Fuel DODGE RAM 1999 Service Repair Manual

Power Steering Pressure Switch
On 2.5L Dakota only, power steering pressure switch sends a
signal to PCM. PCM will raise idle speed to prevent stalling during
high power steering pressure (375-575 psi), low RPM conditions.
Serial Communication Interface (SCI) Receive
SCI receive circuit is a serial communication link used when
diagnosing vehicle using scan tool. PCM receives data and device
activation commands from scan tool on this circuit.
Throttle Position (TP) Sensor
TP sensor monitors opening angle of throttle blade. TP sensor
will vary output voltage from about .26 volt at minimum throttle
opening (idle), to about 4.5 volts at Wide Open Throttle (WOT). PCM
uses this information and other sensor inputs to determine engine
operation. In response, PCM will adjust fuel injection pulse width and
ignition timing.
Transmission Governor Pressure Sensor (A/T Models)
Sensor sends PCM a signal indicating governor pressure. PCM
uses signal as feedback for governor solenoid control.
Transmission Overdrive/Override (OD/OR) Switch (A/T Models)
On models with Overdrive (OD), PCM regulates 3-4 OD upshift
and downshift through OD solenoid. Transmission OD/OR switch is
mounted in instrument panel.
OD/OR switch is normally closed. If OD/OR switch is depressed
and it opens, transmission will not enter OD. Transmission will
downshift if it is in OD and OD/OR switch is depressed.
OD/OR switch circuit includes a transmission fluid
temperature sensor. If this sensor opens, transmission will not shift
into overdrive, or will downshift if already in overdrive.
Transmission Temperature Sensor (A/T Models)
Transmission temperature sensor monitors transmission fluid
temperature and sends an input signal to PCM. Input signal is used for
controlling torque converter clutch operation, overdrive shifts, low
temperature shift compensation, wide open throttle shift strategy and
governor pressure. Transmission temperature sensor is located in
transmission valve body, incorporated into governor pressure sensor.
If transmission fluid temperature is more than 260
F (126C),
PCM forces a 4-3 downshift and engages torque converter clutch until
fluid cools. Once fluid cools to less than 230
F (110C), PCM allows a
3-4 shift. PCM prevents torque converter clutch engagement and
overdrive operation when fluid temperature is less than 50
F (10C).
Vehicle Speed Sensor (VSS)
VSS generates 8 pulses per sensor revolution. VSS input is
used by PCM to determine vehicle speed and distance traveled, and to
maintain set speed during cruise control operation.
PCM interprets speed sensor input along with TP sensor closed
throttle input. This enables PCM to determine if a closed throttle
deceleration or normal throttle idle (vehicle stopped) condition
exists. During deceleration, PCM controls IAC motor to maintain a
desired MAP value. During idle (vehicle stopped), PCM controls IAC
motor to maintain a desired idle speed.
OUTPUT SIGNALS
NOTE: Each vehicle may be equipped with different combinations of
computer-controlled components. The following components may
NOT be used on all models. To determine component location
and output usage on a specific model, see appropriate wiring

diagram in appropriate WIRING DIAGRAMS article. For theory
and operation on each output component, refer to indicated
system.
A/C Clutch Relay
See A/C CLUTCH RELAY under MISCELLANEOUS CONTROLS.
Auto Shutdown (ASD) Relay
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under
MISCELLANEOUS CONTROLS.
Distributorless Ignition System (DIS)
See DISTRIBUTORLESS IGNITION SYSTEM (DIS) under IGNITION
SYSTEM.
Evaporative Canister Purge Control Solenoid (EVAP-CPCS)
See EVAPORATIVE (EVAP) EMISSIONS SYSTEM under EMISSION
SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump Relay
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under
MISCELLANEOUS CONTROLS.
Generator
See GENERATOR under MISCELLANEOUS CONTROLS.
Idle Air Control (IAC) Motor
See IDLE SPEED under FUEL SYSTEM.
Ignition Coil
See IGNITION SYSTEM.
In-Tank Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Limp-In Mode
See LIMP-IN MODE under MISCELLANEOUS CONTROLS.
Malfunction Indicator Light (MIL)
See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.
Radiator Fan Relay
See RADIATOR FAN RELAY under MISCELLANEOUS CONTROLS.
Serial Communications Interface (SCI) Transmit
See SERIAL COMMUNICATIONS INTERFACE (SCI) under SELF-
DIAGNOSTIC SYSTEM.
Shift Indicator Light
See SHIFT INDICATOR LIGHT under MISCELLANEOUS CONTROLS.
Speed Control Servo
See SPEED CONTROL SERVO under MISCELLANEOUS CONTROLS.
Tachometer
See TACHOMETER under MISCELLANEOUS CONTROLS.
Torque Converter Clutch (TCC) Solenoid
See TORQUE CONVERTER CLUTCH (TCC) SOLENOID under
MISCELLANEOUS CONTROLS.

