MAP sensor

The examples and descriptions in this article apply strictly to
four-stroke cycle gasoline engines. Other engine types such as diesel,
or two-stroke cycle can differ in the exact implementation, but the
general theme still applies.

A manifold absolute pressure sensor (MAP) is one of the sensors used in
an internal combustion engine's electronic control system. Engines that
use a MAP sensor are typically fuel injected. The manifold absolute
pressure sensor provides instantaneous manifold pressure information to
the engine's electronic control unit (ECU). This is necessary to
calculate air density and determine the engine's air mass flow rate,
which in turn is used to calculate the appropriate fuel flow. (See
stoichiometry.)

An engine control system that uses manifold absolute pressure to
calculate air mass uses the speed-density method. Engine speed (RPM) and
air temperature are also necessary to complete the speed-density
calculation. Not all fuel-injected engines use a MAP sensor to infer
mass air flow; some use a MAF (mass air flow) sensor. Several makes use
the MAP sensor in OBD II applications to test the EGR valve for
functionality. Most notably General Motors uses this approach.

MAP Sensor Auto Repair Technician Observations

MAP sensors are three wire devices that measure intake manifold vacuum.
In actuality, the MAP sensor measures the difference between intake
manifold pressure/vacuum and atmospheric pressure. This is the reason
why intake vacuum is NOT the same as MAP vacuum. Intake vacuum is
atmospheric pressure minus MAP vacuum. With this in mind, the ECM makes
the appropriate calculations as to the correct injector pulse. The MAP
scanner PID is just MAP vacuum and should not be confused with intake
vacuum. Few manufacturers do put out a manifold vacuum PID and Chrysler
is one of them. In this case, the scanner PID for intake vacuum is a
calculation (atmospheric press. minus MAP vacuum).

An engine's vacuum is a good indicator of load. The MAP sensor outputs a
DC voltage or frequency and its signal is inversely proportional, which
means that as manifold vacuum increases voltage or frequency decreases.
MAP sensors are also used as barometric (BARO) sensors. As soon as the
ignition key is turned on, the ECM reads the MAP voltage or frequency
signal and automatically takes that reading as atmospheric pressure.
Some manufacturers have configured their ECM programming so that the
barometric reading is updated during a WOT condition. Once the engine
starts, the ECM uses the MAP, TPS and RPM signals as main inputs to
calculate fuel control on MAP or SPEED DENSITY SYSTEMS (systems without
a MAF sensor). The ECM modifies injector pulse-width according to the
MAP signal output or engine load. This sensor is also used for ignition
timing and on some systems is a backup for the MAF sensor. With dual MAP
and MAF systems, the MAP sensor is primarily used to monitor the EGR
valve oper ation.

MAP sensors are made of a piezoelectric material. This material is a
form of crystal (Quartz) that when bent changes its internal resistance.
MAP sensors output two different types of signals. Most output a voltage
and usually work with a 5.00 Volt REF. The other type found mostly on
FORDs, output a square wave at a certain frequency (FORD uses 159 Hz).
As manifold vacuum increases the frequency output decreases.

CONDITIONS THAT AFFECT OPERATION

MAP sensors are connected directly to manifold vacuum. This also means
that any condition affecting the engine vacuum will also affect the MAP
sensor reading. Conditions that affect engine vacuum are: EGR stuck
open, clogged catalytic converter, engine mechanical problems, vacuum
leak, ignition timing problems, valve timing adjustments and low fuel
pressure. Also a shorted sensor feeding off the same sensor ground or
5.00 volt ref. line could cause a faulty MAP reading, due to the bad
sensor shorting the MAP signal.

How the MAP value is used

The manifold absolute pressure measurement is used to meter fuel. The
amount of fuel required is directly related to the mass of air entering
the engine. (See stoichiometric.) The mass of air is proportional to the
air density, which is proportional to the absolute pressure and
inversely proportional to the absolute temperature. (See ideal gas law.)
Engine speed determines the frequency, or rate, at which air mass is
leaving the intake manifold and entering the cylinders.

