Rick's Free Auto Repair Advice

MAP sensor — what it does

What does MAP sensor do

A Manifold Absolute Pressure (MAP sensor) gives the engine’s computer some data with which it can calculate the density of the incoming air. Calculating density is not as easy as it sounds. Sure, atmospheric pressure gives us some idea of the air’s weight. But remember, the engine is sucking in air, so it’s creating a vacuum. With atmospheric mappressure pushing air into the engine and the pistons sucking air in, the computer really wants to know the difference between the two. That’s why the reading is called absolute pressure—atmospheric minus vacuum. MAP sensor

When you turn the key, the computer gets a reading on a reading from the MAP sensor. Since the engine isn’t running, that initial reading is actually just barometric pressure. Then, when you start the engine, the computer gets a different reading—atmospheric minus manifold vacuum. Vacuum is highest with your foot off the gas. The engine is sucking in air, but the throttle plate is closed. The air for idle bypasses around the throttle plate through the idle air valve, or bypass air valve. As you accelerate, manifold vacuum drops. The MAP sensor has a diaphragm exposed to atmospheric pressure on one side and manifold vacuum on the other. The diaphragm is attached to a strain gauge that varies resistance and thus the voltage the computer sees. Some manufacturers incorporate electronics to change the varying voltage into a digital wave.

So, the computer knows barametric pressure from start-up, it also knows the outside (ambient) air temperature and the absolute manifold pressure. As you drive, the computer also knows how much the throttle plate is open by looking at the throttle position sensor (TPS) value. It also knows how fast you are going from the vehicle speed sensor. Using all those values and a lookup chart in its memory, it can calculate the density of the incoming air. That’s how it calculates the amount of fuel to add. This type of system is called Speed/density.

What can go wrong with a speed/density system? Well, since the MAP is open to the atmosphere, condensation can form on the diaphragm and freeze, causing either momentary distorted readings or complete failure. The vacuum line from the manifold to the MAP can crack, causing erroneous vacuum readings. Or, a shock from an accident can damage the diaphragm or strain gauge inside the housing (extremely rare, but not unheard of). Test a MAP sensor with a digital voltage meter or an automotive scope, depending on whether the output is varying voltage or a digital wave.

 

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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