ELECTRONIC THROTTLE
The electronic throttle system enables the Engine Control Module (ECM) or Powertrain Control Module (PCM) to control the flow of intake air into the engine.
S-Type
The throttle control system is a closed-loop system controlled by the PCM and the Throttle Actuator Control (TAC) module. There is no mechanical connection between the accelerator pedal and the throttle body. The Accelerator Pedal Position (APP) sensor, located on the accelerator pedal pivot shaft, provides accelerator pedal position input to the PCM. The APP sensor consists of 3 potentiometers, each providing a different analog signal to PCM. The PCM sends the desired throttle angle signal to the TAC module, which is located on the throttle body. The TAC module actuates the throttle motor. An inductive position encoder, located on the throttle motor shaft, provides feedback signals to the TAC module. The Throttle Position (TP) sensor, located on end of throttle shaft, provides PCM with throttle valve position. The TP sensor consists of 3 non-contacting Hall Effect type sensing elements, each providing a different analog signal.
Throttle control system is monitored by the electronic throttle monitor, located in PCM, and the TAC module. Diagnostic information from the TAC module is sent to PCM over the Standard Corporate Protocol (SCP) link. If a malfunction is detected in the throttle control system, system will operate in one of 4 failure modes
- Loss Of Redundancy Failure of one sensor input will cause the Amber warning light to illuminate and FAIL SAFE ENGINE MODE will be displayed on the message center. Throttle control system will remain fully functional.
- Cruise Inhibit Certain failures may cause cruise control system cancellation.
- Limp-Home Failure of 2 or more sensor inputs will cause the Red warning light to illuminate and FAIL SAFE ENGINE MODE will be displayed on the message center. In this mode, there is no throttle control, but idle speed is increased to allow the vehicle to creep.
- Engine Shutdown Serious throttle malfunction may cause engine to shut down.
XJ8, XK8, XJR & XKR
The electronic throttle system is a closed-loop system controlled by the ECM. During normal operation, there is no mechanical connection between the accelerator pedal and the throttle valve. A throttle cable connects accelerator pedal to accelerator lever. Accelerator pedal position sensor provides accelerator lever position input to ECM. ECM uses this input, along with engine operating conditions, to control throttle valve. The throttle valve is opened and closed by the throttle motor via reduction gears. This motor also provides cruise control actuation. The throttle position sensor provides throttle valve position feedback to the ECM.
If a fault develops in the electronic throttle system, ECM will enter limp-home mode. In limp-home mode, the throttle motor power relay is de-energized, deactivating throttle motor. Throttle valve is operated mechanically by the accelerator pedal and limp-home lever. (Scheme 1) Throttle valve opening is limited to a maximum of about 30 degrees. ECM adjusts fuel metering to control engine operation (some cylinders may be cut during small throttle openings).
Scheme 1
SUPERCHARGER (XJR & XKR)
Supercharger system consists of a belt-driven supercharger, internal by-pass valve and an intercooler.
Supercharger compresses air charge entering intake manifold. This creates a surplus volume of intake air, promoting more complete combustion which in turn provides more power.
At idle or at steady highway speeds, by-pass valve opens and excess pressurized air is allowed to recirculate back into supercharger inlet. As engine load increases, manifold vacuum drops. This allows by-pass valve to close and supercharger boost occurs.
VARIABLE INTAKE SYSTEM (S-TYPE V6)
The variable intake system changes intake manifold geometry to maximize engine performance throughout the entire engine speed range. This variable geometry is achieved by the solenoid-operated Intake Manifold Tuning (IMT) valves, which are controlled by the Powertrain Control Module (PCM).
The plenum chamber is split into upper and lower compartments, connected by 2 IMT valves. Either one or both IMT valves can be opened, depending on engine speed. This system works in conjunction with the variable valve timing system.
FAIL-SAFE COOLING SYSTEM (S-TYPE V6)
In the event of coolant loss resulting in engine overheating, PCM controls engine temperature by switching off one or more fuel injectors to inhibit combustion, reducing heat. Vehicle then can be driven, at reduced power, for a short distance. If engine temperature reaches 250-260°F (122-127°C), the TEMP warning light turns on, but engine operation remains normal. If engine temperature reaches 260°F (127°C) or more, PCM will variably cut one cylinder. As temperature continues to rise, the number of cylinders cut will increase, to maximum of 3 cylinders. If engine temperature reaches 300°F (166°C), engine will shut down.
VARIABLE VALVE TIMING
Variable valve timing system improves low and high-speed engine performance, engine idle quality and exhaust emissions. The V8 engine uses a continuously variable system. The V6 engine uses a 2-position system.
