Dodge RAM TRUCK 1500 1995 Service Manual page 1117

B R - — — — — COMPONENT DESCRIPTION/SYSTEM OPERATION—EXCEPT DIESEL 14 - 35
MANIFOLD ABSOLUTE PRESSURE (MAP)
SENSOR—PCM PIN #1—PCM INPUT
The MAP sensor reacts to absolute pressure in the
intake manifold and provides an input voltage to the
powertrain control module (PCM). As engine load
changes, manifold pressure varies, causing the MAP
sensor voltage to change. This change results in a
different input voltage to the PCM. The input voltage
level supplies the PCM with information. This relates
to ambient barometric pressure before engine start­
up, during engine start-up (cranking) and to engine
load while the engine is running. The PCM uses this
input, along with inputs from other sensors, to adjust
air-fuel mixture.
3.9L V-6 or 5.2/5.9L V-8 Engines: The MAP sen­
sor is mounted to the throttle body (Fig. 6). It is con­
nected to the throttle body with an L-shaped rubber
fitting and to the PCM electrically.
S.OL V-10 Engine: The MAP sensor is mounted
into the right side of the intake manifold (Fig. 7).
Fig. 6 MAP Sensor—3.9L V-6 or 5.2/5.9L V-8
Engines—Typical
For component testing, refer to the Diagnostics/Ser­
vice Procedures section of this group.
For removal and installation of this component, re­
fer to the Component Removal/Installation section of
this group.
ENGINE COOLANT TEMPERATURE SENSOR—PCM
PIN #2—PCM INPUT
The engine coolant temperature sensor is a vari­
able, thermistor type. It reacts to temperature
Fig. 7 MAP Sensor—8.0L V-10 Engine—Typical
changes. At cold coolant temperatures, its resistance
is high. As temperatures increase, its resistance will
decrease.
The coolant sensor provides an input voltage to the
powertrain control module (PCM) relating engine
coolant temperature. The PCM uses this input, along
with inputs from other sensors, to determine injector
pulse width and ignition timing. As coolant tempera­
ture varies, the coolant temperature sensor resis­
tance will change, resulting in a different input
voltage to the PCM.
When the engine is cold, the PCM will operate in
the Open Loop Mode. It will demand slightly richer
air-fuel mixtures and higher idle speeds, until nor­
mal operating temperatures are reached. Refer to
Open Loop/Closed Loop Modes Of Operation in this
group for a description of operation.
On the 3.9L V-6 or 5.2/5.9L V-8 engines, the sensor
is installed in the intake manifold near the thermo­
stat housing (Fig. 8).
On the 8.0L V-10 engine, the sensor is installed in
the thermostat housing (Fig. 9).
For component testing, refer to the Diagnostics/Ser­
vice Procedures section of this group.
For removal and installation of this component, re­
fer to the Component Removal/Installation section of
this group.
BATTERY VOLTAGE—PCM PIN #3—PCM INPUT
The battery voltage input provides power to the
powertrain control module (PCM). It also informs the
PCM what voltage level is supplied to the ignition
coil(s) and fuel injectors.
If battery voltage is low, the PCM will increase in­
jector pulse width (period of time that the injector is
energized). This is done to compensate for the re­
duced flow through injector caused by the lowered
voltage.