POWERTRAIN CONTROL MODULE (PCM)
The Powertrain Control Module (PCM) is a preprogrammed, dual microprocessor digital computer that receives various input signals from sensors and switches, and provides the necessary signals to output devices to achieve optimum engine performance conditions. For PCM location, see PCM LOCATION table.
The PCM has a voltage converter that converts battery voltage to a regulated 5-volt output. The regulated 5-volt output is used to power Camshaft Position (CMP) sensor, Crankshaft Position (CKP) sensor, Manifold Absolute Pressure (MAP) sensor, Throttle Position (TP) sensor, Vehicle Speed Sensor (VSS) and logic circuits.
The ignition and fuel injection systems are controlled by the PCM, based on present engine operating conditions. PCM is programmed to provide a precise amount of fuel and the correct ignition timing to meet existing engine speed and load requirements.
The PCM adjusts ignition timing based on inputs it receives from CKP sensor, CMP sensor, MAP sensor, Engine Coolant Temperature (ECT) sensor, TP sensor, VSS, transmission gear selection (A/T only) and brake switch (also known as stoplight switch).
The PCM adjusts idle speed based on inputs it receives from TP sensor, VSS, transmission gear selection (A/T only), A/C clutch switch and brake switch. The PCM also controls the speed (cruise) control system and generator charge rate by controlling the generator field.
| Application | Location |
|---|---|
| Cherokee | Left Front Fender, Near Radiator Support |
| Grand Cherokee | On Firewall, Near Coolant Bottle |
| Wrangler | On Firewall, Near Battery |
PCM LOCATION
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES , which control or produce voltage signals that are monitored by the PCM. The second category covers OUTPUT SIGNALS , which are components controlled by the PCM.
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following
A/C Switch
When A/C switch is in ON position, it sends a signal to PCM to inform PCM that A/C has been selected. PCM adjusts idle speed to a preprogrammed RPM through Idle Air Control (IAC) motor to compensate for increased engine load.
When A/C function is selected (A/C switch on), PCM receives A/C request signal from evaporator switch. This signal indicates evaporator temperature is in the proper range for A/C operation. A/C request signal is used by PCM to activate or deactivate A/C compressor clutch through A/C relay and to determine proper idle speed through IAC motor position.
If A/C low pressure switch opens (indicating a low refrigerant level) or if evaporator switch opens (indicating evaporator is not in proper temperature range), PCM will not receive A/C request signal and PCM will remove ground from A/C relay, deactivating A/C clutch.
On Cherokee equipped with A/C, whenever A/C compressor clutch is energized, PCM will also energize auxiliary radiator (cooling) fan relay. This occurs regardless of engine coolant temperature.
Automatic Shutdown (ASD) Relay Input Signal
A 12-volt sense signal is sent to PCM by ASD relay to indicate that ASD relay has been activated. ASD relay is located in Power Distribution Center (PDC). ASD relay is battery voltage source for heated oxygen sensor heating element, ignition coil, generator field winding and fuel injectors. If PCM does not see a 12-volt signal when ASD relay should be activated, a Malfunction Indicator Light (MIL) Diagnostic Trouble Code (DTC) will be set. See MALFUNCTION INDICATOR LIGHT (MIL) under SELF-DIAGNOSTIC SYSTEM.
Battery Temperature Sensor
PCM uses sensor to determine battery temperature and to control battery charging rate. Temperature data along with battery voltage signal, is used by PCM to vary charging rate. System voltage is higher at colder temperatures and is gradually reduced at warmer temperatures.
Battery Voltage Signal
PCM uses battery voltage level to regulate generator field (rotor) duty cycle and alter fuel injector pulse width according to available voltage. If battery voltage drops, PCM will increase injector on time to compensate for reduced fuel flow of injector caused by the lower voltage. This will permit injector to deliver proper amount of fuel to engine.
Brake Switch
This switch may also be referred to as a brakelight or stoplight switch. Brake switch is mounted on brake pedal support bracket, under instrument panel. PCM uses brake switch to determine when brakes are applied. When brakes are applied (brake switch on), PCM will maintain a preset idle speed using IAC motor. If PCM receives an input signal from brake switch when speed (cruise) control is on, PCM will turn speed control system off.
