Circuit/System Description

The intake air temperature (IAT) sensor is integrated with the mass air flow (MAF) sensor. The IAT sensor is a variable resistor that measures the temperature of the air when it first enters the induction system. The engine control module (ECM) supplies 5 volts to the IAT sensor signal circuit and supplies a ground to the low reference circuit.

The ambient air temperature (AAT) sensor system used on this vehicle consists of two separate sensors mounted in the grille area. The AAT sensors are variable resistors that measure the temperature of the air as it enters the grille opening. This system is uniquely wired as explained by the following:

• The driver side mounted AAT reports information to the body control module (BCM). The BCM supplies 5.8 volts to the sensor signal circuit and a ground for the low reference circuit.
• The passenger side mounted AAT reports information to the engine control module (ECM). However the circuits for this sensor are first routed and connected to the BCM. The internal circuitry of the BCM is a direct pass-through for the circuits that are then routed to the ECM. The ECM supplies 5 volts to the passenger side mounted AAT sensor signal circuit and a ground for the low reference circuit.

The purpose of this diagnostic is to verify the performance of the AAT sensor system by comparing the measured AAT to a modeled AAT, that is derived from the following various inputs:

• Barometric (BARO) pressure
• Mass air flow (MAF)
• Engine speed
• Engine coolant temperature (ECT)
• AAT
• IAT
• Vehicle speed