COMPUTERIZED ENGINE CONTROLS
Computerized engine control system monitors and controls engine operation. Input sensors supply information to the Powertrain Control Module (PCM). PCM processes information from input devices and sends output voltage signals to various control devices. See INPUT DEVICES and OUTPUT SIGNALS . Information received from input devices is processed by PCM to calculate proper fuel delivery, ignition timing, fuel evaporative purge, exhaust gas recirculation and other emission control systems.
POWERTRAIN CONTROL MODULE
Note. Powertrain Control Module (PCM) may also be referred to as Electronic Control Module (ECM) or Vehicle Control Module (VCM). PCM is located below center console.
PCM analyzes all electrical data signals from input devices to control fuel injection, ignition and emissions. PCM includes a backup fail-safe control system. If a malfunction should occur in any PCM controlled systems, in most cases PCM will substitute default settings to maintain necessary engine functions and permit operation of vehicle.
Electronically Erasable Programmable Read Only Memory (EEPROM) is factory-programmed engine calibration data, which modifies PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. It can be reprogrammed using Tech II scan tool. It is permanently soldered into PCM and is not replaceable separately. If battery voltage is removed, EEPROM information is retained.
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 PCM.
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on specific model, see WIRING DIAGRAMS article. The available input signals include the following
A/C Request - A/C On
This provides PCM with a simple ON (A/C Request) signal. PCM make calculations for extra load, fuel enrichment and idle.
Battery Voltage
Battery voltage is monitored by PCM. PCM must have 12 volts to operate. PCM can adjust idle speed, timing or enrich air/fuel ratio to maintain a constant 12 volts.
Camshaft Position Sensor
Camshaft Position (CMP) sensor is located in timing cover or engine block. CMP sensor continuously sends PCM a signal that is used as a 'sync pulse' to trigger injectors in the proper sequence. The signal indicates to PCM the position of No. 1 piston during it's power stroke. PCM is then able to calculate true sequential fuel injection operation. If signal is lost, PCM reverts back to calculated sequential mode based on the last fuel injection pulse received.
Crankshaft Position Sensor
Crankshaft Position (CKP) sensor is located in engine block. CKP sensor detects engine speed and relative position of crankshaft. PCM then uses this information to calculate ignition sequence and timing.
EGR Pintle Position Sensor
This EGR Pintle Position (EGR-PS) sensor is mounted inside linear EGR valve, and informs PCM of EGR valve position. PCM uses this information to control EGR flow.
Engine Coolant Temperature Sensor
Engine Coolant Temperature (ECT) sensor is located in engine block. ECT sensor informs PCM of engine coolant temperature.
Heated Oxygen Sensor
Heated Oxygen Sensors (HO2S) are located in exhaust system. When at operating temperature, sensor produces a voltage signal proportional to exhaust gas oxygen content. By heating up HO2S using a heating element, HO2S will begin functioning earlier than an unheated HO2S. Typically, there are at least two, one in front of and the other behind the catalytic converter (TWC). The HO2S in front of the TWC gives PCM feedback which is used to adjust fuel mixture. The HO2S behind the TWC is used to monitor TWC effectiveness by comparing readings between the two HO2Ss. If both HO2Ss are showing similar readings, TWC failure is indicated.
Intake Air Temperature Sensor
Intake Air Temperature (IAT) sensor is located in fresh air duct in the path of incoming airflow. Sensor sends signals to PCM to indicate ambient air temperatures.
Knock Sensor
Piezo electric Knock Sensor (KS) is located in engine block. If detonation occurs, sensor generates a signal to PCM. PCM uses this signal to determine when to retard ignition timing.
Manifold Absolute Pressure Sensor
Manifold Absolute Pressure (MAP) sensor monitors intake manifold pressure and informs PCM of engine load changes. MAP sensor signal is relative to RPM and throttle position.