Camshaft Position (CMP) Sensor
On Cherokee and Wrangler, CMP sensor is located inside distributor. (Scheme 1) On Grand Cherokee equipped with 4.0L engine, CMP sensor is located on top of oil pump drive shaft assembly. (Scheme 2) On 4.7L, CMP sensor is located on front corner of right cylinder head. This Hall Effect-type sensor works in conjunction with Crankshaft Position (CKP) sensor, providing PCM with inputs necessary to establish and maintain proper fuel injector firing order.
When leading edge of pulse ring enters sync signal generator on camshaft position sensor, resulting change in magnetic field causes a 5-volt reference signal to be induced. When trailing edge of pulse ring leaves sync signal generator on CMP sensor, resulting collapse of magnetic field causes reference signal to drop to zero volts.
Scheme 1
Scheme 2
Crankshaft Position (CKP) Sensor
CKP is a Hall Effect-type sensor. On 2.5L and 4.0L, CKP sensor is mounted on left upper side of transmission bellhousing. Sensor reads slots on outer edge of flywheel/flexplate. There are 2 groups of 4 slots on 2.5L, and 3 groups of 4 slots on 4.0L. On 4.7L, CKP sensor is mounted on right rear side of block, above starter. A tonewheel with notches on its outer edge is bolted to crankshaft. Each slot or notch causes a pulse to be generated as it passes under CKP sensor. Signal generated provides engine speed and CKP information to PCM, which along with other input signals, determines fuel injection sequence, ignition signal and spark timing.
Cruise Control Switches
Cruise control switches include 5 momentary contact switches, controlling 7 different cruise control functions. These switches provide one input to PCM. PCM determines input through resistive multiplexing. PCM also uses brake switch, Park/Neutral (P/N) switch, VSS and TP sensor to control cruise control operation.
Engine Coolant Temperature (ECT) Sensor
On 2.5L and 4.0L, ECT sensor is installed on thermostat housing. On 4.7L, ECT sensor is located on intake manifold, near generator. It provides a variable signal to PCM that is used to calculate injector pulse width and ignition timing. When engine is cold, PCM will operate engine in an open loop cycle, causing slightly richer air/fuel mixture and higher idle speeds. When normal operating temperatures are reached, PCM will operate engine in a closed loop cycle. See MODES OF OPERATION under FUEL CONTROL under FUEL SYSTEM.
Heated Oxygen Sensor (HO2S)
HO2S detects oxygen content of exhaust gases and produces a voltage signal from zero to one volt. PCM uses this signal to determine system output signals which control air/fuel mixture.
Variations in voltage signal from HO2S serve as air/fuel ratio indicators. When oxygen content is low (rich mixture), voltage signal will be approximately one volt. When oxygen content is high (lean mixture), voltage signal will be approximately zero volts. By converting oxygen content to an electrical voltage, HO2S acts as a rich/lean switch.
HO2S contains a ceramic heater in sensor housing. Heater operates on 12 volts. Heater is used under all operating conditions to help HO2S heat up quicker and to maintain correct HO2S operating temperature even under extended idle conditions.
In closed loop operation, PCM monitors HO2S input (along with other sensors) and adjusts injector pulse width accordingly. During open loop operation, PCM ignores HO2S input and adjusts injector pulse width to a preprogrammed value based on other sensor inputs. See MODES OF OPERATION under FUEL CONTROL under FUEL SYSTEM.
Ignition Circuit
When ignition switch is turned to ON position, PCM receives a signal that ignition circuit has been activated. PCM will start monitoring input signals.
Intake Air Temperature (IAT) Sensor
IAT sensor is located on intake manifold, with sensor element extending into air stream. As temperature of intake air changes, sensor resistance changes. Sensor provides an analog voltage signal to PCM. This signal is used to compensate for changes in air density due to temperature. PCM uses IAT sensor inputs along with other inputs to determine injector pulse width.
