Theory Of Operation

Ambient Air Temperature (AAT) sensor performance looks at the outputs of three temperature sensors and compares them under cold start conditions. The AAT reading is a bussed message from the Body Control Module (BCM) to the Powertrain Control Module (PCM). Following a start to run delay time, the outputs of the ambient, engine coolant and intake air temperature sensors are compared. The AAT sensor is a variable resistor that measures the ambient air temperature. The BCM supplies a 5 Volt reference and a ground to the sensors low reference signal circuit. When the AAT is low, the sensor resistance is high. When the AAT is high, the sensor resistance is low.