Mass Airflow Sensor
Mass Airflow (MAF) sensor is located in air cleaner housing. MAF sensor measures rate (volume) of air intake and .
Park/Neutral Switch - A/T & Inhibitor Switch - M/T
Switch is used to inform PCM of selected gear position. Information is used by PCM to allow starter operation and control engine idle speed.
Power Steering Pressure Switch
When power steering pressure is high, Power Steering Pressure (PSP) switch sends a signal to PCM to increase idle speed. PCM will also turn off A/C clutch when high P/S pressure is detected.
Throttle Position Sensor
Throttle Position (TP) sensor is mounted on side of throttle body and informs PCM of changes in throttle position. TP sensor is sometimes combined with a throttle valve switch.
Vehicle Speed Sensor
Vehicle Speed Sensor (VSS) is located at the rear of transfer case or transmission, and informs PCM of vehicle road speed.
OUTPUT SIGNALS
Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to the system indicated after component.
CHECK ENGINE Light or Malfunction Indicator Light
See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.
Distributorless Ignition System
See DISTRIBUTORLESS IGNITION SYSTEM under IGNITION SYSTEM.
EGR Valve
See EXHAUST GAS RECIRCULATION SYSTEM under EMISSION SYSTEMS.
Electronic Spark Advance
See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEM.
Electronic Spark Timing
See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEM.
Fuel Disable Mode - Engine Speed
See FUEL CONTROL under FUEL SYSTEM.
Deceleration Mode
See FUEL CONTROL under FUEL SYSTEM.
Clear Flood Mode
See FUEL CONTROL under FUEL SYSTEM.
Acceleration Mode
See FUEL CONTROL under FUEL SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Regulator
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Idle Air Control Valve
See IDLE SPEED under FUEL SYSTEM.
Fuel pump is located inside fuel tank. Pump and sending unit are integral in design and are replaced as an assembly.
Relay is located in underhood fuse/relay box. When ignition switch is turned to ON position, PCM will activate fuel pump relay to run fuel pump. Fuel pump will operate as long as engine is cranking or running and PCM is receiving ignition reference pulses. If there are no reference pulses, PCM will shut off fuel pump within 2 seconds after ignition is turned on.
Fuel pressure regulator is a vacuum operated relief valve mounted on fuel rail. Fuel pressure to injectors is maintained at about 3 times barometric pressure adjusted for engine load conditions and vehicle speed. Fuel pressure regulator consists of a fuel chamber and a vacuum chamber separated by a diaphragm. (Scheme 1) Fuel chamber has a fuel inlet pipe and a fuel outlet pipe. Proper fuel pressure in maintained in fuel rail, also referred to as fuel distribution pipe. This ensures that appropriate amount of fuel is delivered to injectors under correct pressure. Fuel inlet pipe delivers fuel to regulator from fuel rail. Excess fuel is returned to fuel tank by fuel outlet pipe.
Vacuum chamber is connected to intake manifold by a hose. Any change in fuel pump delivery pressure or intake manifold pressure will cause diaphragm to move. This movement will maintain pressure balance between intake manifold and fuel chamber to ensure a steady supply of fuel to fuel injectors.
Scheme 1
FUEL CONTROL
The purpose of the fuel control system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered by the fuel injectors, which are controlled by the PCM. The PCM checks Engine Coolant Temperature (ECT) and Throttle Position (TP) sensor to determine proper air/fuel ratio for starting. PCM changes the air/fuel ratio to the engine by modifying fuel injector pulse width. Fuel system operates in one of 2 modes: open loop or closed loop.
Open Loop
When engine is cold, PCM ignores signal from heated oxygen sensor (HO2S). Air/fuel ratio is calculated based on inputs from TPS, ECT and MAF or MAP sensors. System will remain in open loop until HO2S reaches operating temperature, coolant temperature reaches preset temperature, a specific period of time has elapsed after engine starts and engine speed has been greater than a specified RPM since start-up.