Manifold Absolute Pressure (MAP) Sensor
On 2.5L and 4.0L, MAP sensor is mounted on side of throttle body and is connected to throttle body by an L-shaped fitting. On 4.7L, MAP sensor is bolted to front of intake manifold. This sensor is used by PCM to calibrate amount of air/fuel mixture supplied to engine. Sensor measures manifold absolute pressure. Ambient barometric pressure is also measured when ignition switch is first turned on and during engine cranking.
Overdrive/Override (OD/OR) Switch (A/T)
OD/OR switch is used on Grand Cherokee. PCM controls 3-4 O/D up-shift and down-shift through OD solenoid, located on transmission. OD/OR switch is a push-button switch, located on gear shift lever. It is used to override PCM control of OD solenoid and keep transmission in a lower gear. An indicator light will come on when OD/OR switch is open and PCM OD control is not allowed.
Park/Neutral (P/N) Switch
On vehicles equipped with automatic transmission, a gear position indicator signal is sent to the PCM when gear selector lever is in Park, Neutral or Drive positions. PCM uses this signal to determine idle speed (varies with gear position), fuel injector pulse width, ignition timing advance and vehicle speed control operation.
Power Steering Pressure (PSP) Switch (2.5L)
A PSP switch is used on 2.5L with power steering. Switch is located on power steering pressure line, next to power steering pump. PSP switch sends a signal to PCM when system pressure rises to more than 250-300 psi (17.6-21.1 kg/cm 2 ). When power steering load is high, PCM increases engine idle speed through IAC motor to prevent stalling.
Serial Communications Interface (SCI) Receive
SCI receive circuit is a serial data circuit that is used when diagnosing vehicle with a scan tool. PCM receives data from scan tool through this SCI receive circuit in Data Link Connector (DLC), located under instrument panel, near steering column.
Throttle Position (TP) Sensor
TP sensor is mounted on throttle body and monitors opening angle of throttle valve. It is a variable resistor operated by opening and closing of throttle plate. PCM uses TP sensor input signal to determine throttle position under all operating conditions and adjusts fuel injector pulse width and ignition timing accordingly.
PCM supplies a 5-volt reference signal to TP sensor. TP sensor output voltage (input signal to PCM) represents throttle blade position. TP sensor output voltage varies from .26 volt at minimum throttle opening (idle) to 4.49 volts at wide open throttle.
Vehicle Speed Sensor (VSS)
On Grand Cherokee, VSS signal is sent through ABS control module by rear wheel speed sensor located on rear axle. On Cherokee and Wrangler, VSS is located on extension housing of transmission or transfer case. PCM uses VSS to determine vehicle speed and distance traveled.
VSS is an 8-pole switch which provides PCM with a pulse or switching rate, proportional to vehicle speed. By comparing number of pulses to time elapsed, PCM determines vehicle speed and distance traveled. VSS generates 8 pulses per sensor revolution. This signal, along with a closed throttle signal from TP sensor, indicates a closed throttle deceleration to PCM.
Under deceleration conditions, PCM adjusts IAC motor to maintain desired MAP value. During idle (vehicle stopped), PCM receives no signal from VSS. PCM adjusts IAC motor to maintain a desired engine speed.
Vehicle Theft Security System (VTSS)
VTSS is used on Grand Cherokee only. VTSS module provides a signal to PCM, through ignition switch circuit, to enable it to start engine. With VTSS module activated, no signal will be sensed by PCM and it will not permit vehicle to start until VTSS is deactivated.
OUTPUT SIGNALS
Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. To determine output usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. For theory and operation on each output component, refer to system indicated after component.
A/C Compressor Clutch Relay
See MISCELLANEOUS CONTROLS .
Automatic Shutdown (ASD) Relay
See FUEL DELIVERY .
Cruise Control Servo
See MISCELLANEOUS CONTROLS .
Evaporative (EVAP) Emission System
See EMISSION SYSTEM .
Fuel Injectors
See FUEL CONTROL .
Fuel Pump Relay
See FUEL DELIVERY .
Generator Field
See MISCELLANEOUS CONTROLS .
