Engine Controls - Theory & Operation: Other Jeep Cherokee III

POWERTRAIN CONTROL MODULE (PCM)

The Powertrain Control Module (PCM) is a preprogrammed, triple microprocessor digital computer. For PCM location, see PCM LOCATION table. The PCM controls A/C compressor clutch, air/fuel mixture, charging system, emission control components, idle speed, ignition timing, speed control (cruise) and some transmission functions. The PCM can adapt it's operation to handle changing operating conditions.

The PCM receives input signals from various sensors and switches. The sensors and switches that provide inputs to PCM are referred to as PCM inputs. The PCM controls various engine and vehicle operations through different system components. These components are referred to as PCM outputs. The PCM adjusts fuel system, charging system and ignition timing based on inputs from various sensors and switches.

PCM LOCATION

INPUT DEVICES

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.

Note. 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 WIRING DIAGRAMS article.

Auto Shutdown Relay

A 12-volt signal indicates to the PCM that the Auto Shutdown (ASD) relay is activated. The ASD relay is used to connect the oxygen sensor heater elements, ignition coil and fuel injectors to a 12-volt power supply. The PCM uses this input to sense that the ASD relay is energized. If the PCM does not see 12 volts at this input when ASD relay is energized, a Diagnostic Trouble Code (DTC) will set.

Battery Voltage

Battery voltage input supplies power to the PCM. It also tells the PCM what battery voltage is available to the fuel injectors and ignition coil. If battery voltage is low, the PCM will increase fuel injector pulse width. This will compensate for the decreased flow throughout the fuel injectors if battery voltage is low.

Brake Switch

When brake switch is pressed, PCM receives an input that the brakes are being applied. After receiving this input, the PCM will control idle speed using the Idle Air Control (IAC) motor. This input is also used to disable vent and vacuum solenoid for speed control servo.

Camshaft Position Sensor (2.5L)

Camshaft Position (CMP) sensor is located in the distributor on right side of engine. The CMP sensor is made up of a Hall Effect switch (sync signal generator) and a rotating pulse ring (shutter) on the distributor shaft. See Distributor (Wrangler 2.5L) under IGNITION SYSTEMS. The pulse ring rotates 180 degrees through the sync signal generator. Signal is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events. CMP sensor signal is also used to synchronize fuel injectors with their respective cylinders.

Camshaft Position Sensor (4.0L)

Camshaft Position (CMP) sensor is mounted on top of oil pump drive shaft assembly and is located on right side of engine, near oil filter. The CMP sensor is made up of a Hall Effect switch (sync signal generator) and a rotating pulse ring (shutter) on top of oil pump drive shaft assembly. The pulse ring rotates 180 degrees through the sync signal generator. Signal is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events. CMP sensor signal is also used to synchronize fuel injectors with their respective cylinders.

Camshaft Position Sensor (4.7L)

Camshaft Position (CMP) sensor is mounted on right front corner of right cylinder head. The CMP sensor is made up of a Hall Effect switch (sync signal generator). The sync signal generator detects notches located on a tonewheel. The tonewheel is located on front of camshaft for right cylinder head. As the tonewheel rotates, the notches pass through the sync signal generator. Signal is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events

Crankshaft Position Sensor

On 2.5L and 4.0L, Crankshaft Position (CKP) sensor is mounted to left side of transmission bellhousing. On 4.7L, CKP sensor is bolted to side of engine cylinder block, above starter motor. On all models, CKP sensor is a Hall Effect-type sensor that reads slots on outer edge of flywheel/flexplate. Each slot causes a pulse to be generated as it passes under CKP sensor. Signal generated provides engine speed and crankshaft position information to the PCM, which along with other input signals, determines fuel injection sequence and ignition timing.

5-Volt Supply

Note. There are 2 different 5-volt supply circuits: a primary circuit and a secondary circuit.