VARIABLE VALVE TIMING SYSTEM (S-TYPE V6)
Variable Valve Timing (VVT) system is a 2-position system operating on the intake camshafts only. There are 30 degrees of crankshaft movement between the retarded and advanced positions. Engine oil pressure operates the system under the control of the Powertrain control module (PCM). System operates in conjunction with the variable intake system. For each intake camshaft there is a VVT unit and a solenoid-operated oil control valve.
When the valve timing solenoid is energized, the oil control valve opens to direct engine oil pressure to the VVT unit. In the unit, the oil pressure overcomes the force of the return spring and moves the piston inward (toward the camshaft), rotating the camshaft to the advanced position.
When the valve timing solenoid is de-energized, the oil control valve closes, reducing oil pressure in the VVT unit. Within the VVT unit, the return spring hold the piston outward, rotating the camshaft to the retarded position.
Maximum system response times are: one second for advancing and 0.7 second for retarding. With valve timing in the retarded mode, the ECM produces a periodic lubrication pulse once every 5 minutes. This may be audible when the vehicle is stationary and the hood is open.
VARIABLE VALVE TIMING SYSTEM (XJ8 & XK8)
Variable Valve Timing (VVT) system is a continuously variable closed-loop system operating on the intake camshafts only. There are 48 degrees of crankshaft movement between the fully retarded and fully advanced positions. Engine oil pressure operates the system under the control of the Engine control module (ECM). For each intake camshaft there is a VVT unit, a bush carrier assembly and a solenoid-operated oil control valve.
When oil pressure is applied to the advance chamber of the VVT unit, the drive ring/piston assembly moves inward (toward the camshaft) and rotates clockwise, turning the camshaft. When oil pressure is applied to the retard chamber, the drive/ring piston assembly moves in the opposite direction. A return spring, located in the retard chamber, reverts the piston to the fully retarded position when the engine is off.
The oil control valve consists of a 4-spool shuttle valve operated by a pulse-width modulated solenoid and return spring. Pulse width is continuously controlled by the ECM, based on engine sensor inputs, including the camshaft position sensors and engine oil temperature sensor.
ENGINE CONTROL MODULE (XJ8, XK8, XJR & XKR)
The Engine Control Module (ECM) controls the engine management system. The ECM is located in the control module enclosure at right rear of the engine compartment. An electric fan ventilates the enclosure with air from the passenger compartment. The ECM maintains optimum engine performance under all operating conditions by metering fuel delivery into each cylinder, and adjusting ignition timing.
The ECM receives inputs from engine related sensors and various vehicle systems and provides output for the following: electronic throttle, fuel pump, fuel injection, ignition system, EVAP system, variable valve timing, EGR system (if equipped), engine starting, HO2S heaters, instrument cluster, A/C compressor clutch, windshield and rear window heaters, radiator cooling fans and diagnostics.
POWERTRAIN CONTROL MODULE (S-TYPE)
The Powertrain Control Module (PCM) performs all of the functions of an Engine Control Module (ECM). PCM also controls the transmission. The PCM is located under the A/C evaporator/blower unit. The PCM harness connectors protrude through the firewall.
INPUT DEVICES
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES , which control or produce voltage signals monitored by the ECM. The second category covers OUTPUT SIGNALS , which are components controlled by the ECM.
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. Available input signals include the following.
Accelerator Pedal Position Sensor (XJ8 & XK8, XJR & XKR)
The Accelerator Pedal Position (APP) sensor is connected to the input shaft within the electronic throttle assembly. The APP sensor consists of a dual-track potentiometer that provides inputs to the ECM of driver demand at the accelerator pedal.
Accelerator Pedal Position Sensor (S-Type)
The Accelerator Pedal Position (APP) sensor is located on the accelerator pedal pivot shaft. The APP sensor consists of 3 rotary carbon track potentiometers, with contacting wipers. Each potentiometer provides an independent analog signal to the PCM.
Air Conditioning Signal
A/C compressor clutch operation is signalled to ECM or PCM to compensate for engine load changes which occur during clutch engagement/disengagement.
Barometric Pressure (BARO) Sensor
The BARO sensor is located within the ECM. The BARO sensor senses ambient atmospheric pressure in the engine management enclosure of the engine compartment.
Camshaft Position (CMP) Sensor
The CMP sensors are located at the rear of each cylinder head. The CMP sensor provides camshaft position input to the ECM or PCM. This input is used for cylinder identification, ignition/fuel synchronization and VVT system feedback (when equipped).
