RAM 1500 Truck 2WD V6-3.9L VIN X (1999)
Ambient Temperature Sensor / Switch HVAC: Testing and Inspection
The thermometer function is supported by the ambient temperature sensor, a wiring circuit, and a portion of the compass mini-trip computer module. If
any portion of the ambient temperature sensor circuit fails, the compass/thermometer display function will self-diagnose the circuit. An "SC" (short
circuit) will appear in the display in place of the temperature, when the sensor is exposed to temperatures above 55° C (131° F), or if the sensor circuit is
shorted. An "OC" (open circuit) will appear in the display in place of the temperature, when the sensor is exposed to temperatures below -40° C (-40° F),
or if the sensor circuit is open.
The ambient temperature sensor circuit can also be diagnosed using the following Sensor Test, and Sensor Circuit Test. If the temperature sensor and
circuit are confirmed to be OK, but the temperature display is inoperative or incorrect, refer to Compass Mini-Trip Computer in the Diagnosis and
Testing.
Sensor Test
1. Turn the ignition switch to the Off position. Disconnect and isolate the battery negative cable. Disconnect the ambient temperature sensor wire
harness connector.
2. Measure the resistance of the ambient temperature sensor. At -40° C (-40° F), the sensor resistance is 336 K Ohms. At 55° C (131° F), the sensor
resistance is 2.488 K Ohms. The sensor resistance should read between these two values. If OK, refer to Sensor Circuit Test in the Diagnosis and
Testing. If not OK, replace the faulty ambient temperature sensor.
Sensor Circuit Test
1. Turn the ignition switch to the Off position. Disconnect and isolate the battery negative cable. Disconnect the ambient temperature sensor wire
harness connector and the overhead console wire harness connector.
2. Connect a jumper wire between the two terminals in the body half 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 roof wire harness overhead
console connector. There should be continuity. If OK, go to Step 4. If not OK, repair the open sensor return circuit or ambient temperature sensor
signal circuit to the ambient temperature sensor as required.
4. Remove the jumper wire from the body half of the ambient temperature sensor wire harness connector. Check for continuity between the sensor
return circuit cavity of the roof wire harness overhead console connector and a good ground. There should be no continuity. If OK, go to Step 5. If
not OK, repair the shorted sensor return circuit as required.
5. Check for continuity between the ambient temperature sensor signal circuit cavity of the roof wire harness overhead console connector and a good
ground. There should be no continuity. If OK, refer to Compass Mini-Trip Computer in the Diagnosis and Testing. If not OK, repair the shorted
ambient temperature sensor signal circuit as required.