Generator Light
See MISCELLANEOUS CONTROLS .
Idle Air Control (IAC) Motor
See IDLE SPEED .
Ignition Coil
See IGNITION SYSTEM .
Injection Timing
See FUEL CONTROL .
Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM .
Radiator Cooling Fan Relay
See MISCELLANEOUS CONTROLS .
Serial Communication Interface (SCI) Transmit
See SELF-DIAGNOSTIC SYSTEM .
Upshift Indicator Light (M/T)
See MISCELLANEOUS CONTROLS .
Fuel injectors are connected to a pressurized fuel rail and positioned in intake manifold above intake valves. Fuel injector wiring harness has a tag to identify each injector (INJ 1, INJ 2, etc.).
Injectors are energized individually in a sequential order controlled by PCM. See INJECTION TIMING . Injectors are connected to 12 volts through ASD relay during start-up, and through charging system during engine operation.
Injector is energized when connected to ground through PCM. PCM also controls amount of time injector is energized (pulse width) by turning ground on and off. Pulse width is based on various inputs and is calculated by PCM.
With injector connected to a pressurized fuel supply, a fine mist will spray from injector nozzle into intake port. Injectors use an electromagnet and spring pressure to open or close fuel metering plunger. When connected to battery voltage, coil of wire in injector becomes an electromagnet. Magnetic field generated will overcome spring pressure and raise plunger off its seat. When injector circuit is opened by PCM, magnetic field collapses and spring pressure forces plunger against its seat.
Whenever an injector is opened, it will always spray a consistent amount of fuel for a given amount of pressure. Because pressure drop across injector is fixed and fuel flow rate constant, only control variable is amount of time injector is open. By controlling time injector is open, PCM can decrease pulse width for engine idle or it can increase pulse width at wide open throttle.
All engines use a Sequential Fuel Injection (SFI) system. Injectors have a specific firing order and fuel injection is timed to piston movement. Spark plugs and injectors are fired in order
- 1-3-4-2 on 2.5L.
- 1-5-3-6-2-4 on 4.0L.
- 1-8-4-3-6-5-7-2 on 4.7L.
In order for PCM to fire injectors in a specific order timed to crankshaft and piston movement, it has to establish a reference point. Establishing a reference point requires PCM inputs from the CKP sensor and CMP sensor. PCM will not allow engine to operate if CKP sensor signal is not received.
On 2.5L and 4.0L, CKP sensor is mounted on left upper side of transmission bellhousing. Sensor reads slots on outer edge of flywheel/flexplate. There are 2 groups of 4 slots on 2.5L, and 3 groups of 4 slots on 4.0L. On 4.7L, CKP sensor is mounted on right rear side of block, above starter. A tonewheel with notches on its outer edge is bolted to crankshaft. Each slot or notch causes a pulse to be generated as it passes under CKP sensor. CKP sensor provides PCM with crankshaft angle and speed. PCM converts crankshaft speed into engine RPM and crankshaft angle into piston position.
Ignition Switch On (Engine Not Running)
This is an open loop mode. PCM pre-positions IAC motor based on engine coolant temperature. PCM determines atmospheric pressure from MAP sensor and determines basic fuel strategy. PCM modifies fuel strategy according to intake air temperature, coolant temperature and throttle position sensor inputs. PCM activates auto shutdown relay, which in turn activates fuel pump for only 3 seconds unless engine is cranked. Fuel pump relay also energizes HO2S heater element for approximately 2 seconds unless engine is cranked, but PCM does not use HO2S input to calibrate air/fuel ratio during this mode.
Engine Start-Up
This is an open loop mode. When starter is engaged, PCM receives input from battery voltage, starter relay, 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 (DTC) 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, CMP sensor, CKP 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, PCM will begin monitoring HO2S input and go into closed loop operation.
Idle
This is a closed loop mode when engine is at operating temperature. 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, CMP sensor, CKP sensor, ECT sensor, IAT sensor and MAP 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.