Transmission Governor Pressure Solenoid
See TRANSMISSION GOVERNOR SOLENOID under MISCELLANEOUS
CONTROLS.
Transmission Overdrive/Override (OD/OR) Switch Indicator
Light
See TRANSMISSION OVERDRIVE/OVERRIDE (OD/OR) SWITCH INDICATOR
under MISCELLANEOUS CONTROLS.
Transmission Overdrive (OD) Solenoid
See TRANSMISSION OVERDRIVE (OD) SOLENOID under MISCELLANEOUS
CONTROLS.
FUEL SYSTEM
FUEL DELIVERY
Auto Shutdown (ASD) Relay
See AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY under
MISCELLANEOUS CONTROLS.
Fuel Pressure Regulator
Fuel pressure regulator is a mechanical device, used to
maintain a constant pressure across fuel injector tip. Spring and
rubber diaphragm will move from an open to closed position keeping
fuel pressure constant. Excess fuel is returned to fuel tank.
Regulator is located in in-tank fuel pump module. Regulator
includes an internal fuel filter. Excess fuel is routed directly into
fuel tank without using a return line. See Fig. 2.
Fig. 2: Cross-Sectional View Of In-Tank Fuel Filter/Fuel Pressure
Regulator (Typical)
Courtesy of Chrysler Corp.
In-Tank Fuel Pump

Fuel pump is a positive displacement, immersible pump with a
permanent magnet electric motor. Fuel is drawn in through a separate
filter/strainer at bottom of fuel pump and pushed through filter to
fuel outlet line (to fuel injectors). Voltage to operate pump is
supplied from fuel pump relay. On some models, fuel pump relay is
activated by ASD relay.
Fuel pump module includes a combination fuel filter/fuel
pressure regulator, fuel pump reservoir, a separate in-tank fuel
filter, pressure relief/rollover valve, fuel gauge sending unit and
fuel supply line. See Fig. 3.
Fig. 3: Identifying Fuel Pump Module Components (Typical)
Courtesy of Chrysler Corp.
FUEL CONTROL
Fuel Injectors
Fuel injectors are electric solenoid valves controlled by
PCM. PCM determines when and length of time (pulse width) injectors
should operate by switching ground path on and off. During start-up,
battery voltage is supplied to injectors through ASD relay. On some
models, battery voltage is supplied by charging system once engine is

operating. When ground is supplied to injector by PCM, armature and
pintle inside injector move a short distance against spring and open a
small orifice. Since fuel is under high pressure, a fine spray is
developed.
Modes Of Operation
As input signals to PCM change, PCM adjusts its response to
output devices. Modes of operation come in 2 types, open loop and
closed loop. In open loop mode, PCM is not using input from HO2S and
is responding to preset programming to determine injector pulse width
and ignition timing. In closed loop mode, PCM adjusts ignition timing
and uses input from HO2S to fine tune injector pulse width.
The following inputs may be used to determine PCM mode:
* A/C Control Positions
* A/C Switch
* Battery Voltage
* Brake Switch
* Camshaft Position (CMP) Sensor
* Crankshaft Position (CKP) Sensor
* Engine Coolant Temperature (ECT) Sensor
* Engine Speed (RPM)
* Heated Oxygen Sensor (HO2S)
* Intake Air Temperature (IAT) Sensor
* Manifold Absolute Pressure (MAP) Sensor
* Park/Neutral (P/N) Switch
* Starter Relay
* Throttle Position (TP) Sensor
* Vehicle Speed Sensor (VSS)
From these inputs, PCM determines which mode vehicle is in
and responds appropriately. Not all inputs are used in all modes or by
all models. Modes of operation are:
* Ignition Switch On (Engine Not Running) - This is an open
loop mode. PCM pre-positions IAC motor based on ECT sensor
input. PCM determines atmospheric pressure from MAP sensor
and determines basic fuel strategy. PCM modifies fuel
strategy according to IAT sensor, ECT sensor and TP sensor
inputs. PCM activates ASD relay, which in turn activates fuel
pump for only 2 seconds unless engine is cranked. PCM also
energizes HO2S heater element for approximately 2 seconds
unless engine is cranked.
* Engine Start-Up - This is an open loop mode. When starter is
engaged, PCM receives input from battery voltage, ignition
switch, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP
sensor and TP sensor. Based on these inputs, voltage is
applied to fuel injectors with PCM controlling injection
sequence, rate, and pulse width. PCM provides ground for
injectors to fire in proper order.
PCM determines proper ignition timing according to input
received from CKP sensor. If PCM does not receive CKP sensor signal
within 3 seconds after engine begins cranking, fuel injection system
is shut down and a Diagnostic Trouble Code (FTC) is set in PCM memory.\
* Engine Warm-Up - This is an open loop mode. PCM determines
injector pulse width using input information from battery
voltage, CKP sensor, CMP sensor, ECT sensor, IAT sensor, MAP
sensor and TP sensor. PCM also monitors A/C request and P/N
switch (A/T only) for fuel calculation. PCM controls engine
idle speed through IAC motor. PCM controls ignition timing
based on CKP sensor input.

