Why is it called "absolute pressure"? Well I guess it is because the intake air pressure in the manifold is essential as how much power we will get when the intake valve is opened. In other words, it is the pressure that is untouched by the atmosphere (only through the throttle body) that is ready in the intake manifold to be vacuumed in.
Air, gas - in general, has three main physical properties that we are all common about: Volume, Temperature, Pressure - and they are the 3 musketeers who can't exist without others. When pressure increases the volume will be squeezed smaller; when temperature rises the gas will likely to expand, and when trapped in a confined space, the expanding gas will build up pressure - those are undeniable gas/air properties and based on this we must have a MAP sensor inside our intake manifold.
Basically, the MAP sensor does the same thing as a MAF sensor - measuring how much air coming in. You might thing the MAP is so much different from a Hot-wire MAF because it measures the volume of incoming air, and MAP is measuring pressure, how? The ECU can't feel the pressure, it's micro-processor is just a 1 dimensional being, the only thing that makes sense to it is "0" and "1". But, because we use the event of massive volume of air coming in or increased air pressure is in the manifold that we turn them to affect our sensors to create electrical signals, the ECU will now receive something different that it can process on. And, think about this: both MAP and MAF give the same type of analog, voltage signal to the ECU, the only difference is the way they operate.
The MAP sensor utilizes a semi-conductor material (it can be silicon) which will become pressurized under high pressure and will start conducting electricity. Using a 5V Reference from the ECU, the Voltage output fed to the ECU will change after the ability to conduct of the material, and it depends on the pressure it gets.
So basically this is similar to a rheostat configuration, and a Vc in, Voltage signal as a divider's output, and an earth.
How do we test a MAP sensor? Aftermarket MAP sensor means the ECU is modified, hence we won't be able to find it inside the intake manifold as we usually do. But normally, there are 2 methods that we can use to test the MAP is Off-car and On-car.
On-car test is simple: find the MAP, identify the wires(Vc; PIM; E2) and then back probe. As the ignition is turned on not the engine, we will be able to retrieve the MAP's maximum reading. This is because 1 bar of atmosphere is able to get inside the manifold through by pass air control, without encountering any vacuum being created because the intake valves are not moving, the cylinders are not moving at all.
Other than that, try turn on the engine and the PIM reading will eventually drop down to minimum readings for idling. This is because maximum vacuum is created by the cylinders hence the absolute pressure inside the manifold is minimum. There are still atmospheric pressure blown in but not enough for the vacuum.
As we open more throttle, more pressure hovers inside the manifold and if we maintain this pressure level we can still have maximum pressure applied to the MAP sensor.
Off-car is even simpler with the vacuum tube removed. Therefore the reading on the MAP is always maximum 1atm anyway. So if we block the hose by a vacuum tube and apply vacuum to it, we can see the voltage reading decreases, just like closing the throttle and let the engine idle.
So overall, MAP sensor and MAF sensor can be either alternatives or complements, depends on the configuration of the engine. They both measure air by sending voltage signal telling the ECU of how much air coming in, just different by the way they utilize which properties of air.
In which case, MAF & MAP sensor is no more useful when it comes into forced-induction engine, where air intake pressure and volume exceed way beyond what of a naturally-aspirated induction. A MAP sensor will change its way into a boost sensor or pressure gauge, measuring pressure which is much higher than atmospheric, and minimum pressure exceeds higher vacuum level. At that point, idling and wide open throttle needs to be monitored more powerfully differently.
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