INTRODUCTION
This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.
SELF-DIAGNOSTIC SYSTEM
ECM constantly looks at information from various input devices to control systems that affect vehicle performance. ECM self-diagnostic capabilities allow for trouble shooting system through a CHECK ENGINE light. CHECK ENGINE light will illuminate to alert driver of a system malfunction. The type of failure (hard or intermittent) will affect how the CHECK ENGINE light illuminates. The specific malfunction can be determined via a 2-digit code displayed by CHECK ENGINE light. To retrieve codes, see TESTS W/CODES article in the ENGINE PERFORMANCE Section.
CHECK ENGINE LIGHT
CHECK ENGINE light is on instrument panel and has the following functions
- It informs driver that a problem has occurred and that vehicle should be taken for service as soon as possible.
- It displays Trouble Codes stored by Electronic Control Module (ECM), which help technician diagnose system problems.
- It indicates Open/Closed Loop operation.
HARD FAILURES
Hard failures cause CHECK ENGINE light to illuminate and remain on until problem is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, Electronic Control Module (ECM) will use a substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, the vehicle runs but driveability will not be optimum.
INTERMITTENT FAILURES
Intermittent failures may cause CHECK ENGINE light to flicker or illuminate and go out after intermittent fault goes away. However, the corresponding trouble code will be retained in Electronic Control Module (ECM) memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from ECM memory. Intermittent failures may be caused by a sensor, connector or wiring related problems. See INTERMITTENTS in H - EFI TESTS W/O CODES article in the ENGINE PERFORMANCE Section.
Impulse & Stylus (DOHC)
The secondary air system is used to control air to secondary intake valves when engine speed is greater than 5000 RPM. System allows for reduced intake air at idle to improve idle quality, and opens at higher RPM to improve engine performance and increase horsepower.
Secondary air system consists of secondary port intake valves mounted in lower intake manifold and a vacuum operated diaphragm that moves secondary valve linkage. Vacuum to diaphragm is turned on or off by a vacuum switching valve that is electrically operated by Electronic Control Module (ECM). (Scheme 1)
Scheme 1
Impulse Turbo
Impulse engine is equipped with an electronically controlled turbocharger system. Turbochargers are cooled by engine cooling system and lubricated by engine oil. Turbocharger system consists of a turbine/compressor, stepping motor and wastegate valve.
Turbocharger works by allowing exhaust gases from exhaust manifold to enter turbine area. Exhaust gases flow through turbine blades, causing it to spin. As throttle opens and engine RPM increases, exhaust flow and turbine speed increase. Inlet air is drawn through air inlet duct into compressor housing, then forced into intake manifold. As turbine speed increases, turbo boost pressure also increases. If boost pressure exceeds a predetermined limit, engine damage can result. Wastegate valve is used to release boost pressure.
Turbocharger wastegate is vacuum controlled by ECM through a wastegate Vacuum Switching Valve (VSV). (Scheme 2)
Scheme 2
COMPUTERIZED ENGINE CONTROLS
Computerized engine control system monitors and controls engine operation. Input sensors supply information to an Electronic Control Module (ECM). ECM processes information from input devices and sends output voltage signals to various control devices. See INPUT DEVICES and OUTPUT SIGNALS in this article. Information received from input devices is processed by ECM to calculate proper fuel delivery, ignition timing, fuel pressure, fuel evaporative purge, exhaust gas recirculation, air injection and wastegate operation.
CONTROL UNIT
On all models except Trooper, ECM is located either under instrument panel left of steering column, or behind driver's side kick panel. On Trooper, ECM is located under center console.
ECM analyzes all electrical data signals from input devices to control fuel injection, ignition and emissions. ECM includes a back-up fail-safe control system. If a malfunction develops in ECM microcomputer, back-up control system will maintain necessary engine functions to permit operation of vehicle. 2.6L, 2.8L and 3.1L engines use an ECM, a separate Programmable Read Only Memory (PROM) calibrator and a Calibration Package (CALPAK).
PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. CALPAK provides fuel delivery back-up so engine runs in case of PROM or ECM failure. Any time ECM is replaced, PROM and CALPAK must both be installed into replacement ECM. If battery voltage is removed, PROM and CALPAK information is retained.
On Impulse (non-turbo) and Stylus, ECM calibrations are stored in the Electronic Erasable Programmable Read Only Memory (EEPROM). EEPROM performs functions of PROM and CALPAK, but is not removable from ECM. When replacing ECM, ensure ECM has correct EEPROM for that vehicle. Impulse (turbo) vehicles use an ECM containing a Memory Calibration (MEM-CAL) unit. MEM-CAL unit contains functions of PROM, CALPAK and Electronic Spark Control (ESC) module.
