Ambient Temp Sensor Description; Operation; Diagnosis And Testing Diagnosis And Testing - Ambient Temperature Sensor; Diagnosis And Testing - Ambient Temperature Sensor Circuit - Dodge RAM 1500 1998 Owner's Manual

8M - 10 MESSAGE SYSTEMS
ELECTRONIC VEHICLE INFO CENTER (Continued)
(2) Install the screws holding the EVIC module in
the overhead console.
(3) Connect the EVIC module electrical connector.
(4) Install the overhead console on the headlin-
er(Refer to 8 - ELECTRICAL/OVERHEAD CON-
SOLE - INSTALLATION).
(5) Connect the battery negative cable.
(6) Check EVIC module function.
NOTE: If a new EVIC module has been installed, the
compass will have to be calibrated and the variance
set. Refer to Compass Variation Adjustment and
Compass Calibration in the Standard Procedures
section of this group for the procedures.
AMBIENT TEMP SENSOR
DESCRIPTION
Ambient air temperature is monitored by the over-
head console. The ambient temperature messages are
received from the Front Control Module (FCM) over
the Programmable Communications Interface (PCI)
J1850 data bus circuit. The FCM receives a hard
wired input from the ambient temperature sensor
(Fig. 7). The ambient temperature sensor is a vari-
able resistor mounted to the underside of the hood,
in the engine compartment.
Fig. 7 Ambient Temperature Sensor - Typical
For more information on the front control module,
refer to Front Control Module in the Electronic
Control Modules section of this manual. For complete
circuit diagrams, refer to Wiring. The ambient tem-
perature sensor cannot be adjusted or repaired and,
if faulty or damaged, it must be replaced.

OPERATION

The ambient temperature sensor is a variable
resistor that operates on a five-volt reference signal
sent to it by the Front Control Module. The resis-
tance in the sensor changes as temperature changes,
changing the temperature sensor signal circuit volt-
age to the Front Control Module. Based upon the
resistance in the sensor, the Front Control Module
senses a specific voltage on the temperature sensor
signal circuit, which it is programmed to correspond
to a specific temperature. The Front Control Module
then sends the proper ambient temperature mes-
sages to the EVIC, CMTC over the PCI J1850 data
bus.
The temperature function is supported by the
ambient temperature sensor, a wiring circuit, the
Front Control Module, the Programmable Communi-
cations Interface (PCI) data bus, and a portion of the
Electronics module. If any portion of the ambient
temperature sensor circuit fails, the Front Control
Module will self-diagnose the circuit.
For complete circuit diagrams, refer to Wiring.
DIAGNOSIS AND TESTING
DIAGNOSIS AND TESTING - AMBIENT
TEMPERATURE SENSOR
(1) Turn the ignition switch to the Off position.
Disconnect and isolate the battery negative cable.
Disconnect the ambient temperature sensor wire har-
ness connector.
(2) Measure the resistance of the ambient temper-
ature sensor. At 24° C (75° F), the sensor resistance
should be approximately 10.3 kilohms. At 30° C (86°
F), the sensor resistance should be approximately
7.57 kilohms. The sensor resistance should decrease
as the temperature rises. If OK, refer to Diagnosis
and Testing - Ambient Temperature Sensor Cir-
cuit in this group. If not OK, replace the faulty
ambient temperature sensor.
NOTE: The ambient temperature sensor is a very
sensitive device. When testing, be certain the tem-
perature sensor has had time to stabilize (room
temperature) before attempting to read the sensor
resistance. Failure to let the ambient temperature
sensor temperature stabilize could result in a mis-
leading test.
DIAGNOSIS AND TESTING - AMBIENT
TEMPERATURE SENSOR CIRCUIT
(1) Turn the ignition switch to the Off position.
Disconnect and isolate the battery negative cable.
Disconnect the ambient temperature sensor wire har-
ness connector and the Front Control Module wire
harness connector.
(2) Connect a jumper wire between the two termi-
nals of the ambient temperature sensor wire harness
connector.
(3) Check for continuity between the sensor return
circuit and the ambient temperature sensor signal
circuit cavities of the Front Control Module wire har-
DR