    (Engine Mass Airflow Rate) ~ RPM ž (Air Density)

        or equivalently

    (Engine Mass Airflow Rate) ~ RPM ž MAP / (absolute temperature)

Example

This example assumes the same engine speed and air temperature.

        * Condition 1:

        An engine operating at WOT (wide open throttle) on top of a very
        high mountain has a MAP of about 15" Hg or 50 kPa (essentially
        equal to the barometer).

        * Condition 2:

        The same engine at sea level will achieve 15" Hg of MAP at less
        than WOT due to the higher barometric pressure.


The engine requires the same mass of fuel in both conditions because the
mass of air entering the cylinders is the same.

If the throttle is opened all the way in condition 2, the MAP will
increase from 15" Hg to nearly 30" Hg (~100 kPa), about equal to the
local barometer, which in condition 2 is sea level. The higher absolute
pressure in the intake manifold increases the air's density, and in turn
more fuel can be burned resulting in higher output.

Anyone who has driven up a high mountain is familiar with the reduction
in engine output as altitude increases.

Vacuum comparison

Vacuum is the difference between the absolute pressures of the intake
manifold and atmosphere. Vacuum is a "gauge" pressure, since gauges by
nature measure a pressure difference, not an absolute pressure. The
engine fundamentally responds to air mass, not vacuum, and absolute
pressure is necessary to calculate mass. The mass of air entering the
engine is directly proportional to the air density, which is
proportional to the absolute pressure, and inversely proportional to the
absolute temperature.

Note: Carburetors are largely dependent on air volume flow and vacuum,
and neither directly infers mass. Consequently, carburetors are precise,
but not accurate fuel metering devices. Carburetors were replaced by
more accurate fuel metering methods, such as fuel injection.

Barometer and vacuum calculations based on MAP

The MAP sensor can be used to directly measure the BAP (barometric
absolute pressure).

    BAP = MAP (When either of the following conditions are true.)

        * o When the engine is not turning. o When operating at WOT
              (nearly equal to the barometric pressure)

Once the BAP is known, the MAP sensor can be used to calculate intake
manifold vacuum.

    BAP - MAP = Manifold Vacuum

        or

    BAP = MAP + Manifold Vacuum

        or

    MAP = BAP - Manifold Vacuum

        * o When the engine is running, the difference between the BAP
              and the MAP is known as intake manifold vacuum. The ECU
              learns the BAP just before cranking the engine, i.e., when
              MAP equals BAP.

As atmospheric pressure decreases with increasing altitude, vacuum must
also decrease to maintain the same MAP in order to maintain the same
torque output. This is accomplished by opening the engine's throttle
more as altitude increases. However, the BAP learned at the beginning of
the trip becomes obsolete as altitude changes.

Sometimes an engine control system will use both a BAP sensor and a MAP
sensor to continuously maintain an accurate barometer and manifold
vacuum. However, neither vacuum nor barometer are necessary for fuel
determination, although they are helpful for other engine functions. The
critical information is the air's density in the intake manifold, and
the speed of the engine, i.e., the speed-density method.

The BAP sensor is often located within the ECU, and the MAP sensor is
usually located near the intake manifold.

(See Earth's atmosphere.)

EGR Testing

With OBD II standards, vehicle manufacturers were required to test the
EGR valve for functionality during driving. Some manufacturers use the
MAP sensor to accomplish this. In these vehicles, they have a MAF sensor
for their primary load sensor. The MAP sensor is then used for
rationality checks and to test the EGR valve. The way they do this is
during a deceleration of the vehicle when there is a high vacuum present
in the intake manifold. During this high vacuum the PCM will open the
EGR valve and then monitor the MAP sensor's values. If the EGR is
functioning properly, the vacuum in the manifold will drop as exhaust
gases enter .