Circuit/System Description

The manifold absolute pressure (MAP) sensor measures the pressure inside the intake manifold. Pressure in the intake manifold is affected by engine speed, throttle opening, air temperature, and barometric pressure (BARO). A diaphragm within the MAP sensor is displaced by the pressure changes that occur from the varying load and operating conditions of the engine. The sensor translates this action into electrical resistance. The MAP sensor wiring includes 3 circuits. The control module supplies a regulated 5 volts to the sensor on a 5-volt reference circuit. The control module supplies a ground on a low reference circuit. The MAP sensor provides a signal voltage to the control module, relative to the pressure changes on the MAP sensor signal circuit. The control module converts the signal voltage input to a pressure value.

Under normal operation, the highest pressure that can exist in the intake manifold is equal to the BARO. This occurs when the vehicle is operated at wide open throttle (WOT), or when the ignition is ON while the engine is OFF. Under these conditions, the control module uses the MAP sensor to determine the current BARO. The lowest manifold pressures occur when the vehicle is idling or decelerating. The MAP can range from 10 kPa when pressures are low to as much as 104 kPa when pressures are high, depending on the BARO.

The control module should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The control module should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at a WOT.

Within the control module, the intake flow rationality diagnostic has the ability to determine an inaccurate input from the 3 systems it monitors. Calibrated within the diagnostic are estimates for the MAP, the mass air flow (MAF), and the throttle position (TP) for all engine operating conditions.

The diagnostic uses the following system of models and calculations that are then compared to the actual measured values from the MAF, MAP, and the TP sensors and to each other to determine the appropriate DTC to fail.

• The throttle model describes the flow through the throttle body and is used to estimate the MAF through the throttle body as a function of BARO, TP, intake air temperature (IAT), and estimated MAP.
• The intake manifold model describes the intake manifold and is used to estimate the MAP as a function of the MAF into the manifold from the throttle body, and the MAF out of the manifold caused by engine pumping. The flow into the manifold from the throttle uses the MAF estimate calculated from the above throttle model.