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the control unit. The second category covers OUTPUT SIGNALS, which are components controlled by the control unit.
INPUT DEVICES
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 in the ENGINE PERFORMANCE Section. The available input signals include the following
Airflow Sensor (AFS)
AFS is located in air cleaner housing. Airflow sensor measures rate (volume) of air intake.
Cam Angle Sensor (Hall Effect Switch)
Hall Effect switch signals ECM when No. 1 cylinder is 25° ATDC on compression stroke. ECM uses this information to start fuel injector sequence.
Coolant Temperature Sensor (CTS)
CTS is located in engine block. CTS informs ECM of engine coolant temperature.
Crank Angle Sensor (CAS)
CAS is located in distributor housing. CAS detects engine speed and relative position of each piston. With this information, ECM calculates ignition timing and dwell angle.
Detonation (Knock) Sensor
Detonation sensor is located on cylinder head. If a detonation occurs, detonation sensor sends a signal to ECM to retard ignition timing.
EGR Temperature Sensor
EGR temperature sensor is mounted in engine exhaust passage, next to EGR valve. It provides ECM with EGR temperature information.
Manifold Absolute Pressure (MAP) Sensor
MAP sensor monitors intake manifold vacuum and informs ECM of engine load.
Manifold Air Temperature (MAT) Sensor
MAT sensor monitors intake manifold air temperature and notifies ECM.
Oxygen (O2) Sensor
O2 sensor is located in exhaust manifold and informs ECM of amount of oxygen in exhaust gases.
Park/Neutral Switch (A/T), Inhibitor Switch (M/T)
Switch is used to inform ECM of gear selection. Information is used by ECM to allow starter operation and control engine idle speed.
Power Steering Pressure Switch (PSPS)
When power steering pressure is high, PSPS sends a signal to ECM to increase idle speed. ECM will also turn off A/C clutch when high P/S pressure is detected.
Throttle Position Sensor (TPS)
TPS is mounted on throttle body throttle shaft and informs ECM of incremental changes in throttle position. TPS is sometimes combined with a throttle valve switch.
Throttle Valve Switch (TVS)
TVS is mounted on throttle body throttle shaft and informs ECM of changes in throttle position.
Vehicle Speed Sensor (VSS)
VSS is located in speedometer and informs ECM 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.
Air Management Valve (AMV)
See AIR INJECTION under EMISSION SYSTEMS.
Air Switching Valve (ASV)
See AIR INJECTION under EMISSION SYSTEMS.
Air Regulator
See IDLE SPEED under FUEL SYSTEM.
See SELF-DIAGNOSTIC SYSTEM.
Direct Ignition System (DIS)
See IGNITION SYSTEM.
Dropping Resistor
See FUEL CONTROL under FUEL SYSTEM.
EGR Duty Solenoid
See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.
EGR Valve
See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.
Electronic Spark Advance
See TIMING CONTROL SYSTEMS.
Electronic Spark Control
See TIMING CONTROL SYSTEMS.
Electronic Spark Timing
See TIMING CONTROL SYSTEMS.
Fast Idle Solenoid
See IDLE SPEED under FUEL SYSTEM.
Fuel-Cut System
See FUEL CONTROL under FUEL SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Regulator
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pressure-Up System
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Idle Air Control (IAC) Valve
See IDLE SPEED under FUEL SYSTEM.
See FUEL CONTROL under FUEL SYSTEM.
Power Transistor
See IGNITION SYSTEM.
Vacuum Switching Valve (VSV)
See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.
Fuel pump is located inside fuel tank. Pump and sending unit are integral in design and must be replaced as an assembly. (Scheme 3)
Scheme 3
Fuel Pump Relay (Impulse & Stylus)
When ignition is turned on, ECM energizes fuel pump relay. Relay will operate fuel pump for a few seconds to pressurize fuel system. After engine start, relay contacts are held closed by ECM. If engine running signals to ECM are interrupted, ECM will de-energize relay.
Fuel Pump Relay (Except Impulse & Stylus)
When ignition switch is turned on, ECM will activate fuel pump relay to run fuel pump. Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If there are no reference pulses, ECM will shut off fuel pump within 2 seconds after ignition is turned on. Should fuel pump relay, or 12-volt relay from ECM fail, fuel pump will run through oil pressure switch, which will activate back-up relay circuit. Oil pressure must reach 4 psi (0.28 kg/cm 2 ) before back-up relay is activated.