These circuits provide a 5-volt power supply to: Crankshaft Position (CKP) sensor, Camshaft Position (CMP) sensor, oil pressure sensor, Vehicle Speed Sensor (VSS) and transmission pressure sensor (RE automatic transmission). These circuits also provide a 5-volt reference voltage to Manifold Absolute Pressure (MAP) sensor and Throttle Position (TP) sensor.

Fuel Level Sensor

Fuel level sensor is located on fuel pump module and can be replaced separately. Fuel level sensor is used as an input to operate fuel gauge and for certain On-Board Diagnostic (OBD-II) emission requirements. When fuel level decreases, voltage increases to the PCM. When fuel level increases, voltage decreases to the PCM. The PCM will monitor voltage from fuel level sensor to prevent false misfire and fuel system monitor DTCs from being set. If the voltage is less than approximately 15 percent of tank capacity this monitor is turned on. If vehicle is equipped with a Leak Detection Pump (LDP), this monitor is turned on if fuel capacity is more than 85 percent of fuel tank capacity.

Engine Coolant Temperature Sensor

The Engine Coolant Temperature (ECT) sensor is a 2-wire Negative Thermal Coefficient (NTC) sensor. ECT is located in thermostat housing or near front of intake manifold. (Scheme 1)or (Scheme 2). As engine coolant temperatures increases, voltage decreases. When engine coolant temperature decreases, voltage increases. When ignition is on, PCM sends a regulated 5-volt signal to ECT sensor. When engine is cold, PCM operates in open loop. PCM will command richer air/fuel mixtures and higher idle speeds. When engine reaches operating temperature, PCM will use input from the ECT sensor for the following calculations.

1. ASD Relay Shutdown Times
2. EVAP Purge Solenoid On/Off Times
3. Fuel Injector Pulse Width
4. Fuel Injector Pulse Width During Engine Cranking
5. Idle Air Control Motor Key-On Steps
6. Ignition Spark Advance Curve
7. Leak Detection Pump Operation
8. O2 Sensor Closed Loop Times
9. Radiator Fan Relay On/Off Times
10. Target Idle Speed

The ECT sensor resistance values are the same as the Intake Air Temperature (IAT) sensor.

Scheme 1

Scheme 2

Oxygen (O2) Sensor

Oxygen sensors are used to measure the amount of oxygen in the exhaust gases. Depending on emissions package either 2 or 4 sensors are used. All 4-cylinder engines use 2 sensors regardless of emissions package. O2 sensors are placed upstream and downstream of catalytic converter. Sensors upstream of catalytic converter are referred to as 1/1. Sensors downstream of catalytic converter are referred to as 1/2. If vehicle has more than one sensor upstream of catalytic converter, they will be referred to as 1/1 (left bank) and 1/2 (right bank). If vehicle has more than one sensor downstream of catalytic converter, they will be referred to as 2/1 (left bank) and 2/2 (right bank).

O2 sensors produce their own voltage which varies from zero to one volt. The voltage varies in correlation to oxygen content in exhaust gas. If a lot of oxygen is present in exhaust gas, voltage produced will be near zero volts. If very little oxygen is present in exhaust gas, voltage produced will be near one volt. The PCM uses this voltage to adjust air/fuel mixture. O2 sensors have an opening (near sensor wiring harness) to monitor air outside the sensor. The sensors compare outside oxygen content with oxygen content in the exhaust gas and produce a voltage accordingly.

For O2 sensors to produce a voltage they must maintain an operating temperature of 930-1100°F (499-593°C). O2 sensors use a heater circuit to heat up quickly and maintain operating temperature. The heater circuit is a 12-volt circuit. Power for the heater circuit is controlled through the Auto Shutdown (ASD) relay on certain non-California emissions packages. The ASD relay is controlled by the PCM. On other non-California emissions packages, heater circuit is controlled through O2 sensor heater relays. These relays are also controlled by the PCM. On all emissions packages, the O2 sensor heater is a Positive Thermal Coefficient (PTC) element. As temperature increases, resistance increases. When temperature decreases, resistance decreases.