PCM also operates A/C compressor clutch (if A/C is requested)\
through A/C clutch relay. When engine reaches operating temperature,
vehicle will go into idle mode and PCM will begin monitoring HO2S
input and go into closed loop operation.
* Idle - When engine is at operating temperature, this is a
closed loop mode. In idle mode, PCM now adds HO2S signal to
array of inputs used in ENGINE WARM-UP mode. PCM maintains
correct air/fuel ratio by adjusting injector pulse width and
ignition timing. PCM also controls A/C clutch operation (if
A/C is requested).
* Cruise - When engine is at operating temperature, this is a
closed loop mode. Using information from A/C switch, battery
voltage, CKP sensor, ECT sensor, IAT sensor, MAP sensor and
CMP sensor. PCM also monitors A/C request and P/N switch (A/T
only), TP sensor and VSS signals for fuel calculation. PCM
monitors HO2S and adjusts air/fuel ratio as needed. PCM
controls engine idle speed through IAC motor. PCM controls
spark advance as necessary.
* Acceleration - This is an open loop mode. When PCM
recognizes an abrupt increase in throttle position or
manifold pressure as a demand for increased engine output, it
increases injector pulse width in response to increased fuel
demand. HO2S signals are ignored.
* Deceleration - This is an open loop mode when engine is at
operating temperature and under deceleration. When PCM
receives inputs signaling a closed throttle and an abrupt
decrease in manifold pressure, it reduces injector pulse
width to lean air/fuel mixture. Under certain RPM and closed
throttle position conditions, HO2S signals are ignored and
PCM cuts off fuel injection until idle speed is reached. PCM
also drives IAC motor for smooth transition to idle mode.
* Wide Open Throttle - This is an open loop mode. When PCM
senses wide open throttle, it grounds fuel injectors in
sequence, it ignores HO2S input and it controls pulse width
to supply a pre-determined amount of additional fuel. PCM
also adjusts spark advance and disengages A/C clutch for
approximately 15 seconds.
* Ignition Switch Off - This is an open loop mode. PCM drives
IAC motor into position in anticipation of next start-up. All
outputs are turned off, no inputs are monitored and PCM shuts
down.
Sequential Fuel Injection (SFI)
Individual, electrically pulsed injectors (one per cylinder)
are located in intake manifold runners. These injectors are next to
intake valves in intake manifold. PCM controls injection timing based
on crankshaft position signal input. PCM regulates air/fuel mixture by
length of time injector stays open (pulse width) based on inputs from
HO2S, ECT sensor, MAP and other sensors.
IDLE SPEED
NOTE: DO NOT attempt to correct a high idle speed condition by
turning factory sealed throttle body throttle plate set
screw. This will not change idle speed of warm engine, but
may cause cold start problems due to restricted airflow.
Idle Air Control (IAC) Motor
IAC motor adjusts idle speed to compensate for engine load
and ambient temperature by adjusting amount of air flowing through by-
pass in back of throttle body. PCM uses ECT sensor, VSS, TP sensor and

