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 Totally Integrated Power Module (TIPM) to the Powertrain Control Module (PCM). The AAT sensor is a variable resistor that measures the ambient air temperature. The TIPM 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. Following a start to run delay time, the outputs of the ambient, engine coolant and intake air temperature sensors are compared. If the engine coolant and intake air temperature sensors agree and the ambient air temperature does not agree, the ambient air temperature sensor is declared as irrational. If declared irrational a second comparison will be done after a short drive cycle.