O2 sensors work by the same principal, however a sensor used upstream of a catalytic converter cannot be interchanged with a sensor used downstream of a catalytic converter. There are physical differences preventing interchangeability. An upstream O2 sensor is used by the PCM to monitor oxygen content before the catalyst. The O2 sensor voltage input is used by the PCM to adjust air/fuel mixture to make the downstream O2 sensor voltage correct. The upstream O2 sensor is also used to determine catalyst efficiency. A downstream O2 sensor is also used by the PCM to monitor oxygen content after the catalyst. The O2 sensor voltage input is used by the PCM to determine air/fuel mixture adjustments as the downstream oxygen content changes. As the air/fuel mixture is changed, the PCM monitors upstream O2 sensor voltage and changes fuel delivery until upstream O2 sensor voltage changes enough to correct downstream O2 sensor voltage. The downstream O2 sensor is also used to determine catalyst efficiency.

Ignition Circuit Sense

This circuit is used to tie the ignition switch to the PCM. When ignition switch is in RUN or START positions, battery voltage is supplied to the PCM. This is how the PCM "senses" the ignition is on. This circuit is also used to "wake up" the PCM. Voltage can be as low as 6 volts and the PCM will still function. Voltage is supplied the PCM circuit through an 8-volt regulator. This allows the PCM to perform fuel, ignition and emissions functions. The 8-volt regulator voltage also supplies power to a 5-volt regulator.

Intake Air Temperature Sensor

Intake Air Temperature (IAT) sensor is mounted in near throttle body. (Scheme 3)- (Scheme 6). The IAT sensor is a 2-wire Negative Thermal Coefficient (NTC) sensor. As intake air temperature increases, voltage decreases. As intake air temperature decreases, voltage increases. The IAT sensor provides an input to the PCM indicating intake air density based on temperature. The IAT sensor input is used by the PCM to calculate fuel injector pulse width and ignition timing spark advance. The resistance values are the same as the Engine Coolant Temperature (ECT) sensor.

Scheme 3

Scheme 4

Scheme 5

Scheme 6

Manifold Absolute Pressure Sensor

Manifold Absolute Pressure (MAP) sensor is located near throttle body or near front of intake manifold. (Scheme 1), (Scheme 3), (Scheme 4) or (Scheme 6). The MAP sensor contains a silicone based sensing unit to provide information on manifold pressure. The PCM uses this input to calculate fuel injector pulse width and ignition spark advance. When manifold pressure is the same as barometric pressure, fuel injector pulse width is adjusted to maximum. The PCM supplies the MAP sensor with a 5-volt reference signal. There is a return voltage signal from the MAP sensor which indicates manifold pressure. A zero pressure reading equals.5 volt and full scale is 4.5 volts. Voltage swing is 4 volts. This indicates a pressure swing of zero to 15 psi.

The PCM uses MAP sensor input as the number one contributing factor for adjusting fuel injector pulse width. Air density changes with altitude. The PCM must know air density to properly adjust air/fuel mixture. This input also helps PCM adjust for changing barometric pressure. Altitude and barometric pressure have an inverse relationship. As altitude increases, barometric pressure decreases. When altitude decreases, barometric pressure increases. After engine start, the PCM looks at MAP sensor input every 12 milliseconds. The PCM compares this input with what the input was at key-on. The difference between current input and input when key-on is current manifold pressure.

When ignition is first turned on with engine off, PCM updates barometric pressure. If vehicle is started and driven to a different altitude, barometric pressure needs to be updated. The update takes place in the MAP sensor memory cell. When barometric pressure is updated, the PCM can adjust air/fuel mixture more efficiently. During vehicle operation, if PCM sees Wide Open Throttle (WOT) (based on throttle position and engine RPM), barometric pressure will update. The PCM uses MAP sensor input to calculate the following.

1. Automatic Transmission Shift Point Strategies (Certain Models)
2. Barometric Pressure
3. Deceleration Fuel Shutoff
4. Engine Load
5. Fuel Injector Pulse Width
6. Idle Speed
7. Ignition Spark Advance
8. Manifold Pressure

The MAP sensor produces a voltage from a single piezoresistive element located in the center of a diaphragm. The diaphragm and element are both made of silicone. When manifold pressure changes it causes the diaphragm to deflect which stresses the silicone. Resistance in the silicone changes when stressed. The MAP sensor also contains electronics to compensate and condition the signal for temperature.