various switch input operations to adjust IAC motor to obtain optimum
idle conditions. Deceleration stall is prevented by increasing airflow
when throttle is closed suddenly.
IGNITION SYSTEM
NOTE: Pickup equipped with 8.0L engine uses Distributorless
Ignition system (DIS). All other models use a Hall Effect
ignition system.
The PCM completely controls ignition system. During
crank/start mode, PCM will set a fixed amount of spark advance for an
efficient engine start. Amount of spark advance or retard is
determined by inputs that PCM receives from ECT sensor, engine vacuum
and engine RPM. During engine operation, PCM can supply an infinite
number of advance curves to ensure proper engine operation.
DISTRIBUTORLESS IGNITION SYSTEM (DIS)
DIS eliminates mechanical ignition components that can wear
out. PCM has complete ignition control and uses a coil pack, CMP
sensor and CKP sensor to control ignition timing. CMP sensor reads
slots in cam timing sprocket. PCM uses this information along with
information from CKP sensor to determine if fuel injectors and
ignition coils are properly sequenced for correct cylinders.
Basic timing is determined by CKP sensor position and is not
adjustable. One complete engine revolution may be required for PCM to
determine crankshaft position during cranking.
Molded ignition coils are used. Each coil fires 2 paired
spark plugs at the same time. One cylinder is on compression stroke
and other cylinder is on exhaust stroke.
HALL EFFECT IGNITION SYSTEM
This system is equipped with a Hall Effect distributor. See
Fig. 1 . Shutter(s) attached to distributor shaft rotate through
distributor Hall Effect switch, also referred to as a CMP sensor,
which contains a distributor pick-up (a Hall Effect device and
magnet). As shutter blade(s) pass through pick-up, magnetic field is
interrupted and voltage is toggled between high and low. PCM uses this
cylinder position data from CMP sensor, along with engine speed (RPM)
and CKP sensor data, to control ignition timing and injector pulse
width to maintain optimum driveability.
EMISSION SYSTEMS
Vehicles are equipped with different combinations of emission
system components. Not all components are used on all models. To
determine component usage on a specific model, see EMISSION
APPLICATIONS - TRUCKS article.
AIR INJECTION SYSTEM
This system adds a controlled amount of air to exhaust gases,
through air relief valve and check valves, to assist oxidation of
hydrocarbons and carbon monoxide in exhaust stream. Air is injected at
catalytic converters.
CRANKCASE VENTILATION (CCV) SYSTEM
CCV system performs same function as a conventional Positive

Crankcase Ventilation (PCV) system, but does not use a vacuum
controlled valve. See POSITIVE CRANKCASE VENTILATION (PCV).
EVAPORATIVE (EVAP) EMISSIONS SYSTEM
This system stores fuel vapors from fuel tank, preventing
vapors from reaching the atmosphere. As fuel evaporates inside fuel
tank, vapors are routed through vent hoses to charcoal canister where
they are stored until engine is started.
Evaporative Canister Purge Control Solenoid (EVAP-CPCS)
Charcoal canister purging is controlled by PCM through an
EVAP-CPCS. During engine warm-up and for a short period after hot
restarts, PCM energizes EVAP-CPCS, interrupting engine vacuum signal
to charcoal canister.
After engine reaches a predetermined operating temperature
and PCM internal timer has expired, PCM will de-energize EVAP-CPCS,
allowing engine vacuum to purge charcoal canister. EVAP-CPCS will also
be de-energized during certain idle conditions so PCM can update fuel
delivery calibration.
POSITIVE CRANKCASE VENTILATION (PCV)
PCV system uses a vacuum operated valve. A closed engine
crankcase breather/filter, with a hose connecting it to air filter
housing, provides source of air for system. Crankcase blow-by gases
are removed from crankcase through PCV valve with manifold vacuum.
These gases are introduced into incoming air/fuel mixture and become
part of the calibrated mixture.
A non-vacuum operated Crankcase Ventilation (CCV) system is
used on some engines, see CRANKCASE VENTILATION (CCV) SYSTEM.
SELF-DIAGNOSTIC SYSTEM
The PCM monitors several different circuits of engine control
system. If a problem is sensed with a monitored circuit, PCM will
store a Diagnostic Trouble Code (FTC) to aid technician in diagnosis
of system. The Malfunction Indicator Light (MIL), or a scan tool can
be used to read DTCs. For additional information, see SELF-DIAGNOSTICS
- JEEP, TRUCKS & RWD VANS article.
MALFUNCTION INDICATOR LIGHT
Malfunction Indicator Light (MIL) comes on and remains on for\
3 seconds as a bulb test each time ignition switch is turned to ON
position. If PCM receives an incorrect signal or receives no signal
from battery voltage input, charging system, ECT sensor, MAP sensor or
TP sensor, MIL will come on. MIL will also come on if certain
emission-related faults exist. This warns driver that PCM is in limp-
in mode and immediate repairs are necessary. See LIMP-IN MODE under
MISCELLANEOUS CONTROLS. MIL can also be used to display Diagnostic
Trouble Codes (DTCs). For additional information, see SELF-DIAGNOSTICS\
- JEEP, TRUCKS & RWD VANS article.
SERIAL COMMUNICATIONS INTERFACE (SCI)
SCI circuit is used by PCM to send data to and receive data
and sensor activation signals from scan tool. Scan tool uses signals
sent on SCI to display fault messages or Diagnostic Trouble Codes
(DTCs), sensor voltages and device states (On/Off). Scan tool uses S\
CI
to send solenoid and switch activation commands to PCM so that devices
and circuits can be tested. SCI is also used to write SRI mileage to