Fuel Pressure Regulator (PFI)
Fuel pressure regulator governs flow of fuel to injectors. Pressure varies depending on different vehicle speed and load conditions. To improve hot restartability on some models, ECM controls fuel pressure regulator vacuum switching valve (VSV) to change vacuum supplied to pressure regulator. (Scheme 4)
Scheme 4
Fuel pressure regulator consists of a fuel chamber and a vacuum chamber separated by a diaphragm. (Scheme 5) Fuel chamber has a fuel inlet pipe and a fuel outlet pipe. Fuel inlet pipe delivers fuel from fuel distributor pipe. Appropriate amount of fuel is then delivered to fuel injector.
Scheme 5
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.
Fuel Pressure Regulator (TBI)
Fuel pressure regulator, built into fuel meter cover on throttle body, governs flow of fuel to injectors. Pressure remains constant at all speeds and load conditions.
Fuel pressure regulator consists of a fuel chamber and an air chamber separated by a diaphragm-operated relief valve and a calibrated spring. Fuel passes injectors and is stopped by diaphragm until pressure builds sufficiently to overcome calibrated spring. Excess fuel is returned to fuel tank by fuel return pipe.
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 ECM. The main control sensor is the oxygen (O2) sensor located in the exhaust manifold. The O2 sensor tells the ECM the amount of oxygen in the exhaust gases and the ECM changes the air/fuel ratio to the engine by modifying fuel injector pulse width. Fuel system operates in either "open loop" or "closed loop" mode.
Open Loop
When engine is cold and engine speed is greater than 400 RPM, ECM operates in open loop mode. In open loop, ECM calculates air/fuel ratio based upon coolant temperature and Manifold Absolute Pressure (MAP) sensor readings. Engine remains in open loop operation until O2 sensor reaches operating temperature, coolant temperature reaches preset temperature and a specific period of time has elapsed after engine starts.
Closed Loop
When O2 sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, ECM operates in closed loop. In closed loop, ECM controls air/fuel ratio based upon O2 sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in the O2 sensor circuit, vehicle once again enters open loop mode.
Battery Voltage Correction
ECM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. ECM is able to perform these commands because of a built-in memory/learning function.
Dropping resistor is connected in series between power source and fuel injector(s). The dropping resistor reduces applied voltage to stabilize injection starting condition during initial stage.
Fuel is metered into cylinders by electrically controlled solenoid valves in injectors. ECM controls on/off time (duty cycle) of fuel injectors to regulate air/fuel ratio. (Scheme 6)
Scheme 6
Controlled by throttle position sensor, this system reduces or shuts off fuel flow during vehicle deceleration to reduce exhaust emissions. With engine at operating temperature, vehicle at cruising speed, and throttle position sensor returned to idle position, ECM reduces impulses to fuel injectors.
Fuel Pressure-Up System (PFI)
Fuel pressure-up system slightly increases fuel pressure on hot restarts for improved starting and idle stability. Pressure rise is accomplished by cutting off vacuum signal to fuel pressure regulator. ECU controls vacuum signal through Vacuum Switching Valve (VSV).
O2 sensor is located in exhaust manifold and informs ECM of amount of oxygen in exhaust gases.
Air regulator operates as an air by-pass during cold fast idle conditions. When engine is cold, air regulator allows additional flow of air to enter intake manifold.
Temperature sensitive electric element closes regulator as engine temperature increases. Air regulator is independent from I-TEC system and is operated by ignition switch through fuel pump relay.
When A/C is turned on, solenoid increases engine idle speed to prevent engine stalling.
IAC system is used to control idle speed. IAC system consists of an air control valve, ECM and sensors. ECM controls idle speed by opening and closing IAC valve to allow air to by-pass throttle body. It does this by sending voltage pulses to proper motor winding in IAC motor. This will cause motor shaft and valve to move in and out of motor a given distance for each pulse received. ECM pulses are referred to as counts.
Each time ignition is turned off, ECM will reset IAC valve. This is done by sending enough counts to seat valve. Fully seated valve is ECM reference point. A given number of counts are then issued to open valve and normal ECM control of IAC valve will begin from this point. To increase idle speed, ECM will increase counts to retract IAC valve to allow more airflow through idle air passage and by-pass throttle plate until idle speed reaches proper RPM. To decrease idle speed, ECM will reduce counts to extend IAC valve to reduce air flow through idle passage around throttle plate. This will reduce ECM counts.
