Speed Density
Engine is equipped with a Mass Airflow (MAF) sensor and an Intake Air Temperature (IAT) sensor. See Fig 1 .
The MAF sensor is an air flow meter that measures the amount of air entering the engine. The Engine Control Module (ECM) uses the MAF sensor voltage signal to provide the correct fuel delivery for a reduction in emissions. The ECM uses the MAF sensor signal to control fuel delivery until a calibrated amount of engine air flow is attained. The MAF sensor has an ignition 1 voltage circuit, a signal circuit and a low reference circuit. The MAF sensor produces an output voltage based on the inlet air flow through the air induction system. This output voltage will display on scan tool as a grams per second (g/s) value. The ECM will calculate a predicted MAF value. ECM compares actual MAF sensor voltage signal to the predicted MAF value. This comparison will determine if signal is stuck or is too low or too high for a given operating condition.
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 ECM 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, ECM detects a high voltage on the IAT signal circuit. With lower sensor resistance, ECM detects a lower voltage on the IAT signal circuit. If ECM detects an out-of-range signal voltage, a DTC will set. To diagnose IAT sensor circuit, go to SELF-DIAGNOSTICS - 6.6L SIERRA & SILVERADO - DIESEL article.