Closed Loop
When HO2S reaches operating temperature, coolant temperature reaches a preset temperature, and a specific period of time has passed since engine start-up, PCM operates in closed loop to control air/fuel ratio based on HO2S signals (in addition to other input parameters). PCM maintains air/fuel ratio as close as possible to 14.7:1. If HO2S cools down (due to excessive idling) or a fault occurs in the HO2S circuit, vehicle once again enters open loop mode.
Fuel is metered into cylinders by electrically controlled solenoid valves in injectors. PCM controls on/off time (duty cycle) of fuel injectors to regulate air/fuel ratio.
PCM monitors engine speed and shuts off fuel injectors when engine speed increases to more than 6400 RPM. Injectors are turned back on when RPM decreases to less than 6150.
Flooded engine can be cleared by depressing accelerator pedal fully while cranking engine. PCM will de-energize fuel injectors. Fuel injectors will remain de-energized as long as throttle remains open 80 percent or more and engine speed is less than 800 RPM.
PCM will reduce amount of fuel injected when it detects a decrease in throttle position and airflow. PCM may cut off fuel completely for short periods if deceleration is very rapid.
PCM provides extra fuel when a rapid increase in throttle position and airflow is detected.
See HEATED OXYGEN SENSOR under INPUT DEVICES.
Battery Voltage Correction
PCM compensates for low battery voltage by increasing injector pulse width, ignition dwell time and increasing idle RPM. PCM is able to perform these commands because of built-in memory/learning function.
Idle Air Control (IAC) valve is located in throttle body, and controls engine idle speed, while preventing stalls due to changes in engine load. IAC valve controls by-pass air around throttle plate by moving pintle in (to decrease air flow) or out (to increase airflow).
Proper engine RPM and positioning of IAC valve pintle is calculated by PCM based on battery voltage, coolant temperature and engine load. PCM sends voltage pulses to proper motor winding in IAC valve motor. This will cause motor shaft and IAC valve to move in or out of IAC valve motor a predetermined amount for each pulse received. PCM pulses are referred to as counts.
IAC valve should be disconnected only when ignition switch is in OFF position. Each time ignition is turned off, PCM will reset IAC valve. This is done by sending enough counts to seat valve. Fully seated valve is the PCM reference point. A given number of counts are then issued to open valve, and normal PCM control of IAC valve will begin from this point. To increase idle speed, PCM will increase counts to retract IAC valve to allow more air to flow through idle air passage and by-pass throttle plate until idle speed reaches proper RPM. To decrease idle speed, PCM will reduce counts to extend IAC valve to reduce the flow of air through idle passage around throttle plate. This will reduce PCM counts.
Note. Distributorless Ignition System (DIS) may also be referred to as Electronic Ignition (EI) system.
Distributorless Ignition System (DIS) consists of multiple coils or coil packs, Ignition Control Module (ICM), crankshaft position sensor and PCM. On Passport 2.2L and Rodeo 2.2L, waste-spark ignition is used. Coil packs consist of 2 independently mounted ignition coils, each firing two cylinders at a time. On all other engines, each cylinder has its own independent ignition coil mounted on top of the spark plug. ICM receives ignition control signals from PCM which in turn triggers corresponding ignition coils. PCM controls spark timing and ignition during crank and run modes.
Crankshaft position sensor magnetic pick-up provides a signal to PCM to identify correct firing sequence and crank signals to trigger each coil at proper time. PCM controls timing and fuel injector pulse width using crankshaft position, engine RPM, engine temperature and MAP sensor input signals.
IGNITION TIMING CONTROL SYSTEMS
Ignition timing is entirely controlled by PCM. PCM monitors information such as crankshaft position, engine speed, coolant temperature and MAP sensor readings, computes optimum ignition timing and triggers coils to fire using Ignition Control Module (ICM).
EMISSION SYSTEMS
Note. For emission systems usage, see EMISSION APPLICATIONS article.