Impulse (Turbo)
Direct ignition system consists of a coil pack, ignition module, cam angle sensor and ECM. Coil pack consists of 2 interchangeable ignition coils. Ignition module controls primary circuit to coils and spark timing below 400 RPM and if ECM by-pass circuit becomes open or grounded.
Cam angle sensor magnetic pick-up provides a cam signal to ECM to identify correct firing sequence and crank signals to trigger each coil at proper time. System uses Electronic Spark Timing (EST) and control wires from ECM. ECM controls timing using crankshaft position, engine RPM, engine temperature and manifold absolute pressure.
Optical
A crank angle sensor is built into distributor housing. Crank angle sensor uses a photo electric pick-up and rotor plate to measure engine speed and piston position. (Scheme 7) The ECM controls a power transistor unit to trigger ignition coil. On Impulse and Stylus, a power transistor unit is mounted on side of ignition coil. On Amigo, Pickup and Trooper 2.6L, power transistor unit is in-line between ECM and ignition coil.
Scheme 7
Magnetic
Electronic ignition system consists of a magnetic pick-up coil, pole piece and electronic module inside distributor. (Scheme 8) Ignition coil is externally mounted.
When distributor rotates, an alternating current is induced in pick-up coil windings. Current passes through electronic module to ECM, which in turn signals electronic module in distributor to trigger ignition primary circuit. Collapse of magnetic field in ignition coil primary windings induces a high voltage surge in secondary windings to fire spark plugs.
Scheme 8
Amigo, Pickup, Rodeo & Trooper (PFI)
Power transistor unit is in-line between ECM and ignition coil. Transistor is controlled by ECM and triggers ignition coil.
ELECTRONIC SPARK ADVANCE (I-TEC SYSTEM)
Electronic Control Module (ECM) uses inputs from crank angle sensor to control distributor advance curve. ECM is part of I-TEC engine control system. Other sensor inputs to ECM controlling ignition timing advance are, coolant temperature sensor, vehicle speed sensor, MAP sensor, airflow sensor, throttle valve switch and detonation (knock) sensor.
ELECTRONIC SPARK TIMING (EST)
Spark advance curve is totally controlled by Electronic Spark Timing (EST) circuitry of Electronic Control Module (ECM). No vacuum or centrifugal advance mechanisms are used in this system. ECM monitors crankshaft position, engine RPM, engine load, atmospheric (barometric) and manifold pressure, engine detonation, engine temperature and transmission gear position. ECM uses this data to compute desired spark advance.
When engine is cranking, ignition module sends a reference (RPM) signal to ECM on reference wire of 4-wire EST connector. When engine speed exceeds 450 RPM, ECM applies 5 volts on by-pass wire to switch ignition timing control to ECM.
Note. Reference signal also triggers fuel injection system. If ECM does not receive a reference signal during cranking, vehicle will not start.
ECM expects to see no voltage on EST line when timing is controlled by ignition module (less than 450 RPM). If ECM senses voltage on this line, it will set a Code 42 and it will not go into EST mode. While system is running on ignition module, signal on EST wire will be grounded through ignition module. When engine speed is greater than 450 RPM, voltage will be applied to by-pass wire. EST voltage will vary as EST is no longer grounded to ignition module.
If by-pass wire is open or grounded, ignition module will not go into EST mode. Instead, EST voltage will be low and a Code 42 will set. If EST line is grounded, ignition module will switch to EST but, there will be no EST signal and Code 42 will set. If by-pass signal is missing because of ECM failure or open circuits, engine will run at base timing plus a small amount of RPM advance built into ignition module.
ELECTRONIC SPARK CONTROL (ESC)
On vehicles using ECMs containing MEM-CAL units, the ECM supplies a 5-volt DC reference signal on the knock sensor signal line. Internal circuitry of the knock sensor pulls this voltage down to about 2.5 volts. When knock occurs, the knock sensor produces an AC voltage signal which rides on the 2.5-volt DC signal to the ECM. The voltage and frequency of this signal depend upon knock signals received by the sensor. The ECM retards spark timing until signals from detonation sensor cease.
EMISSION SYSTEMS
Note. For emission systems usage, see EMISSION APPLICATIONS article.
AIR INJECTION
Purpose of air injection system is to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions by supplying filtered air through air check valve into exhaust manifold.
Amigo, Pickup 2.6L & Rodeo 2.6L
Air injection system consists of a belt driven air pump, Air Management Valve (AMV), silencer, relief valve, check valve and necessary plumbing. (Scheme 9)
To reduce hydrocarbon (HC) and carbon monoxide (CO) emissions, system pumps air into exhaust ports to accelerate oxidation. AMV directs air to either check valve or atmosphere depending on signal from ECM.