Deceleration
This is an open loop mode with engine at operating temperature. During hard deceleration, PCM receives inputs from A/C switch, battery voltage, ECT sensor, CKP sensor, IAT sensor, MAP sensor, TP sensor, CMP sensor, P/N sensor and VSS signal. PCM will ignore HO2S signal and enter a fuel cut-off strategy when vehicle is under hard deceleration with proper RPM and closed throttle conditions. If hard deceleration does not exist, PCM will determine proper injector pulse width. PCM will adjust IAC motor and ignition timing based on above inputs.
Wide Open Throttle (WOT)
This is an open loop mode. PCM receives inputs from battery voltage, CKP sensor, ECT sensor, IAT sensor, MAP sensor, TP sensor and CMP sensor. When PCM senses WOT, it grounds fuel injectors in sequence, it ignores HO2S input and it controls pulse width to supply a predetermined amount of additional fuel.
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.
ASD relay is located in Power Distribution Center (PDC), near radiator coolant recovery bottle. For ASD relay location within PDC, refer to PDC cover label.
ASD relay supplies battery voltage to fuel pump, fuel injectors, generator field winding, HO2S heater element and ignition coil. Relay contacts are normally open.
Power is supplied to relay coil when ignition is turned on. PCM controls ground circuit, which energizes relay coil and closes relay contacts.
PCM will only ground relay when ignition switch is in RUN or START positions, and activity is sensed through CMP sensor in distributor and CKP sensor. If PCM senses RPM signal has stopped, it will remove ground from relay coil, which will cause contacts to open and remove power from circuit.
Fuel Pump Module
All models are equipped with an in-tank fuel pump module which includes a fuel strainer, gauge float, gauge sending unit, pressure regulator and pump. (Scheme 3)
Turbine-type electric pump is driven by a permanent magnet, 12-volt electric motor. Voltage to operate pump is supplied by PCM through a common ASD relay and fuel pump relay circuit. Fuel is drawn in through a separate filter/strainer at bottom of fuel pump and pushed through pressure regulator to fuel outlet line (to fuel injectors).
Fuel pressure regulator is a mechanical device calibrated to maintain fuel system operating pressure at 44.2-54.2 psi (3.1-3.7 kg/cm 2 ) across injector tips. If fuel pressure is more than specified, an internal rubber diaphragm and calibrated springs will move from open to closed position, returning excess fuel directly into fuel tank through regulator without using a return line. When fuel pump is not operating, some fuel pressure is maintained by a fuel pump outlet check valve to help with engine start-up.
Scheme 3
PCM energizes fuel pump using fuel pump relay. Fuel pump relay is located in PDC, near radiator coolant recovery bottle. For fuel pump relay location within PDC, refer to PDC cover label. Fuel pump relay coil is powered when ignition switch is in RUN or START position. Fuel pump relay is controlled by PCM switching fuel pump relay and common ASD relay coil ground on and off.
Fuel pump circuit is completed during cranking and whenever engine is running. If ignition switch is turned to RUN position, fuel pump will operate for one second and then shut off. If PCM does not receive a crank or run signal, it deactivates fuel pump by opening relay coil ground circuit. One second time limit is used to prevent unnecessary operation of fuel pump once system is pressurized. If engine is running, PCM maintains fuel pump relay coil ground allowing continuous operation of fuel pump.
IAC motor is mounted on throttle body and is used by PCM to adjust engine idle speed. Throttle plate regulates off-idle engine speed by controlling amount of air allowed to enter intake manifold, and is mechanically operated by accelerator cable.
PCM and IAC motor adjust engine idle by regulating size of an air by-pass passage that routes air past closed throttle plate. Amount of air flowing through by-pass depends on engine operating conditions at idle. PCM uses ECT sensor, TP sensor, VSS and various switch input operations to adjust IAC motor to obtain optimum idle conditions.
When engine is cold, PCM increases engine speed by retracting IAC motor pintle, thus allowing more air to enter intake manifold. To maintain proper air/fuel mixture, more fuel is also injected into intake manifold. Richer air/fuel mixture, in turn, raises engine idle speed. As engine warms, PCM will extend motor pintle into air passage to reduce amount of air by-passing throttle plate. Deceleration stall is prevented by increasing airflow when throttle is closed suddenly.