Oil Pressure Sensor

On Cherokee, oil pressure sensor is located on right side of engine near camshaft position sensor and has a Black 3-pin connector. On Grand Cherokee, oil pressure sensor is located at right rear of engine, above starter motor (4.0L) or on right side of engine on oil filter housing (4.7L). On Wrangler, oil pressure sensor is located on right side of engine near distributor (2.5L) or camshaft position sensor (4.0L) and has a Black 3-pin connector. On all models, the oil pressure sensor is a solid-state sending unit. The sensor uses 3 circuits: 5-volt power supply, sensor ground and sensor signal. The sensor signal circuit varies with engine oil pressure. This signal is transferred "bussed" to the instrument panel on either a CCD or PCI bus circuit (depending on vehicle line). The transferred signal is used to operate oil pressure gauge and CHECK GAUGES light.

Power Steering Pressure Switch

Power Steering Pressure (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.

Throttle Position Sensor

Throttle Position (TP) sensor is mounted on throttle body and monitors opening angle of throttle valve. (Scheme 3)- (Scheme 6). It is a variable resistor operated by opening and closing of the 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

On Grand Cherokee, Vehicle Speed Sensor (VSS) signal is sent through Controller Anti-Lock Brake (CAB) 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. On all models, 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.

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 WIRING DIAGRAMS article. For theory and operation on each output component, refer to system indicated after component.

A/C Clutch Relay

See A/C CLUTCH RELAY under MISCELLANEOUS CONTROLS.

Automatic Shutdown Relay

See FUEL DELIVERY .

Cruise Control Servo

See CRUISE CONTROL SERVO under MISCELLANEOUS CONTROLS.

Evaporative Emission System

See EMISSION SYSTEMS & SUB-SYSTEMS .

Fuel Injectors

See FUEL CONTROL .

Fuel Pump Relay

See FUEL DELIVERY .

Generator Field

See GENERATOR FIELD under MISCELLANEOUS CONTROLS.

Generator Light

See GENERATOR LIGHT under MISCELLANEOUS CONTROLS.

Idle Air Control Motor

See IDLE SPEED .

Ignition Coil

See IGNITION SYSTEMS .

Injection Timing

See FUEL CONTROL .

Malfunction Indicator Light

See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.

Radiator Cooling Fan Relay

See RADIATOR COOLING FAN RELAY under MISCELLANEOUS CONTROLS.

Serial Communication Interface (SCI) Transmit

See SERIAL COMMUNICATIONS INTERFACE (SCI) TRANSMIT under SELF-DIAGNOSTIC SYSTEM.

Upshift Indicator Light (M/T)

See UPSHIFT INDICATOR LIGHT under 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. 1-3-4-2 on 2.5L.
2. 1-5-3-6-2-4 on 4.0L.
3. 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. On all models, 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 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

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 Wide Open Throttle (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.

Auto Shutdown (ASD) relay is located in Power Distribution Center (PDC). PDC is located on right side of engine compartment, next to battery. 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(s). 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 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 and pump. (Scheme 7)or (Scheme 8). On Cherokee and Wrangler, fuel filter/fuel pressure regulator is mounted on top of fuel pump module. On Grand Cherokee, fuel filter/pressure regulator is located in front of fuel tank, above rear axle. (Scheme 9)

On all models, 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 fuel filter/pressure regulator to fuel outlet line (to fuel injectors).

Fuel filter/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. On Cherokee and Wrangler, excess fuel is returned directly into fuel tank through regulator without using a return line. On Grand Cherokee, excess fuel is returned to fuel tank through a fuel return line. On all models, 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 7

Scheme 8

Scheme 9

PCM energizes fuel pump using fuel pump relay. Fuel pump relay is located in PDC. PDC is located on right side of engine compartment, next to battery. 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.

