RAM 1500 Truck 4WD V8-5.9L VIN Z (2002)
Ambient Temperature Sensor / Switch HVAC: Description and Operation
AMBIENT TEMPERATURE SENSOR
The ambient air temperature is monitored by the Front Control Module (FCM) and displayed by the Compass/mini-trip Computer
/Electronic Vehicle Information Center (CMTC/EVIC). The FCM receives a hardwire input from the Ambient Temperature Sensor (ATS).
The ATS is a variable resistor that operates on a 5-volt reference signal circuit hardwired from the FCM. The resistance in the ATS changes as the
outside temperature rises or falls. The FCM senses the change in reference voltage through the ATS resistor. Based on the resistance of the ATS, the
FCM is programmed to correspond to a specific temperature. The FCM stores and filters the ambient temperature data and transmits this data to the
CMTC/EVIC via the Programmable Communication Interface (Vehicle Communication Bus) (PCI) Bus. The ATS cannot be adjusted or repaired and, if
faulty or damaged, it must be replaced.
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 FCM, and the
CMTC/EVIC display.
If the CMTC/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 CMTC/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 DTC's will be recorded in the FCM. The ATS can be diagnosed using the Sensor Test.
Test the ATS circuits using the diagnostics at Body Control System. 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.
