1999 DODGE NEON Service Repair Manual

POWERTRAIN CONTROL MODULE
The Powertrain Control Module (PCM) is a digital
computer containing a microprocessor (Fig. 2). The
PCM receives input signals from various switches
and sensors that are referred to as PCM Inputs.
Based on these inputs, the PCM adjusts various
engine and vehicle operations through devices that
are referred to as PCM Outputs.
PCM Inputs:
²Air Conditioning Controls
²Battery Voltage
²Battery Temperature Sensor
²Brake Switch
²Camshaft Position Sensor
²Crankshaft Position Sensor
²Engine Coolant Temperature Sensor
²Fuel Level Sensor
²Ignition Switch
²Intake Air Temperature Sensor
²Knock Sensor
²Manifold Absolute Pressure (MAP) Sensor
²Oxygen Sensors
²Power Steering Pressure Switch
²SCI Receive
²Speed Control Switches
²Throttle Position Sensor
²Transmission Park/Neutral Switch (automatic
transmission)
²Vehicle Speed Sensor
PCM Outputs:
²Air Conditioning WOT Relay
²Auto Shutdown (ASD) Relay
²Charging Indicator Lamp
²Data Link Connector
²Duty Cycle EVAP Canister Purge Solenoid
²EGR Solenoid
²Fuel Injectors
²Fuel Pump Relay²Generator Field
²Idle Air Control Motor
²Ignition Coils
²Malfunction Indicator (Check Engine) Lamp
²Radiator Fan Relay
²Speed Control Solenoids
²Tachometer
²Torque Convertor Clutch Solenoid
Based on inputs it receives, the PCM adjusts fuel
injector pulse width, idle speed, ignition spark
advance, ignition coil dwell and EVAP canister purge
operation. The PCM regulates the cooling fan, air
conditioning and speed control systems. The PCM
changes generator charge rate by adjusting the gen-
erator field. The PCM also performs diagnostics.
The PCM adjusts injector pulse width (air-fuel
ratio) based on the following inputs.
²Battery voltage
²Coolant temperature
²Intake air temperature
²Exhaust gas content (oxygen sensor)
²Engine speed (crankshaft position sensor)
²Manifold absolute pressure
²Throttle position
The PCM adjusts ignition timing based on the fol-
lowing inputs.
²Coolant temperature
²Intake air temperature
²Engine speed (crankshaft position sensor)
²Knock sensor
²Manifold absolute pressure
²Throttle position
²Transmission gear selection (park/neutral
switch)
The PCM also adjusts engine idle speed through
the idle air control motor based on the following
inputs.
²Air conditioning sense
²Battery voltage
²Battery temperature
²Brake switch
²Coolant temperature
²Engine speed (crankshaft position sensor)
²Engine run time
²Manifold absolute pressure
²Power steering pressure switch
²Throttle position
²Transmission gear selection (park/neutral
switch)
²Vehicle distance (speed)
The Auto Shutdown (ASD) and fuel pump relays
are mounted externally, but turned on and off by the
PCM.
The crankshaft position sensor signal is sent to the
PCM. If the PCM does not receive the signal within
approximately one second of engine cranking, it deac-
Fig. 2 Powertrain Control Module (PCM)
14 - 24 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

tivates the ASD relay and fuel pump relay. When
these relays deactivate, power is shut off from the
fuel injectors, ignition coils, heating element in the
oxygen sensors and the fuel pump.
The PCM contains a voltage converter that
changes battery voltage to a regulated 9 volts direct
current to power the camshaft position sensor, crank-
shaft position sensor and vehicle speed sensor. The
PCM also provides a 5 volt direct current supply for
the manifold absolute pressure sensor and throttle
position sensor.
AIR CONDITIONING PRESSURE TRANSDUCERÐ
PCM INPUT
The Powertrain Control Module (PCM) monitors
the A/C compressor discharge (high side) pressure
through the air conditioning pressure transducer.
The transducer supplies an input to the PCM. The
PCM engages the A/C compressor clutch if pressure
is sufficient for A/C system operation.
AUTOMATIC SHUTDOWN (ASD) SENSEÐPCM
INPUT
The ASD sense circuit informs the PCM when the
ASD relay energizes. A 12 volt signal at this input
indicates to the PCM that the ASD has been acti-
vated. This input is used only to sense that the ASD
relay is energized.