A factory adjusted set screw is used to limit position of throttle plate. DO NOT use this screw to adjust idle speed.
HALL EFFECT SYSTEM
All engines use a Powertrain Control Module (PCM) based ignition system. Base ignition timing is NOT adjustable with this system. On Cherokee and Wrangler, ignition system consists of CKP sensor, distributor (including rotor and CMP sensor), ECT sensor, ignition coil, MAP sensor, and TP sensor. On Grand Cherokee, a distributor is not used.
See INPUT DEVICES .
See INPUT DEVICES .
Distributor (Cherokee & Wrangler)
Note. Grand Cherokee is not equipped with a distributor. CMP sensor is located on top of oil pump drive shaft assembly. See INPUT DEVICES .
Distributor consists of camshaft position sensor, cap and rotor. (Scheme 4) Distributor does not use centrifugal or vacuum advance mechanisms to advance ignition timing. Ignition timing advance is electronically controlled by PCM. See INPUT DEVICES.
Scheme 4
Ignition Coil (Cherokee & Wrangler)
Ignition coil is constructed of epoxy-embedded windings and is not oil filled. Battery voltage is supplied to ignition coil positive terminal from ASD relay. PCM receives inputs from appropriate sensors. Based on these inputs, it then determines the proper ignition timing and interrupts the ignition coil ground signal to trigger secondary voltage of ignition coil.
Ignition Coils (Grand Cherokee)
On 4.0L, a coil rail assembly is bolted to cylinder head. Coil rail includes 3 coils, which are connected to spark plugs through cables that are integral to coil rail. One electrical connector, located at rear of coil rail, is used for all 3 coils. Coil rail must be replaced as an assembly. Battery voltage is supplied to all 3 coils through ASD relay. PCM operates coils and adjusts ignition timing by controlling coil ground circuits.
On 4.7L, 8 coils are used. Each coil is mounted directly on spark plug. Battery voltage is supplied to all 8 coils through ASD relay. PCM operates coils and adjusts ignition timing by controlling coil ground circuits.
See INPUT DEVICES .
See INPUT DEVICES .
PCM opens and closes ignition coil ground circuit to adjust ignition timing according to engine operating conditions. Amount of electronic spark advance provided by PCM is determined by CKP sensor (engine RPM), ECT sensor, MAP sensor and TP sensor inputs. For additional information, see POWERTRAIN CONTROL MODULE (PCM) under COMPUTERIZED ENGINE CONTROLS.
See INPUT DEVICES .
CRANKCASE VENTILATION (CCV) SYSTEM
CCV system is used on 2.5L and 4.0L engines. It performs same function as a conventional PCV system, but does not use a vacuum controlled valve. See POSITIVE CRANKCASE VENTILATION (PCV) . On 2.5L, a fitting on driver's side of valve cover contains a metered orifice that is connected to manifold vacuum. On 4.0L, a molded vacuum tube connects manifold vacuum to top, rear of valve cover. Molded vacuum tube contains a fixed, calibrated orifice that meters amount of crankcase vapors drawn out of engine.
On both engines, a fresh air supply hose from air cleaner is connected to top, front of valve cover. When engine is running, fresh air enters engine and mixes with crankcase vapors. Manifold vacuum then draws crankcase vapors/air mixture through fixed orifice and into engine by intake manifold vacuum.
POSITIVE CRANKCASE VENTILATION (PCV)
4.7L engine is equipped with a closed Positive Crankcase Ventilation (PCV) system. This system consists of a PCV valve with a hose connected to intake manifold. Fresh air source is provided by a closed engine crankcase breather/filter with a hose connected to air cleaner housing. Filtered air is routed into crankcase through crankcase breather/filter, forcing crankcase vapors out through PCV valve and into intake manifold.
EVAP system stores fuel vapors from fuel tank, preventing vapors from reaching atmosphere in an EVAP canister. See EVAP CANISTER . EVAP system is a combination electrical and vacuum operated system. A duty cycle EVAP canister purge solenoid, controlled by PCM, regulates rate of vapor flow from EVAP canister to intake manifold.
