Dodge ZB 2005, Service Manual

Page: 809 / 1873

MAP SENSOR
DESCRIPTION
The MAP sensor (Fig. 15) mounts to the drivers
side intake manifold plenum.
OPERATION
The MAP serves as a PCM input, using a silicon
based sensing unit, to provide data on the manifold
vacuum that draws the air/fuel mixture into the com-
bustion chamber. The PCM requires this information
to determine injector pulse width and spark advance.
When MAP equals Barometric pressure, the pulse
width will be at maximum.
Also like the cam and crank sensors, a 5 volt ref-
erence is supplied from the PCM and returns a volt-
age
signal to the PCM that reflects manifold
pressure. Like the cam and crank sensors ground is
provided through the sensor return circuit.
The MAP sensor input is the number one contrib-
utor to pulse width. The most important function of
the MAP sensor is to determine barometric pressure.
The PCM needs to know if the vehicle is at sea level
or is it in Denver at 5000 feet above sea level,
because the air density changes with altitude. It will
also help to correct for varying weather conditions. If
a hurricane was coming through the pressure would
be very, very low or there could be a real fair
weather, high pressure area. This is important
because as air pressure changes the barometric pres-
sure changes. Barometric pressure and altitude have
a direct inverse correlation, as altitude goes up baro-
metric goes down. The first thing that happens as
the key is rolled on, before reaching the crank posi-
tion, the PCM powers up, comes around and looks at
the MAP voltage, and based upon the voltage it sees,
it knows the current barometric pressure relative to
altitude. Once the engine starts, the PCM looks at
the voltage again, continuously every 11 milliseconds,
and compares the current voltage to what it was at
key on. The difference between current and what it
was at key on is manifold vacuum.
During key On (engine not running) the sensor
reads (updates) barometric pressure. A normal range
can be obtained by monitoring known good sensor in
your work area.
As the altitude increases the air becomes thinner
(less oxygen). If a vehicle is started and driven to a
very different altitude than where it was at key On
the barometric pressure needs to be updated. Any
time the PCM sees Wide Open throttle, based upon
TPS angle and RPM it will update barometric pres-
sure in the MAP memory cell. With periodic updates,
the PCM can make its calculations more effectively.
The PCM uses the MAP sensor to aid in calculat-
ing the following:
•
Barometric pressure
•
Engine load
•
Manifold pressure
•
Injector pulse-width
•
Spark-advance programs
•
Idle speed
•
Decel fuel shutoff
The MAP sensor signal is provided from a single
piezoresistive element located in the center of a dia-
phragm. The element and diaphragm are both made
of silicone. As the pressures changes the diaphragm
moves causing the element to deflect which stresses
the silicone. When silicone is exposed to stress its
resistance changes. As manifold vacuum increases,
the MAP sensor input voltage decreases proportion-
ally. The sensor also contains electronics that condi-
tion
the signal and provide temperature
compensation.
The PCM recognizes a decrease in manifold pres-
sure by monitoring a decrease in voltage from the
reading stored in the barometric pressure memory
cell. The MAP sensor is a linear sensor; as pressure
changes, voltage changes proportionately. The range
of voltage output from the sensor is usually between
4.6 volts at sea level to as low as 0.3 volts at 26 in. of
Hg (Table 1). Barometric pressure is the pressure
exerted by the atmosphere upon an object. At sea
level on a standard day, no storm, barometric pres-
sure is 29.92 in Hg. For every 100 feet of altitude
barometric pressure drops .10 in. Hg. If a storm goes
through it can either add, high pressure, or decrease,
low pressure, from what should be present for that
Fig. 15 MAP & COOLANT TEMP. SENSOR
1 - Fuel Rail
2 - MAP Sensor
3 - Coolant Temperature Sensor
4 - Fuel Injector
14 - 36 FUEL INJECTION
ZB