When energized, the ASD relay supplies battery
voltage to the fuel injectors, ignition coils and the
heating element in each oxygen sensor. If the PCM
does not receive 12 volts from this input after
grounding the ASD relay, it sets a Diagnostic Trouble
Code (DTC).
BATTERY VOLTAGEÐPCM INPUT
The PCM monitors the battery voltage input to
determine fuel injector pulse width and generator
field control.
If battery voltage is low the PCM will increase
injector pulse width (period of time that the injector
is energized).
BATTERY TEMPERATURE SENSORÐPCM INPUT
The PCM uses the temperature of the battery area
to control the charge rate. The signal is used to reg-
ulate the system voltage. The system voltage is
higher at cold temperatures and is gradually reduced
as temperature is increased.
BRAKE SWITCHÐPCM INPUT
When the brake switch is activated, the PCM
receives an input indicating that the brakes are
being applied. The brake switch is mounted on the
brake pedal support bracket.
CAMSHAFT POSITION SENSORÐPCM INPUT
The PCM determines fuel injection synchronization
and cylinder identification from inputs provided by
the camshaft position sensor (Fig. 3) or (Fig. 4) and
crankshaft position sensor. From the two inputs, the
PCM determines crankshaft position.
The camshaft position sensor attaches to the rear
of the cylinder head. A target magnet attaches to the
rear of the camshaft and indexes to the correct posi-
tion. The target magnet has four different poles
arranged in an asymmetrical pattern (Fig. 5). As the
target magnet rotates, the camshaft position sensor
senses the change in polarity (Fig. 6). The sensor out-
put switch switches from high (5.0 volts) to low (0.5
volts) as the target magnet rotates. When the north
pole of the target magnet passes under the sensor,
the output switches high. The sensor output switches
low when the south pole of the target magnet passes
underneath.
The sensor also acts as a thrust plate to control
camshaft endplay.
Fig. 3 Camshaft Position SensorÐSOHC
Fig. 4 Camshaft Position SensorÐDOHC
PLFUEL SYSTEM 14 - 25
DESCRIPTION AND OPERATION (Continued)

CRANKSHAFT POSITION SENSORÐPCM INPUT
The PCM determines what cylinder to fire from the
crankshaft position sensor input and the camshaft
position sensor input. The second crankshaft counter-
weight has two sets of four timing reference notches
including a 60 degree signature notch (Fig. 7). From
the crankshaft position sensor input the PCM deter-
mines engine speed and crankshaft angle (position).
The notches generate pulses from high to low in
the crankshaft position sensor output voltage. When
a metal portion of the counterweight aligns with the
crankshaft position sensor, the sensor output voltage
goes low (less than 0.5 volts). When a notch aligns
with the sensor, voltage goes high (5.0 volts). As a
group of notches pass under the sensor, the outputvoltage switches from low (metal) to high (notch)
then back to low.
If available, an oscilloscope can display the square
wave patterns of each voltage pulses. From the width
of the output voltage pulses, the PCM calculates
engine speed. The width of the pulses represent the
amount of time the output voltage stays high before
switching back to low. The period of time the sensor
output voltage stays high before switching back to
low is referred to as pulse width. The faster the
engine is operating, the smaller the pulse width on
the oscilloscope.
By counting the pulses and referencing the pulse
from the 60 degree signature notch, the PCM calcu-
lates crankshaft angle (position). In each group of
timing reference notches, the first notch represents
69 degrees before top dead center (BTDC). The sec-
ond notch represents 49 degrees BTDC. The third
notch represents 29 degrees. The last notch in each
set represents 9 degrees before top dead center
(TDC).
The timing reference notches are machined at 20É
increments. From the voltage pulse width the PCM
tells the difference between the timing reference
notches and the 60 degree signature notch. The 60
degree signature notch produces a longer pulse width
than the smaller timing reference notches. If the
camshaft position sensor input switches from high to
low when the 60 degree signature notch passes under
the crankshaft position sensor, the PCM knows cylin-
der number one is the next cylinder at TDC.
The crankshaft position sensor mounts to the
engine block behind the alternator, just above the oil
filter (Fig. 8).