Idle Air Control (IAC) Motor

IAC motor is mounted on throttle body and is used by PCM to adjust engine idle speed. (Scheme 3)- (Scheme 6). 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.

TIMING CONTROL SYSTEMS

Ignition timing is electronically controlled by Powertrain Control Module (PCM). Base ignition timing is NOT adjustable with this system. PCM controlled ignition system consists of engine coolant temperature sensor, crankshaft position sensor, distributor (2.5L), camshaft position sensor, ignition coil(s), manifold absolute pressure sensor and throttle position sensor.

Camshaft Position Sensor

See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

Distributor (Wrangler 2.5L)

Distributor consists of camshaft position sensor, pulse ring, cap and rotor. (Scheme 10) Distributor does not use centrifugal or vacuum advance mechanisms to advance ignition timing. Ignition timing advance is electronically controlled by PCM. See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

Scheme 10

See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

Ignition Coil (2.5L)

On 2.5L, a single ignition coil is used. 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, PCM then determines the proper ignition timing and interrupts the ignition coil ground signal to trigger secondary voltage of ignition coil.

Ignition Coils (4.0L)

On 4.0L, a one-piece coil rail assembly is used. Coil rail assembly is located on top of engine, right of valve cover. Coil rail includes 3 coils, which are connected to spark plugs through cables that are integral to coil rail assembly. One electrical connector, located at rear of coil rail, is used for all 3 coils. Individual coils cannot be replaced separately. If any coils are defective, coils and 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.

Ignition Coils (4.7L)

On 4.7L, 8 individual 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 under COMPUTERIZED ENGINE CONTROLS.

See INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

Powertrain Control Module

Powertrain Control Module (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 under COMPUTERIZED ENGINE CONTROLS.

CLOSED CRANKCASE VENTILATION

Closed Crankcase Ventilation (CCV) system is used on 2.5L and 4.0L engines. (Scheme 11)and (Scheme 12). It performs same function as a conventional PCV system, but does not use a vacuum controlled valve. 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.

Scheme 11

Scheme 12

POSITIVE CRANKCASE VENTILATION

The 4.7L engine is equipped with a closed Positive Crankcase Ventilation (PCV) system. (Scheme 13) 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.

Scheme 13

Evaporative (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

The Leak Detection Pump (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. On Cherokee, EVAP canister is located underneath left side of vehicle near front of rear axle. On Grand Cherokee, EVAP canister is located in engine compartment, left of brake booster. On Wrangler, EVAP canister is located on left side of engine compartment. On all models, 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 valve at 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. Rollover valve is not serviced separately. If rollover valve is defective, fuel tank must be replaced.

TRIP INDICATOR

The trip is essential for running monitors and turning off the Malfunction Indicator Light (MIL). A trip is defined as a set of vehicle operating conditions that must be met for a specific monitor to run. All trips begin with an ignition key cycle. Good trip counters are: specific good trip, fuel system good trip and misfire good trip.

1. Specific Good Trip The term good trip has different meanings depending on the circumstances. If the MIL is off, a good trip is defined as when the oxygen sensor monitor and the catalyst monitor have completed in the same drive cycle. If the MIL is on and a DTC was set by the fuel system monitor or misfire monitor, vehicle must be operated in the similar conditions window for a specified amount of time. If MIL is on and a DTC was set by a PCM task manager commanded once-per-trip monitor (oxygen sensor/heater monitor, catalyst monitor, purge flow monitor, leak detection monitor or EGR monitor), a good trip is when the monitor is passed on the next engine start-up. If the MIL is on and any other emissions DTC is set (not an OBD-II monitor), a good trip is when the oxygen sensor monitor and catalyst monitor have completed, or 2 minutes of engine run time has occurred (if the oxygen sensor monitor and catalyst monitor has stopped running). NOTE: For more information on similar conditions window, see «SIMILAR CONDITIONS WINDOW»(ref-63416-S18583695132001020500000) .
2. Fuel System Good Trip To count a good trip (3 required) and turn off the MIL, the following conditions must be met. Engine must be in closed loop, must be operating in similar conditions window and short term multiplied by long term must be less than threshold value.
3. Misfire Good Trip If operating in similar conditions window and 1000 engine revolutions have occurred with no misfires, the PCM will count one good trip (3 required) in order to turn off MIL.
4. Warm-Up Cycles Once the MIL has been turned off by the good trip counter, the PCM will automatically switch to a warm-up cycle counter that can be viewed by the DRBIII(R) scan tool. Warm-up cycles are used to clear DTCs and freeze frame data from PCM memory. Forty warm-up cycles are necessary to clear DTCS and freeze frame data. A warm-up cycle is defined as the engine is started, an increase of 40°F (4°C) in engine coolant temperature exists after engine is started and engine coolant temperature reaches at least 160°F (71°C).