PCM.
MISCELLANEOUS CONTROLS
NOTE: Although not strictly considered part of engine performance
system, some controlled devices can adversely affect
driveability if they malfunction.
A/C CLUTCH RELAY
A/C clutch relay is controlled by PCM. When A/C or Defrost
mode is selected and PCM receives A/C request signal from evaporator
switch, PCM will cycle clutch on and off through A/C clutch relay.
When this relay is energized during engine operation, PCM will
determine correct engine idle speed through IAC motor.
When PCM senses low idle speed or wide open throttle through
TP sensor, PCM will de-energize A/C clutch relay, preventing A/C
operation.
AUTO SHUTDOWN (ASD) RELAY & FUEL PUMP RELAY
ASD relay and electric fuel pump relay are energized when
ignition is on. These relays are controlled through PCM by switching a
common ground circuit on and off. Following components are controlled
by ASD and fuel pump relays:
* Electric Fuel Pump
* Fuel Injectors
* Generator Field Winding
* Ignition Coil(s)
* HO2S Heating Element
When ignition switch is turned to RUN position, PCM energizes
ASD relay and electric fuel pump relay which powers these components.
If PCM does not receive a CMP and CKP sensor signal within one second
of engine cranking (start-up), PCM will turn ground circuit off and
de-energize ASD relay.
GENERATOR
Powertrain Control Module (PCM) regulates charging system
voltage.
LIMP-IN MODE
Limp-in mode is the attempt by PCM to compensate for failure
of certain components by substituting information from other sources
so that vehicle can still be operated. If PCM senses incorrect data or
no data at all from MAP sensor, TP sensor, ECT sensor or battery
voltage, system is placed into limp-in mode and Malfunction Indicator
Light (MIL) on instrument panel comes on.
If faulty sensor comes back on line, PCM will resume closed
loop operation. On some vehicles, MIL will remain on until ignition is
shut off and vehicle is restarted. To prevent damage to catalytic
converter, vehicle should NOT be driven for extended periods in limp-
in mode.
RADIATOR FAN RELAY
Electric cooling fan is used only on Dakota. Using
information supplied by A/C signal (if equipped), ECT sensor, and VSS,\

PCM controls operation of electric cooling fan. PCM operates fan
through radiator fan relay by grounding or ungrounding relay control
circuit. PCM regulates engine idle speed through IAC motor when fan is
on.
SHIFT INDICATOR LIGHT
PCM provides ground for shift indicator light on models
equipped with manual transmission. Based on engine speed, throttle
position, and vehicle speed, PCM turns shift indicator light on to
advise driver to shift to a higher gear for optimum fuel economy.
SPEED CONTROL SERVO
System is electrically actuated and vacuum operated. Controls
are located on steering wheel. Controls consist of 3 buttons: OFF/ON,
RESUME/ACCEL and SET/DECEL. Speed control servo is controlled by PCM.
System will operate at 35-85 MPH.
TACHOMETER
PCM provides signal to drive tachometer.
TORQUE CONVERTER CLUTCH (TCC) SOLENOID
PCM controls torque converter lock-up through TCC solenoid.
PCM controls lock-up according to various operating conditions.
TRANSMISSION GOVERNOR SOLENOID
PCM controls solenoid to regulate line pressure for shift
control.
TRANSMISSION OVERDRIVE/OVERRIDE (OD/OR) SWITCH INDICATOR
LIGHT
PCM controls indicator light on OD/OR switch on models
equipped with overdrive automatic transmission.
TRANSMISSION OVERDRIVE (OD) SOLENOID
On models equipped with OD transmission, PCM controls 3-4 OD
upshift and downshift through OD solenoid. PCM determines optimum OD
shift scheduling for all operating conditions.