ENGINE COOLANT TEMPERATURE SENSORÐPCM
INPUT
The combination coolant temperature sensor has
two elements. One element supplies coolant temper-
ature signal to the PCM. The other element supplies
coolant temperature signal to the instrument panel
gauge cluster. The PCM determines engine coolant
temperature from the coolant temperature sensor.
As coolant temperature varies the coolant temper-
ature sensors resistance changes resulting in a differ-
ent input voltage to the PCM and the instrument
panel gauge cluster.
When the engine is cold, the PCM will provide
slightly richer air- fuel mixtures and higher idle
speeds until normal operating temperatures are
reached.
SOHC
The coolant sensor threads into the rear of the cyl-
inder head, next to the camshaft position sensor (Fig.
9). New sensors have sealant applied to the threads.
Fig. 5 Target MagnetÐTypical
Fig. 6 Target Magnet Polarity
14 - 26 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

DOHC
The coolant sensor threads into the intake mani-
fold next to the thermostat housing (Fig. 10). New
sensors have sealant applied to the threads.
FUEL LEVEL SENSORÐPCM INPUT
The fuel level sensor (fuel gauge sending unit)
sends a signal to the PCM to indicate fuel level. The
purpose of this feature is to prevent a false setting of
misfire and fuel system monitor trouble codes if the
fuel level is less than approximately 15 percent of its
rated capacity. It is also used to send a signal for fuel
gauge operation via the CCD bus circuits.
HEATED OXYGEN SENSOR (O2S SENSOR)ÐPCM
INPUT
As vehicles accumulate mileage, the catalytic con-
vertor deteriorates. The deterioration results in a
less efficient catalyst. To monitor catalytic convertor
Fig. 7 Timing Reference Notches
Fig. 8 Crankshaft Position Sensor
Fig. 9 Engine Coolant Temperature SensorÐSOHC
Fig. 10 Engine Coolant Temperature SensorÐDOHC
PLFUEL SYSTEM 14 - 27
DESCRIPTION AND OPERATION (Continued)

deterioration, the fuel injection system uses two
heated oxygen sensors. One sensor upstream of the
catalytic convertor, one downstream of the convertor.
The PCM compares the reading from the sensors to
calculate the catalytic convertor oxygen storage
capacity and converter efficiency. Also, the PCM uses
the upstream heated oxygen sensor input when
adjusting injector pulse width.
When the catalytic converter efficiency drops below
emission standards, the PCM stores a diagnostic
trouble code and illuminates the malfunction indica-
tor lamp (MIL).
The O2S sensors produce voltages from 0 to 1 volt,
depending upon the oxygen content of the exhaust
gas in the exhaust manifold. When a large amount of
oxygen is present (caused by a lean air/fuel mixture),
the sensors produces a low voltage. When there is a
lesser amount present (rich air/fuel mixture) it pro-
duces a higher voltage. By monitoring the oxygen
content and converting it to electrical voltage, the
sensors act as a rich- lean switch.
The oxygen sensors are equipped with a heating
element that keeps the sensors at proper operating
temperature during all operating modes. Maintaining
correct sensor temperature at all times allows the
system to enter into closed loop operation sooner.
Also, it allows the system to remain in closed loop
operation during periods of extended idle.
In Closed Loop operation the PCM monitors the
O2S sensor input (along with other inputs) and
adjusts the injector pulse width accordingly. During
Open Loop operation the PCM ignores the O2 sensor
input. The PCM adjusts injector pulse width based
on preprogrammed (fixed) values and inputs from
other sensors.
The Automatic Shutdown (ASD) relay supplies bat-
tery voltage to both the upstream and downstream
heated oxygen sensors. The oxygen sensors are
equipped with a heating element. The heating ele-
ments reduce the time required for the sensors to
reach operating temperature.
UPSTREAM OXYGEN SENSOR
The input from the upstream heated oxygen sensor
tells the PCM the oxygen content of the exhaust gas.
Based on this input, the PCM fine tunes the air-fuel
ratio by adjusting injector pulse width.
The sensor input switches from 0 to 1 volt, depend-
ing upon the oxygen content of the exhaust gas in
the exhaust manifold. When a large amount of oxy-
gen is present (caused by a lean air-fuel mixture), the
sensor produces voltage as low as 0.1 volt. When
there is a lesser amount of oxygen present (rich air-
fuel mixture) the sensor produces a voltage as high
as 1.0 volt. By monitoring the oxygen content and
converting it to electrical voltage, the sensor acts as
a rich-lean switch.The heating element in the sensor provides heat to
the sensor ceramic element. Heating the sensor
allows the system to enter into closed loop operation
sooner. Also, it allows the system to remain in closed
loop operation during periods of extended idle.
