Tip & How-To about Mercedes-Benz E-Class
1.9) MAP- Manifold Absolute air Pressure (sensor)
What is it? The MAP sensor is a device tomeasure the relative air pressure drop in the inlet manifold, as compared toatmospheric air pressure, when the engine is running.
Where is it located? The MAP sensor is most frequently sited either remotely on the middle ofthe fire wall (bulkhead) at the back of the engine compartment, on the innerwing (fender), on the side of the air ducting or directly bolted on to theintake manifold plenum. A vacuum hosefrom the inlet manifold connects remotely located MAPs, but if sited on themanifold plenum there is a direct connection on its underside.
How does it work? When the engine is at idle the throttle plate is fully closed and thevacuum within the inlet manifold is at its greatest, 20 inches (~500mm) mercury.On older MAP designs, sealed bellows (as in a weather barometer) expand orcontract in response to pressure change and this moves an armature in or out ofan electrical coil. The voltage output fromthe coil reflects the position of the armature and therefore the air pressure.
Modern MAP sensors aresmall, membrane-mounted, solid-state, Piezo electronic, strain-gauge sensor devicesproviding the ECU with electrical signal corresponding to the absolute airpressure within the inlet manifold. The air pressure taken in conjunction withair temperature (IAT) provides the ECU with information on the overallmass of air taken in by the engine and it can therefore calculate theappropriate amount of fuel to inject, providing power whilst minimizingemissions. This device also helps adjustthe engine to accommodate pressure decrease with increase in altitude. In normally aspirated engines MAPs measureinlet pressure decrease in the inlet manifold relative to atmospheric pressure butin turbo charged engines MAPs are also capable of measuring the positive (over)pressure of air (boost) delivered to the inlet manifold from the turbo charger.
Symptoms of a faulty MAP
Associated OBD2 error codes DTCs: P0105 - P109; P0121, P0129; P0236 - P0238;P1296, P1297
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