MAP and MAF Sensors
Engine load is monitored by the ECM and the TCM through the MAP and MAF sensors. The MAP or manifold absolute pressure sensor senses engine load through pressure in the intake manifold. A faulty MAP sensor will really effect engine performance but can also effect a transmissions shift timing. This is because the ECM uses this signal as an indication of engine load. Another important sensor used to indicate engine load and also has an effect on shift timing is the MAF sensor.
A MAF mass air flow sensor measures the volume of air passing through the air tube to indicate engine load. There are several types found on vehicles today. These include the cold wire, hot wire, Karmen Vortex, and the vane type MAF sensors.
The vane type uses a paddle or door that moves as the air rushes by. This type of sensor utilizes a potentiometer (variable resister) to send a variable or changing voltage signal to the ECM in proportion to air volume.
The cold wire MAF sensor uses thin metal strips that vibrate as the air passes by. This vibration changes in frequency as does the signal sent to the ECM.
The hot wire MAF sensor contains a heated filament similar to a toasters. The sensors resistance changes or decreases as cool air passes by the filament. The changes in current to maintain the sensors heat are converted into a voltage signal and sent to the ECM to adjust fuel trim.
Most MAF sensors have an internal IAT (Intake Air Temperature) sensor located inside of the sensor. The ECM uses the MAF sensors signal along with temperature and pressure signals to make accurate air fuel adjustments. These signals used in correlation with the closed loop signal from the oxygen sensor provide extremely accurate fuel trim conditions (14.7:1) stoichiometry.
Both the MAP and MAF sensors provide the ECM and TCM with valuable information regarding engine load. The transmissions control unit uses this information for shift control. A mass air flow sensor left unplugged or in need of cleaning will effect a vehicles shift control dramatically.