Fuel Evaporation Emission Control System
Fuel Evaporation Emission Control (EVAP) system prevents escape of gasoline vapors (hydrocarbons) from fuel tank into atmosphere. To reduce hydrocarbon (HC) emissions, evaporated fuel from fuel tank is absorbed into a charcoal canister to hold vapors, and is purged by PCM control when engine coolant temperature is greater than 140°F (60°C). Vapors are pulled into intake manifold for combustion in cylinders. (Scheme 2) Purging of canister is controlled by a PCM-operated EVAP purge solenoid (normally closed). Attached to some charcoal canisters is an EVAP canister vent solenoid which allows fresh air to enter charcoal canister, helping to purge fuel vapors. EVAP vent solenoid closes to seal off evaporative emission system for leak testing.
Scheme 2
Enhanced Fuel Evaporation Emission Control System
This system goes one step further in managing fuel vapors. PCM monitors pressure/vacuum in fuel tank by means of a Fuel Tank Pressure Sensor (FTPS). Enhanced EVAP system is required to detect leaks as small as 0.04" (1.0mm) between fuel filler cap and purge solenoid. If any EVAP system leaks are detected, or if fuel filler cap is left off or loose after refueling, PCM will store pertinent Diagnostic Trouble Code (DTC). EVAP system diagnostics will conduct up to eight different tests under various conditions to monitor integrity of EVAP system.
EXHAUST GAS RECIRCULATION SYSTEM
Exhaust Gas Recirculation (EGR) system causes Oxides of Nitrogen (NOx) to be reduced by recycling burned exhaust gases through EGR valve and back into intake manifold. The reduced amount of burnable mixture introduced into the cylinder causes lower combustion chamber temperatures, thereby reducing NOx.
Linear EGR valve system is used. ECM continuously monitors EGR position through an EGR Pintle Position Sensor (EGR-PS), and continuously adjusts the valve in order to obtain correct flow. PCM uses ECT, TPS and MAF or MAP information to set EGR valve position.
POSITIVE CRANKCASE VENTILATION SYSTEM
Positive Crankcase Ventilation (PCV) system prevents blow-by gases that build up in crankcase from escaping into atmosphere. Fresh air is routed to crankcase from throttle body and mixed with blow-by gases, then drawn back into intake manifold to be burned. PCV system consists of a valve cover baffle plate, PCV valve or orifice, and oil separator (most models). Oil separator removes oil particles from blow-by gases.
MALFUNCTION INDICATOR LIGHT
Also called CHECK ENGINE light, Malfunction Indicator Light (MIL) comes on when ignition is turned on. Light remains on for several seconds after engine has started. If an abnormal sensor input signal occurs, light comes on and Diagnostic Trouble Code (DTC) is stored in memory. If the abnormal input signal returns to normal, PCM turns light off, but DTC remains stored in memory until cleared. If engine is turned off and started again, light will not come on until PCM detects another malfunction during system operation.
HARD FAILURES
Hard failures (emission-related faults) cause MIL to illuminate and remain on until system or component passes the same test for 3 consecutive drive cycles without a fault. If vehicle is experiencing a misfire malfunction which may cause damage to Three-Way Catalytic Converter (TWC), MIL will flash once per second. This will continue until vehicle is outside of speed and load conditions which could cause possible catalyst damage, and MIL will stop flashing and remain on steady. If light comes on and remains on (light may flash) during vehicle operation, determine cause of malfunction using diagnostic (code) charts. See appropriate SELF-DIAGNOSTICS article. If a sensor fails, Powertrain Control Module (PCM) will use a substitute value in its calculations to continue engine operation. In this condition (limp-in mode), the vehicle will run, but driveability will not be optimum.
INTERMITTENT FAILURES
Intermittent failures may cause MIL to flicker or illuminate and go out after intermittent fault goes away. However, corresponding trouble code will be retained in Powertrain Control Module (PCM) memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from PCM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article.