In Closed Loop, the PCM adjusts injector pulse
width based on the upstream heated oxygen sensor
input along with other inputs. In Open Loop, the
PCM adjusts injector pulse width based on prepro-
grammed (fixed) values and inputs from other sen-
sors.
The upstream oxygen sensor threads into the out-
let flange of the exhaust manifold (Fig. 11).
DOWNSTREAM OXYGEN SENSOR
The downstream heated oxygen sensor threads into
the outlet pipe at the rear of the catalytic convertor
(Fig. 12). The downstream heated oxygen sensor
input is used to detect catalytic convertor deteriora-
tion. As the convertor deteriorates, the input from
the downstream sensor begins to match the upstream
sensor input except for a slight time delay. By com-
paring the downstream heated oxygen sensor input
to the input from the upstream sensor, the PCM cal-
culates catalytic convertor efficiency.
IGNITION CIRCUIT SENSEÐPCM INPUT
The ignition circuit sense input tells the Power-
train Control Module (PCM) the ignition switch has
energized the ignition circuit. Refer to the wiring dia-
grams for circuit information.
INTAKE AIR TEMPERATURE SENSORÐPCM INPUT
The Intake Air Temperature (IAT) sensor measures
the temperature of the intake air as it enters the
engine. The sensor supplies one of the inputs the
PCM uses to determine injector pulse width and
spark advance.
Fig. 11 Upstream Heated Oxygen Sensor
14 - 28 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

As Intake Air temperature varies the Intake Air
Temperature sensors resistance changes resulting in
a different input voltage to the PCM.
The IAT sensor and Manifold Absolute Pressure
(MAP) switch are a combined into a single sensor
that attachs to the intake manifold (Fig. 13) or (Fig.
14).
KNOCK SENSORÐPCM INPUT
The knock sensor threads into the side of the cyl-
inder block in front of the starter (Fig. 15). When the
knock sensor detects a knock in one of the cylinders,
it sends an input signal to the PCM. In response, the
PCM retards ignition timing for all cylinders by a
scheduled amount.
Knock sensors contain a piezoelectric material
which sends an input voltage (signal) to the PCM. As
the intensity of the engine knock vibration increases,
the knock sensor output voltage also increases.
MANIFOLD ABSOLUTE PRESSURE (MAP)
SENSORÐPCM INPUT
The PCM supplies 5 volts direct current to the
MAP sensor. The MAP sensor converts intake mani-
fold pressure into voltage. The PCM monitors the
MAP sensor output voltage. As vacuum increases,
MAP sensor voltage decreases proportionately. Also,
as vacuum decreases, MAP sensor voltage increases
proportionately.
At key on, before the engine is started, the PCM
determines atmospheric air pressure from the MAP
sensor voltage. While the engine operates, the PCM
determines intake manifold pressure from the MAP
sensor voltage. Based on MAP sensor voltage and
inputs from other sensors, the PCM adjusts spark
advance and the air/fuel mixture.
The MAP/Intake Air Temperature sensor mounts to
the intake manifold (Fig. 13) and (Fig. 14).
Fig. 12 Downstream Heated Oxygen Sensor
Fig. 13 / MAP/Intake Air Temperature SensorÐSOHC
Fig. 14 MAP/Intake Air Temperature SensorÐDOHC
Fig. 15 Knock Sensor
PLFUEL SYSTEM 14 - 29
DESCRIPTION AND OPERATION (Continued)

POWER STEERING PRESSURE SWITCHÐPCM
INPUT
A pressure sensing switch is located on the power
steering gear. The switch (Fig. 16) provides an input
to the PCM during periods of high pump load and
low engine RPM; such as during parking maneuvers.
When power steering pump pressure exceeds 2758
kPa (400 psi), the switch is open. The PCM increases
idle air flow through the IAC motor to prevent
engine stalling. When pump pressure is low, the
switch is closed.
SENSOR RETURNÐPCM INPUT
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.