Scheme 9
Trooper 2.6L
Air injection system consists of a belt driven air pump, relief valve, Air Switching Valve (ASV), silencer, Vacuum Switching Valve (VSV), check valve and necessary plumbing. (Scheme 10)
To reduce hydrocarbon (HC) and carbon monoxide (CO) emissions, system draws air into exhaust ports to accelerate oxidation. ASV switches air passages from air pump through a VSV actuated by an electric signal supplied by Electronic Control Module (ECM).
Scheme 10
Trooper 2.8L
Air injection system consists of a belt driven air pump, Electric Air Control Valve (EACV) with solenoid, check valves and necessary plumbing. (Scheme 10) When engine is cold or in wide open throttle condition, ECM energizes EACV solenoid and air is directed to exhaust manifold ports. When coolant temperature increases, EACV solenoid is de-energized and air goes into air cleaner. At higher engine speeds, air is directed to air cleaner through pressure relief valve even though EACV solenoid may be energized. During engine deceleration, air is directed to air cleaner. If engine is operating under a rich condition, or CHECK ENGINE light Illuminates, EACV solenoid is de-energized and air is diverted to air cleaner.
CRANKCASE VENTILATION
Positive Crankcase Ventilation (PCV) system is designed to prevent contaminating hydrocarbon (HC) build-up in crankcase from escaping into atmosphere. PCV system is a closed type. It consists of a valve cover baffle plate, PCV valve and oil separator (except some models). Oil separator removes oil particles from blow-by gases. Crankcase vapors are routed from crankcase through oil separator to PCV valve, and into intake manifold to be reburned in combustion chamber.
EVAPORATIVE EMISSION SYSTEM
Purpose of Fuel Evaporation Emission Control (EVAP) system is to prevent escape of gasoline vapors (hydrocarbons) from fuel tank into atmosphere. To reduce hydrocarbon (HC) emissions, evaporated fuel from fuel tank is routed through a charcoal canister into intake manifold for combustion in cylinders. (Scheme 11) Purging of canister is controlled by a Vacuum Switching Valve (VSV) operated by ECM.
Scheme 11
EXHAUST GAS RECIRCULATION (EGR) SYSTEM
EGR system reduces oxides of nitrogen (NOx) by recycling (through EGR valve) some exhaust gas back into intake manifold to lower combustion chamber temperature.
Amigo & Pickup
On Federal 2.6L, EGR system consists of an EGR valve, thermal vacuum valve and backpressure transducer. On California 2.6L, EGR system consists of an EGR valve, EGR cut solenoid, EGR duty solenoid, vacuum regulator and a vacuum tank. On 3.1L, EGR system consists of an EGR valve and Electronic Vacuum Regulator Valve (EVRV) to regulate EGR flow. ECM controls vacuum to EGR valve through EVRV using coolant temperature, throttle position and distributor signal inputs.
Impulse & Stylus
EGR system consists of an EGR valve, backpressure transducer and EGR solenoid (non-turbo) or vacuum switching valve (turbo).
Rodeo & Trooper
On PFI models, EGR system consists of an EGR valve, backpressure transducer, thermal vacuum valve and vacuum switching valve. On TBI models, EGR system consists of and EGR valve and an Electronic Vacuum Regulator Valve (EVRV) to regulate EGR flow. ECM controls vacuum to EGR valve through EVRV using coolant temperature, throttle position and distributor signal inputs.
When throttle valve is opened, a vacuum is applied to EGR valve vacuum diaphragm. When vacuum in vacuum diaphragm chamber reaches a specified value, diaphragm overcomes holding force of spring and moves to its fully raised position. This upward motion of diaphragm opens EGR valve allowing exhaust gas to be pulled into intake manifold.
EGR Backpressure Transducer
Backpressure transducer is a device that modulates a vacuum signal from throttle valve to EGR valve via EGR solenoid or vacuum switching valve. An air bleed in backpressure transducer is opened during low exhaust gas pressure, preventing operation of EGR valve. During high exhaust gas pressure, air bleed is closed and vacuum signal from throttle valve is sent directly to EGR valve.
EGR duty solenoid is operated by ECM to control vacuum to the EGR valve or backpressure transducer.
Scheme 12
Electronic Vacuum Regulator Valve (EVRV)
ECM controls vacuum to EGR valve through EVRV using inputs from coolant temperature, throttle position and distributor signal.
VSV is controlled by ECM to supply vacuum signal to canister purge, fuel pressure regulator, air management valve, EGR valve and turbocharger wastegate, under specified conditions.