal circuit, which it is programmed to correspond to a specific temperature. The BCM then sends the proper ambient temperature messages to the CMTC over the PCI data bus.

The thermometer function is supported by the ambient temperature sensor, a wiring circuit, the Body Control Module (BCM), the Programmable Communications Interface (PCI) data bus, and a portion of the Compass/Mini-Trip Computer module.

The ambient temperature sensor circuit can also be diagnosed by referring to Diagnosis and Testing - Ambient Temperature Sensor, and Diagnosis and Testing - Ambient Temperature Sensor Circuit. If the temperature sensor and circuit are confirmed to be OK, but the temperature display is inoperative or incorrect, refer to Diagnosis and Testing - Compass/Mini-Trip Computer. For complete circuit diagrams, refer to the appropriate wiring information.


Refer to Body Control System.
Body Control Module

Refer to Electronic Vehicle Information Center (EVIC).
Driver/Vehicle Information Display

Refer to The Programmable Communication Interface Multiplex System (PCI Bus).
Information Bus


Ambient Temperature Sensor Fault Codes
The outside temperature function is supported by the ambient temperature sensor (ATS), a signal and ground circuit hardwired to the BCM, and the EVIC display.

If the EVIC display indicates 54 °C (130 °F) or the ATS sense circuit is shorted to ground, the temp display will be 54 °C (130 °F) to indicate a SHORT circuit condition.

If the EVIC display indicates -40 °C (- 40 °F) or the ATS sense circuit is open, the temp display will be -40 °C (- 40 °F) to indicate an OPEN circuit condition.

If there is an OPEN or SHORT circuit condition, it must be repaired before the CMTC/EVIC VFD can be tested.

The ATS is supported by the FCM. Ambient Temperature Sensor DTCs will be recorded in the FCM. The ATS can be diagnosed using the following Sensor Test. Test the ATS circuits using the diagnostics in the Body Diagnostic Procedures. If the EVIC passes the self test, and the ATS, the circuits, and PCI bus communications are confirmed to be OK, but the EVIC temperature display is inoperative or incorrect, replace the FCM.