Crankshaft Position (CKP) Sensor
On V8 engines, CKP sensor is located at the rear of the bedplate, near flywheel. On V6 engines, CKP sensor is located on the front cover, near crankshaft pulley. The CKP provides ECM or PCM with information on engine speed and crankshaft position. ECM uses this information to control fuel injection pulse rate and ignition timing. CKP sensor is also used for misfire detection.
Cylinder Head Temperature (CHT) Sensor (V6)
The CHT sensor is located on the left bank cylinder head, between ignition coils No. 2B and 3B. This sensor provides input to the PCM for cylinder head temperature and is used for the fail-safe cooling system, in place of the engine coolant temperature sensor. Sensor allows cylinder head temperature to be measured even if coolant has been lost.
Engine Coolant Temperature (ECT) Sensor (V8)
ECT sensor is located on engine coolant outlet duct. The ECT sensor is a Negative Thermal Coefficient (NTC) thermistor. Electrical resistance within ECT sensor increases as coolant temperature decreases.
ECM or PCM monitors ECT sensor resistance value and triggers temperature enrichment circuits (when necessary) to extend injector "on" time during cold starting and warm-up.
Heated Oxygen Sensor (HO2S)
An HO2S is located upstream and downstream of each catalytic converter (total of 4). HO2S provide input to the ECM or PCM proportional to the oxygen content of the exhaust gases leaving the engine. Heater element improves response time of the sensor during warm-up.
Intake Air Temperature (IAT) Sensor
IAT sensor is integrated into the same housing as the Mass Airflow (MAF) sensor. On supercharged models, a second IAT sensor is located in the outlet duct of the right bank intercooler. On all models, sensor is a Negative Thermal Coefficient (NTC) thermistor with a shrouded bead for fast response to changing air temperatures. Sensor resistance decreases as air temperature increases. IAT sensor provides an air temperature signal to ECM or PCM.
Knock Sensor (KS)
On V8 engines, knock sensors are located at the center of the cylinder block on the inboard side of each cylinder bank. On V6 engines, right bank knock sensor is located on the side of engine block, near starter. The left bank knock sensor is located on the inboard side of cylinder bank. On all engines, sensors are piezo-electric devices that provide inputs to the ECM or PCM to detect and locate detonation during combustion.
Manifold Air Pressure (MAP) Sensor
MAP sensor is only used on the 2001 XJR and XKR. Sensor measures air pressure at the induction elbow and sends a signal to the ECM. ECM uses this input to monitor EGR operation.
Mass Airflow (MAF) Sensor
MAF sensor uses a hot-wire type sensing element. Incoming air passing through sensor causes hot wire to cool. As a result, ECM or PCM must apply additional current to maintain hot wire at a preprogrammed temperature. ECM or PCM measures airflow by monitoring amount of additional current required to maintain hot wire at preprogrammed temperature.
Throttle Position (TP) Sensor
TP sensor is located on the end of the throttle valve shaft. Sensors provides inputs to the ECM or PCM of the position of the throttle valve.
OUTPUT SIGNALS
Note. Vehicles are equipped with various combinations of computer-controlled components. Not all components listed are used on every vehicle. For theory and operation on each output component, refer to system indicated after component.
EGR System
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Electronic Throttle System
See ELECTRONIC THROTTLE under AIR INDUCTION SYSTEM.
Evaporative Emission Control System
See EMISSION SYSTEMS .
Fuel Injectors
See FUEL INJECTORS under FUEL CONTROL.
Fuel Pump Relay
See FUEL DELIVERY .
Idle Speed Control
See IDLE SPEED .
Ignition Coils
See IGNITION SYSTEM .
Malfunction Indicator (CHECK ENGINE) Light
See MALFUNCTION INDICATOR LIGHT (MIL) under SELF-DIAGNOSTIC SYSTEM.
Variable Intake System
See VARIABLE INTAKE SYSTEM (S-TYPE V6) under AIR INDUCTION SYSTEM.
Fuel Pump
Output signal from ECM or PCM controls relay that operates fuel pump. When ignition is turned on, pump will operate for 2 seconds without ignition switch being turned to crank engine, after which time pump will be de-energized. In addition, if engine stops with ignition on, pump will be de-energized after 2 seconds. Pump operates continuously while ignition switch is set to crank engine, or when engine is running. Supercharged vehicles are equipped with a secondary fuel pump. Both fuel pumps operate under certain conditions.
