Heating Ventilation Air Conditioning (HVAC)

CIRCUIT DESCRIPTION

The in-car temperature sensor uses a thermistor to control the signal voltage to the ECC Head. The ECC Head applies a voltage on CKT 734 to the sensor. When the sensor is cold its resistance is high, therefore the ECC Head will see a high signal voltage. As the sensor warms, its resistance decreases and the signal voltage is pulled low through the sensor ground, CKT 909. This signal voltage will vary between 5 volts (open circuit) and 0 volts (shorted circuit). Code 13 will set if the signal voltage indicates less than -34°F (open circuit) or over 87°F (shorted circuit). These conditions can be observed in ECC diagnostic parameter -25.

TEST DESCRIPTION

Numbers below refer to circled numbers on the diagnostic chart.

1. If the shorted circuit reading changes to an open circuit reading after disconnecting the sensor, the ECC Head and wiring are OK.

2. If the open circuit reading changes to a shorted circuit reading after jumpering the sensor harness terminals, the ECC Head and wiring are OK.

3. By applying a ground to various points in the circuits, an open can be isolated by observing whether the parameter display can be changed from the open reading to the shorted reading.

NOTES ON INTERMITTENTS

If Code 113 is being set. manipulate the related wiring while observing ECC parameter -25. If the failure is induced, the reading will jump from its normal value to a reading outside the range of -34°C to 87°C

If the value displayed by parameter -25 is not reasonably close to the actual temperature of the air at the sensor, check for poor terminal contact or replace the sensor.