PCM energizes purge solenoid by switching ground circuit on and off. PCM does not energize purge solenoid during engine cold start warm-up (open loop) or hot start time delay. When engine reaches specified temperature (closed loop) and programmed time delay ends, PCM will energize purge solenoid. Purge solenoid will be energized and de-energized 5-10 times per second, depending on operating conditions. PCM varies vapor flow rate by controlling amount of time solenoid is energized (pulse width).
LEAK DETECTION PUMP (LDP)
LDP incorporates 2 primary functions: it detects a leak in EVAP system, and it seals EVAP system so leak detection test can be run.
Primary LDP components are a 3-port solenoid that activates both primary functions, a pump with an LDP switch that monitors pump movement, 2 check valves and spring diaphragm, and a canister vent seal containing a spring-loaded vent seal valve.
Immediately after a cold start, with engine temperature of 40-86°F (4.4-30.0°C), 3-port solenoid is briefly energized allowing engine vacuum to enter pump and draw diaphragm up. This allows air to be drawn into LDP cavity. When solenoid is de-energized, it vents engine vacuum, permitting diaphragm to drop down, forcing air out of LDP. Repeated energizing and de-energizing cycles create flow in pump fashion.
After passing leak detection phase of test, system pressure is maintained until EVAP purge system is activated, creating an artificial leak. If cycle rate increases due to flow of EVAP purge system, LDP test passes. If LDP test does not pass, an obstruction exists in system.
EVAP CANISTER
As fuel evaporates inside fuel tank, vapors are routed through vent hoses to EVAP canister, where they are stored until engine is started. EVAP canister is filled with granules of an activated carbon mixture. Fuel vapors entering canister are absorbed by granules. EVAP canister has one inlet connected to pressure relief/rollover valves of fuel tank through hoses and tubes. See PRESSURE RELIEF/ROLLOVER VALVE .
PRESSURE RELIEF/ROLLOVER VALVE
Fuel tanks are equipped with a pressure relief/rollover valve. Dual-function valve relieves fuel tank pressure and prevents fuel flow through fuel tank vent hoses in case of a vehicle rollover. Valve consists of a plunger, spring, orifice and guide plate. Valve is normally open, allowing fuel vapors to vent to EVAP canister where they are stored.
If bottom of plunger is contacted by sloshing fuel in fuel tank, plunger seats in guide plate, preventing liquid fuel from reaching EVAP canister. In a vehicle rollover, valve is inverted. This forces plunger against guide plate and fuel is prevented from flowing through valve orifice and into fuel tank vent tube.
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 sets a DTC that affects tailpipe emissions, it illuminates MIL. Some monitors require 2 consecutive trips, with a detected fault, to illuminate MIL. MIL stays on continuously when PCM has entered a Limp-In mode or identified a failed emission component. MIL will flash or illuminate continuously when PCM detects an engine misfire.
PCM may turn off MIL if malfunction is not detected for 3 consecutive trips, or if PCM does not detect a malfunction during 3 consecutive misfire or fuel system tests. PCM performs these tests when engine is operating within 375 RPM and 10 percent of load when malfunction was first detected.
Misfire Monitor
PCM monitors for engine misfire during most operating conditions by looking at changes in crankshaft speed. If a misfire occurs, crankshaft speed will vary more than normal.
Fuel System Monitor
By comparing oxygen sensor input to long-term adaptive memory, PCM can determine whether vehicle will pass an emissions test. If a malfunction occurs, and PCM cannot maintain the optimum air/fuel ratio, MIL will be illuminated.
Catalyst Monitor
Catalyst monitor uses 2 oxygen sensors to monitor efficiency of catalytic converter. By monitoring oxygen storage capacity of a catalyst, efficiency can be calculated. PCM compares number of lean-to-rich switches between upstream and downstream oxygen sensors. When catalyst efficiency deteriorates and emissions increase beyond legal limit, MIL will be illuminated.
TRIP DEFINITION
"Trip" has different meanings depending on what the circumstances are. When MIL is off, a trip is defined as when the Oxygen Sensor Monitor and Catalyst Monitor have been completed in the same drive cycle.