Returnless Fuel System (S-Type)
The returnless fuel system is used to reduce fuel tank vapor. System does not use a return line or pressure regulator. Fuel pressure is controlled by PCM. A pressure relief valve, located at the front of the fuel rail, allows system to be depressurized before fuel system service.
Air-Assisted Fuel Injection (Normally-Aspirated V8 Engines)
All normally-aspirated V8 engines are equipped with Air-Assisted Injection (AAI). AAI improves the atomization of fuel when cold, thus improving combustion when cold and reducing hydrocarbon emissions. The system uses manifold vacuum to draw air through the injector nozzles. During cold start and part throttle conditions, the Air Assist Control (AAC) valve, located on the throttle body, allows air to flow through the air supply rail to the injectors. AAC valve closes at higher engine loads, as manifold vacuum decreases.
The AAC valve is driven by a pulse-width modulated signal controlled by the Engine Control Module (ECM) or Powertrain Control Module (PCM). The valve is fully open when engine coolant temperature is 140°F (60°C) or less. The valve is fully closed when engine coolant temperature is 158°F (70°C) or more.
Solenoid-operated fuel injectors are controlled by ECM or PCM. Amount of fuel required is determined from a base air/fuel ratio which is then adjusted for specific operating conditions. ECM or PCM varies number and duration of injector pulses per engine cycle to regulate fuel flow. Injector timing is synchronized with engine rotation, except during starting and acceleration. All normally-aspriated V8 engines are equipped with air-assisted injectors, which are modified with a plastic shroud over the nozzle.
Battery Voltage Correction
Operational characteristics of fuel injectors vary with battery voltage. ECM monitors battery voltage and compensates injector drive pulse duration for variations from a nominal voltage.
IDLE SPEED
Note. Idle speed control is accomplished through the electronic throttle.
IGNITION SYSTEM
Ignition timing and spark distribution are controlled by ECM or PCM according to sensor inputs. Ignition timing is controlled primarily as a function of engine load and speed. Engine load is sensed by Mass Airflow (MAF) sensor. Engine speed is sensed by a Crankshaft Position (CKP) sensor. The ECM or PCM processes inputs from the sensors and accesses ignition timing from the ignition timing strategy. Individual ignition coils are located above each spark plug. ECM or PCM incorporates the primary circuit for each coil and provides switching for each primary circuit. The correct firing sequence and timing of the ignition coils is determined by ECM or PCM from cylinder synchronization input provided by the Camshaft Position (CMP) sensor.
V6
Under closed or part throttle conditions, crankcase ventilation is provided by a Positive Crankcase Ventilation (PCV) system. An oil separator is bolted to the top of the engine block between the cylinder banks and is connected to intake manifold vacuum via a PCV valve and hose. Full load ventilation is provided by breather hoses between each camshaft cover and the air intake duct. (Scheme 2)
Scheme 2
V8
Engine ventilation is through a part load and a full load breather. (Scheme 3) The part load breather is a flexible composite hose connected between the oil separator in left bank camshaft cover and the induction elbow. A restrictor in the outlet from the oil separator prevents reverse flow.
The full load breather is a flexible composite hose connected between the oil separator in right bank camshaft cover and the air intake duct.
Ends of the breather hoses incorporate quick release connectors. The oil separators consist of wire gauze packed into an open ended enclosure below the breather outlet.
Scheme 3
EXHAUST GAS RECIRCULATION (EGR)
To lower oxides of nitrogen (NOx) exhaust gas emissions, an Exhaust Gas Recirculation (EGR) system is used on S-Type (V6), XJR and XKR.
S-Type (V6)
EGR system consists of a vacuum-operated EGR valve, EGR Vacuum Regulator (EVR), Differential Pressure Feedback EGR (DPFE) sensor and exhaust gas feedback pipe with internal orifice. (Scheme 4) Exhaust gas from the right bank exhaust manifold flows through the feedback pipe to the EGR valve. The EGR vacuum regulator controls the vacuum signal applied to the EGR valve. The differential pressure feedback EGR sensor is connected to the feedback pipe with hoses connected to either side of the orifice. The difference in pressure between each side of the orifice is measured by the DPFE sensor, which sends a signal to the PCM. The PCM provides a pulse-width modulated signal to the EGR vacuum regulator, based on inputs from the DPFE sensor and other sensors.
Scheme 4
XJR & XKR
EGR system consists of electronic EGR valve and transfer pipe and is controlled by the ECM. (Scheme 5) The EGR valve is a 4-pole stepper motor installed on the rear of the induction elbow and is cooled by coolant flow from the electronic throttle. The transfer pipe connects to the right exhaust manifold. There are no temperature or position feedback signals from the EGR valve. The ECM monitors EGR operation using a MAP sensor.