SIMILAR CONDITIONS WINDOW

The similar conditions window displays information about the engine operation during a monitor. Manifold absolute pressure (engine load) and engine RPM information is stored in this window when a failure occurs. There are 2 different similar conditions windows, fuel system and misfire.

Fuel System

1. Fuel System Similar Conditions Window An indicator that ABSOLUTE MAP WHEN FUEL SYS FAIL and RPM WHEN FUEL SYS FAIL are all in the same range when failure occurred. This is indicated by switching from NO to YES.
2. Absolute MAP When Fuel Sys Fail Stored MAP reading at time of failure. Tells what engine load the failure occurred.
3. Absolute MAP A real-time reading of engine load.
4. RPM When Fuel Sys Fail Stores the engine RPM at time of failure.
5. Engine RPM A real-time reading of engine RPM.
6. Adaptive Memory Factor The PCM uses both Short Term Compensation and Long Term Adaptive to calculate Adaptive Memory Factor for total fuel correction.
7. Upstream O2S Volts A real-time reading of O2 sensor to indicate performance of sensor.
8. Similar Conditions Window (SCW) Time In Window The PCM uses this timer to indicate that after all Similar Conditions have been met, if engine has been running good enough in SCW without failure detected. This timer is used to increment a Good Trip.
9. Fuel System Good Trip Counter This trip counter is used to turn off the MIL for fuel system DTCs. A Fuel System Good Trip increment is when engine is in Similar Conditions Window, Adaptive Memory Factor must be less than calibrated threshold and Adaptive Memory Factor must stay below that threshold for a specific amount of time.
10. Test Done This Trip Indicates the monitor has already run and completed during the current trip.

Misfire

1. Same Misfire Warm-Up State Indicates if misfire occurred when engine was warmed-up more than 160°F (71°C).
2. In Similar Misfire Window Indicates that Absolute MAP When Misfire Occurred and RPM When Misfire Occurred are all in the same range when failure occurred. Indicated by switching from NO to YES.
3. Absolute MAP When Misfire Occurred Stored MAP reading at time of failure. Tells what engine load the failure occurred.
4. Absolute MAP A real-time reading of engine load.
5. RPM When Misfire Occurred Stores the engine RPM at time of failure.
6. Engine RPM A real-time reading of engine RPM.
7. Adaptive Memory Factor The PCM uses both Short Term Compensation and Long Term Adaptive to calculate Adaptive Memory Factor for total fuel correction.
8. 200 Rev Counter Counts 0-100 720 degree cycles.
9. Similar Conditions Window (SCW) Cat 200 Rev Counter Counts when in Similar Conditions.
10. Similar Conditions Window (SCW) FTP 100 Rev Counter Counts 0-4 when in Similar Conditions.
11. Misfire Good Trip Counter Counts up to 3 to turn off MIL.
12. Misfire Data Data collected during test.
13. Test Done This Trip Indicates YES when test is done.

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 (DTC), 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.

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).

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 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

All Cherokee and Wrangler vehicles are equipped with an upshift indicator light (arrow up symbol), but light is only functional on vehicles equipped with a manual transmission. Upshift indicator light is located at right upper side of 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.