When any emissions DTC is set, MIL will be illuminated. When MIL is on, it takes 3 good trips to extinguish MIL. For Fuel System Monitor or Misfire Monitor (continuous monitors), vehicle must be operated in similar conditions for a specified amount of time to be considered a Good Trip.
If a Non-Continuous OBD-II monitor fails 2 consecutive times and illuminates MIL, rerunning that monitor on next start-up, and passing the monitor, is considered to be a Good Trip. Non-Continuous OBD-II monitors include Oxygen Sensor Monitor, Catalyst Monitor, Purge Flow Monitor, Leak Detection Pump Monitor, EGR Monitor and Oxygen Sensor Heater Monitor.
If any other Emission DTC is set (not an OBD-II monitor), a Good Trip is completed when Oxygen Sensor Monitor and Catalyst Monitor have been completed, or 2 minutes of engine run time if Oxygen Sensor Monitor or Catalyst Monitor have been stopped from running.
It can take up to 2 consecutive failures to illuminate MIL. After MIL is illuminated, it takes 3 Good Trips to extinguish MIL. After MIL is off, PCM will erase DTC after 40 warm-up cycles. A warm-up cycle is counted when ECT sensor has crossed 160°F (71°C) and increased by at least 40°F (4.4°C).
SERIAL COMMUNICATIONS INTERFACE (SCI) TRANSMIT
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 SCI 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. See CHRYSLER CORP. - SELF-DIAGNOSTICS - JEEP, TRUCKS & RWD VANS article.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/C clutch relay is controlled by PCM. When A/C 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. PCM also de-energizes relay if engine coolant temperature is more than 257°F (125°C). On Cherokee equipped with A/C, whenever A/C compressor clutch is energized, PCM will also energize auxiliary radiator (cooling) fan relay. This occurs regardless of engine coolant temperature.
Generator output is controlled by PCM based on battery voltage and temperature input. PCM keeps charging system output voltage at 12.9-15.0 volts. Charging system output voltage is controlled by generator voltage regulator (field control), located within PCM. Voltage determined by PCM as final goal for charging system is called "control" voltage. Control voltage will be used to determine generator field control and to detect if charging system is operating properly.
If sensed voltage is lower than control voltage, PCM will alter duty cycle and ground generator (rotor) field for a longer period of time and create a higher generator output which should raise sensed voltage level. If sensed voltage is higher than control voltage, PCM will alter duty cycle and lower generator output, which should lower sensed voltage level.
Generator light (battery symbol) on base instrument cluster will come on if PCM senses a low charging condition. Light may also momentarily come on at low idle with all accessories turned on. Once PCM compensates for accessory electrical load, generator light should go out.
An electric radiator cooling fan is used on Cherokee 4.0L models with A/C and/or heavy-duty cooling. PCM operates cooling fan, regardless of temperature, whenever A/C compressor clutch is activated. Normal operation of the radiator cooling fan is controlled by PCM through ECT sensor input.
When engine coolant temperature reaches 217°F (103°C), PCM supplies radiator cooling fan relay with a ground path. Radiator cooling fan relay contacts close and allow battery voltage from ignition switch to reach radiator cooling fan motor. When coolant temperature drops to 208°F (98°C), PCM will remove radiator cooling fan relay ground path.
Vacuum operated cruise control system is electronically controlled by PCM. Controls are located on steering wheel and consist of 3 control switches. OFF/ON, SET/COAST and RESUME/ACCEL switches send signals to PCM. PCM controls vacuum and vent solenoids, located inside cruise control servo, when operating cruise control system. System will operate at 35-85 MPH. When brake pedal is depressed enough to activate brake switch, PCM will release vacuum and disengage cruise control.
UPSHIFT INDICATOR LIGHT
Vehicles equipped with M/T have an upshift light (arrow up symbol) located in instrument cluster. Light is controlled by PCM, which turns on light to inform driver when to shift to next higher gear for best fuel economy. PCM determines which gear should be used by observing and remembering RPM and MAP values.