Scheme 5
EVAPORATIVE EMISSION CONTROL SYSTEM (EXCEPT S-TYPE)
The EVAP system is designed to reduce the amount of fuel vapor that is vented to the atmosphere. Fuel tank is vented through the charcoal canister, where vapor is stored, until it is purged into the intake manifold by the canister purge valve. The ECM or PCM controls the flow of fuel vapor by controlling the duty cycle of the canister purge valve.
Vehicle is equipped with one of 3 EVAP systems. On all systems, before the ECM or PCM allows canister purge operation, the following conditions must be met
- After battery disconnection, engine management adaptions must be reinstated.
- Engine has run for at least 8 seconds.
- Engine coolant temperature is 158°F (70°C) or more.
- Engine is not running in the fuel cut-off condition.
- Adaptive fuel correction function does not indicate a rich or lean failure.
- EVAP system leak test has not failed.
- No faults are detected in MAF sensor, ECT sensor, canister purge valve or canister close valve.
Single Canister System
This system uses a single charcoal canister with a pressure control valve between the canister and the fuel tank rollover valve. (Scheme 6) Pressure control valve is controlled by engine vacuum. When engine is off, pressure control valve is closed, maintaining a slight pressure in fuel tank. Any pressure increase causes the valve to open and release vapor to the canister. When engine is running, manifold vacuum holds the valve open. This maintains atmospheric pressure within the tank and allows vapor to flow to the canister.
Scheme 6
Running Loss System
This system uses 2 charcoal canisters connected in series to reduce hydrocarbon emissions, and a normally open pressure control valve. (Scheme 7) A tank pressure sensor and canister close valve are used by the on-board diagnostic system to test for leaks in the system.
The pressure control valve allows continuous vapor flow to the canisters during normal running and prevents fuel from entering the vent line while refueling. Fuel tank vapor flows through the valve to the left canister. A third pipe connects the valve to the filler neck. When refueling, the difference in pressure between the tank and the filler neck closes the valve, preventing vapor flow. When the fuel filler cap is reinstalled, the pressure equalizes and allows the valve to open.
The canister close valve is a solenoid-operated valve controlled by the ECM. The valve is normally open and is closed only during the EVAP system leak test. The tank pressure sensor provides a signal to the ECM.
Scheme 7
Running Loss With On-Board Refueling Vapor Recovery (ORVR) System
This system allows vapor generated during refueling to be collected by charcoal canisters. System uses 2 charcoal canisters connected in series to reduce hydrocarbon emissions. A tank pressure sensor and canister close valve are used by the on-board diagnostic system to test for leaks in the system. (Scheme 8) During normal running, vapor is collected and purged in the same manner as the Running Loss System.
The ORVR system has the following features
- A narrow fuel filler pipe providing a liquid seal when refueling, which prevents fuel vapor from venting to the atmosphere.
- There is no vent between the tank and the filler pipe. A check valve at the lower end of the filler pipe is used to prevent fuel from surging up the filler pipe while refueling.
- A Fuel Level Vent Valve (FLVV) consisting of a 2-stage shut-off valve with rollover protection and a pressure relief valve.
- A grade vent valve with rollover protection and an outlet pipe connected to the FLVV outlet pipe.
- Larger bore vapor pipes.
During refueling, the tank is vented through the FLVV, vapor hoses and charcoal canisters. The FLVV incorporates a float valve which is closed by the rising fuel level, creating a backpressure and causing refueling to stop. When the FLVV is closed (tank is full), any increase in pressure is relieved by the grade vent valve, by-passing the FLVV. When the FLVV is open, vapor vents to the canisters normally. The pressure relief valve allows vapor venting through the filler cap if a blockage or failure occurs.
Scheme 8
EVAPORATIVE EMISSION CONTROL SYSTEM (S-TYPE)
The EVAP control system consists of a charcoal canister, a canister purge valve, a canister vent solenoid and a fuel tank pressure sensor. When the purge valve is closed, fuel tank vapor is vented through the rollover valve into the canister. The canister stores the fuel vapor until the PCM energizes the purge valve, allowing the vapor to be drawn into the intake manifold.
MALFUNCTION INDICATOR LIGHT (MIL)
All models are equipped with a MIL in instrument cluster. MIL will turn on when ignition switch is turned to ON position (bulb check) and when an engine management fault is detected. For additional information, see appropriate SELF-DIAGNOSTICS article.