The MAP (Manifold Absolute Pressure) sensor is fitted to the inlet manifold after the air filter box and after the throttle body. Its purpose is to measure the pressure of the filtrated air and provide this information to the car computer (ECU – Engine Control Unit).
The purpose of measuring the air pressure inside the inlet manifold is to identify the engine load. The engine load is detected by the changes of the air pressure. The MAP sensor is therefore key to optimal combustion and therefore optimal engine performance.
Some engines use a MAP sensor to measure air and some use a MAF sensor (which is fitted before the throttle body). In some unique engine designs both are used, and in this case the MAF is used to manage the air/fuel ratio and the MAP is used to verify the required pressure changes (for example from the opening/closing of the EGR valve).
A comparison of a MAP sensor engine design vs a MAF sensor engine design shows a strength in MAP sensor in terms of air leaks. In a MAF engine design, a leak after the MAF will not be compensated for, whereas a MAP design will compensate for any leaks and is therefore less prone to allow a suboptimal combustion. Engine designs where both MAF and MAP sensor are used will also use the strength of the MAP sensor and compensate for leaks. In any case, if the combustion is suboptimal the O2 sensor (Oxygen sensor) or A/F sensor will detect it and report back to the ECU to adjust the fuel ratio to compensate for the lean or rich state.
When the engine is idling the air pressure will be very low. When the air pressure is read as low by the MAP sensor, the ECU see it as the engine being under a fairly low load. As the throttle is opened, the pressure increases and the ECU see it as a greater engine load. The MAP sensor signal is used to identify the amount of fuel to introduce in order to have the optimal combustion.
The MAP sensor consists of two chambers separated by a flexible diaphragm. One chamber is the "reference air" (which may be sealed or vented to the outside air), and the other is the vacuum chamber which is connected to the intake manifold on the engine by a rubber hose or direct connection.
A pressure sensitive electronic circuit inside the MAP sensor monitors the movement of the diaphragm and generates a voltage signal that changes in proportion to pressure. This produces an analog voltage signal that ranges from 1 to 5 volts.
The voltage will increase/decrease in proportion to the changes in the pressure. Low air pressure will start from 1V increasing to 5V for high air pressure. Output changes approximately 0.7V to 1V for every 5 inches Hg of change in vacuum. Hg is Inches of mercury, a measurement unit for pressure. At atmospheric pressure (engine off) the voltage is around 3V and at idle the voltage is around 1.5V. A quick snap at the throttle will show less than 1.5V during deceleration of the engine as the pressure drops due to vacuum increasing. A scan tool sometimes will calculate the pressure from the voltage and display it as Hg, Kpa, Bar or PSI (which are all different measuring units for pressure).
MAP sensors consists of three wires, one wire for the signal, one for power (5 volt) and one for the signal. A MAP sensor that reads 1 or 2 volts at idle may read 4.5 volts to 5 volts at wide open throttle.
To identify the ground, measure continuity with the key removed from the ignition.
To identify the power wire, measure volts between the battery ground and the remaining wire(s) with key in ignition position. The wire that shows +5V will be the power wire.
The last wire will be the signal wire.