Sensor Return—Pcm Input; Sci Receive—Pcm Input - Chrysler 2000 LHS Owner's Manual

LH
DESCRIPTION AND OPERATION (Continued)
Fig. 8 MAP Sensor—3.2/3.5L Engine
1 – MAP/AIR TEMPERATURE SENSOR
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 12 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
you 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
• Shift-point
strategies
only, via the CCD bus)
• 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
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
altitude. You should make a habit of knowing what
the average pressure and corresponding barometric
pressure is for your area. Always use the Diagnostic
Test Procedures Manual for MAP sensor testing.
SENSOR RETURN—PCM INPUT
OPERATION
The sensor return circuit provides a low electrical
noise ground reference for all of the systems sensors.
The sensor return circuit connects to internal ground
circuits within the Powertrain Control Module
(PCM).
SCI RECEIVE—PCM INPUT
OPERATION
SCI Receive is the serial data communication
receive circuit for the DRB scan tool. The Powertrain
Control Module (PCM) receives data from the DRB
through the SCI Receive circuit.
FUEL SYSTEM
(F4AC1 transmissions
and provide temperature
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