SPEED CONTROL SERVOSÐPCM OUTPUT
The PCM controls the speed control vacuum servo.
The PCM supplies power, through the brake switch,
to the servo. Based on the speed control switch
inputs to the PCM and the speed control strategy,
the PCM provides ground to the servo vacuum or
vent circuit as required. When the PCM supplies a
ground to the servo vacuum circuit, the speed control
system opens the throttle plate to obtain or maintain
the selected road speed. When the PCM supplies a
ground to the servo vent circuit, the speed control
system releases the throttle plate. Refer to Group 8H
for speed control information.
SCI RECEIVEÐPCM INPUT
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.
PARK/NEUTRAL POSITION SWITCHÐPCM INPUT
The park/neutral position switch is located on the
automatic transaxle housing (Fig. 17). Manual tran-
saxles do not use park/neutral switches. The switch
provides an input to the PCM to indicate whether
the automatic transaxle is in Park/Neutral, or a drive
gear selection. This input is used to determine idle
speed (varying with gear selection) and ignition tim-
ing advance. The park/neutral input is also used to
cancel vehicle speed control. The park/neutral switch
is sometimes referred to as the neutral safety switch.
THROTTLE POSITION SENSORÐPCM INPUT
The throttle position sensor mounts to the side of
the throttle body (Fig. 18) and (Fig. 19).
The Throttle Position Sensor (TPS) connects to the
throttle blade shaft. The TPS is a variable resistor
that provides the PCM with an input signal (voltage).
The signal represents throttle blade position. As the
position of the throttle blade changes, the resistance
of the TPS changes.
The PCM supplies approximately 5 volts DC to the
TPS. The TPS output voltage (input signal to the
powertrain control module) represents throttle blade
position. The TPS output voltage to the PCM varies
from approximately 0.35 to 1.03 volts at minimum
throttle opening (idle) to a maximum of 3.1 to 4.0
volts at wide open throttle.
Along with inputs from other sensors, the PCM
uses the TPS input to determine current engine oper-
ating conditions. The PCM also adjusts fuel injector
pulse width and ignition timing based on these
inputs.
VEHICLE SPEED SENSORÐPCM INPUT
The vehicle speed sensor is located in the transmis-
sion extension housing (Fig. 20) and (Fig. 21). The
sensor input is used by the PCM to determine vehicle
speed and distance traveled.
Fig. 16 Power Steering Pressure SwitchFig. 17 Park/Neutral Switch
14 - 30 FUEL SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

The vehicle speed sensor generates 8 pulses per
sensor revolution. These signals, in conjunction with
a closed throttle signal from the throttle position sen-
sor, indicate a closed throttle deceleration to the
PCM. Under deceleration conditions, the PCM
adjusts the Idle Air Control (IAC) motor to maintain
a desired MAP value.
When the vehicle is stopped at idle, a closed throt-
tle signal is received by the PCM (but a speed sensor
signal is not received). Under idle conditions, the
PCM adjusts the IAC motor to maintain a desired
engine speed.
The vehicle speed sensor signal is also used to
operate the following functions or systems:
²Speedometer
²Speed control²Daytime Running Lights (Canadian Vehicles
only).AIR CONDITIONING CLUTCH RELAYÐPCM
OUTPUT
The PCM controls the air conditioning clutch relay
ground circuit. The A/C clutch relay coil side contains
a 10 amp fuse between the buss bar in the Power
Distribution Center (PDC) and the relay. The power
side of this relay is fused with a 40 amp fuse. When
the PCM receives an air conditioning input, it
grounds the A/C compressor clutch relay and the
radiator fan relay.
When the PCM senses low idle speeds or wide open
throttle through the throttle position sensor, it
removes the ground for the A/C compressor clutch
relay. When the relay de-energizes, the contacts open
preventing air conditioning clutch engagement. Also,
if the PCM senses a part throttle launch condition, it
Fig. 18 Throttle Position Sensor and Idle Air Control
MotorÐSOHC
Fig. 19 Throttle Position Sensor and Idle Air Control
MotorÐDOHC
Fig. 20 Vehicle Speed SensorÐAutomatic
Transmission
Fig. 21 Vehicle Speed SensorÐManual
Transmission
PLFUEL SYSTEM 14 - 31
DESCRIPTION AND OPERATION (Continued)