Speed Density
Engine is equipped with a Mass Airflow (MAF) sensor, a Manifold Absolute Pressure (MAP) sensor and an Intake Air Temperature (IAT) sensor. The IAT/MAF sensor is a combination sensor. See Fig 1 or Figure .
The MAF sensor is an airflow meter that measures the amount of air entering the engine. The PCM uses the MAF sensor signal to provide the correct fuel delivery for all engine speeds and loads. A small quantity of air entering the engine indicates a deceleration or idle condition. A large quantity of air entering the engine indicates an acceleration or high load condition. The MAF sensor has an ignition 1 voltage circuit, a ground circuit and a signal circuit.
The MAP sensor responds to pressure changes in the intake manifold. The pressure changes occur based on the engine load. The MAP sensor has the following circuits: 5-volt reference, low reference circuit and MAP sensor signal circuit. The PCM supplies 5 volts to the MAP sensor on the 5-volt reference circuit. PCM also provides a ground on the low reference circuit. MAP sensor provides a signal to PCM on MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. PCM should detect a high signal voltage at a high MAP, such as when ignition is on, with engine off, or at a Wide Open Throttle (WOT). The MAP sensor is also used in order to determine the Barometric Pressure (BARO). This occurs when the ignition is on, with engine off. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range.
The IAT sensor is a variable resistor. The IAT sensor has a signal circuit and a low reference circuit. The IAT sensor measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit and a ground for the IAT low reference circuit. When IAT sensor is cold, the sensor resistance is high. When air temperature increases, sensor resistance decreases. With high sensor resistance, PCM detects a high voltage on the IAT signal circuit. With lower sensor resistance, PCM detects a lower voltage on the IAT signal circuit. If PCM detects an out-of-range signal voltage, a DTC will set. To diagnose, go to SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.