DTC 04

CIRCUIT DESCRIPTION

The outside air temperature sensor (2) is a thermistor that provides a signal voltage to the control assembly microprocessor (15).

The microprocessor (15) supplies a reference voltage (about 5 volts) on CKT 735 to the sensor. When the outside air temperature is cold, the thermistor resistance is high; therefore, the microprocessor (15) will receive a high signal voltage input. If the outside air temperature is high, the thermistor resistance is low; therefore, the microprocessor (15) will receive a low signal voltage input.

TEST DESCRIPTION

The following numbers refer to circled numbers on the diagnostic chart.

1. This test will determine if the microprocessor (15) has defaulted to a default value of 128 which represents 10°C (50°F) and Code 04 will set if low resistance (short) is present for more than 1.6 seconds.

2. This test will determine if the outside air temperature sensor (2) is within its correct range. The resistance varies with temperature so the Diagnostic Aid Correction Chart must be used if outside temperature is not 16°C (59°F).

3. This test will determine if a short to power or ground exists in the 5-volt reference wire.

4. This test will determine if the microprocessor (15) is functioning properly. The disconnected harness should set a sensor open Code 03.

5. A 12-volt short to power would cause an open in CKT 198 (sensor ground) and grounding through 5-volt reference voltage CKT 735.

DIAGNOSTIC AIDS

A "scan" tool can be used if a Code 04 is not set. Check resistance (or counts) of sensor hot and cold. Compare the reading with the Diagnostic Aid Correction Chart. The outside air temperature sensor (2) is to be replaced if out of parameter, hot or cold.

If the outside air temperature sensor (2) is within parameter, carefully check harness and connections for short to power or ground in CKT 735.