Heating Ventilation Air Conditioning (HVAC)

CIRCUIT DESCRIPTION

The coolant temperature sensor uses a thermistor to control the signal voltage to the ECC Head. The ECC Head applies a voltage on CKT 905 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 55 will set if the signal voltage indicates over 156°C (shorted circuit). This condition can be observed in ECC diagnostic parameter -21.

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.

NOTES ON INTERMITTENTS

If Code 155 is being set, manipulate the related wiring while observing ECC parameter -21. If the failure is induced, the reading will jump from its normal value to a reading over 87°C.

If the value displayed by parameter -21 is not reasonably close to the actual temperature of the coolant, check for poor terminal contact or replace the sensor.