INTRODUCTION
This article covers the basic description and operation of engine performance related systems and components. Before diagnosing vehicles or systems you are not completely familiar with, read through this article.
ENGINE CONTROL UNIT (ECU)
The ECU controls fuel injector pulse width and ignition timing based on inputs from various sensors and switches. Sensors monitor exhaust gas oxygen content, air/fuel temperature, coolant temperature, manifold absolute pressure, battery voltage, crankshaft position (engine speed) and throttle position. Switches provides ECU with informations on power steering pump output pressure, wide open throttle conditions and transmission gear selection (automatic transmission only). Battery voltage is also monitored to determine injector pulse width.
Note. The various components are grouped into 2 categories. The first category, INPUT DEVICES, consists of components which control or produce voltage signals monitored by ECU. The second category, OUTPUT SIGNALS, includes components that are controlled by ECU (this is usually accomplished by ECU grounding individual circuits).
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. The available input signals include the following
A/C CONTROLS
Tells ECU that the A/C is on, and indicates when the compressor clutch needs to be engaged. ECU changes engine idle speed to compensate increased engine load during compressor operation.
AIR TEMPERATURE SENSOR
This is a thermostatic control sensor for the air cleaner system. Sensor provides heated air for carburetor or throttle body (depending on vehicle application) during engine warm-up.
BATTERY VOLTAGE
Ensures proper voltage is applied to fuel injector. ECU varies voltage applied to injector to compensate for fluctuations in battery voltage.
CLOSED THROTTLE SWITCH
Switch is integral with Idle Speed Actuator (ISA) motor and enables ECU to increase or decrease throttle stop angle (by extending or retracting ISA) in response to operating conditions.
COOLANT TEMPERATURE SENSOR/SWITCH
Coolant temperature sensor/switch is installed in the intake manifold coolant and monitors engine coolant temperature. Coolant temperature information is transmitted to the ECU through input voltage generated by sensor/switch. ECU uses this information to adjust injector pulse width, compensate fuel condensation in the manifold, control warm-up idle speed, increase advance when coolant is cold, and energize EGR valve solenoid to prevent vacuum to EGR valve (cold engine).
DETONATION (OR KNOCK) SENSOR
Sensor generates input signal to ECU when detonation occur. ECU uses this information to adjust ignition advance to eliminate detonation at appropriate cylinders.
DISTRIBUTOR (PICKUP COIL & TRIGGER WHEEL) 5.9L
Ignition coil is opened and closed electronically by ignition module. Distributor pickup coil and trigger wheel provide input signal for ignition module. Trigger wheel, located on distributor shaft, has one tooth for each cylinder. Wheel is mounted so teeth rotate past pickup coil one at a time. Trigger wheel teeth induce alternating current (pulse) into pickup coil as they pass by, triggering ignition module. Ignition module opens and closes primary coil circuit according to position of trigger wheel teeth.
DISTRIBUTOR (SYNC SIGNAL GENERATOR) 4.0L & 4.2L
Sync signal generator is in the distributor and denotes position of pistons one and six. The sync signal works in conjunction with engine speed sensor to provide ECU with inputs to establish and maintain correct injector firing order.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
MAP sensor monitors absolute pressure in the intake manifold and provides input voltage to ECU. Change in engine load varies manifold pressure, and causes MAP sensors resistance to vary. Change in sensor resistance causes change in input voltage to ECU. The changing voltage provides ECU with information on ambient barometric pressure while engine is running. ECU uses this information to adjust air/fuel mixture.
MANIFOLD AIR TEMPERATURE (MAT) SENSOR
Installed in the intake manifold, with sensor element extending into the air/fuel stream. MAT sensor provides ECU with input voltage indicating air/fuel stream temperature.
NEUTRAL SAFETY SWITCH
Switch tells ECU that automatic transaxle is NOT in PARK or NEUTRAL but in DRIVE, and prevents engine starter from engaging.
OXYGEN SENSOR
Note. For additional oxygen sensor information, see TITANIA OXYGEN SENSOR TESTING article.
Sensor is located in the exhaust manifold and monitors oxygen content in exhaust gas. Sensor transmits this information by generating input voltage to ECU. ECU uses information to vary air/fuel mixture accordingly. Oxygen sensor may also contain an internal heating element in order to reach operating temperature earlier. This element is controlled by the ECU through an oxygen sensor heater relay.
POWER STEERING PRESSURE SWITCH
Switch monitors power steering pump pressure. When pressure exceeds 275 psi (parking maneuvers), ECU increases engine idle speed to prevent stalling.
SPEED SENSOR/CPS (CRANKSHAFT POSITION SENSOR)
Sensor is attached to flywheel housing. Sensor monitors crankshaft angle, then generates input signal to ECU to provide it with this information. ECU converts rate of angle change to RPM, and converts crankshaft angle into piston position. Sensor senses piston position and engine speed by detecting the flywheel teeth as they pass by during engine operation.
START SIGNAL
Starter motor relay sends input (start signal) to ECU. ECU will then monitor all inputs to determine current operating conditions.
TPS (THROTTLE POSITION SENSOR)
Sensor is a variable resistor mounted on throttle body and connected to throttle shaft. Sensor provides ECU with input voltage to indicate throttle valve position.
WIDE OPEN THROTTLE (WOT) SWITCH
Switch is attached to throttle body and provides input to ECU to indicate wide-open-throttle condition. ECU will enrich air/fuel ratio to correspond to increased air flow.
OUTPUT SIGNALS
Note. Each vehicle may be equipped with different combinations of computer controlled components. The following listed components may NOT be used on all models. For theory and operation on each output component, refer to the indicated system under appropriate heading in this article.
A/C CLUTCH RELAY
See MISCELLANEOUS CONTROLS.
B+ LATCH RELAY
See FUEL CONTROL under FUEL SYSTEM.
EGR/EVAPORATIVE CANISTER PURGE SOLENOID
See EVAPORATIVE EMISSION SYSTEM under EMISSION SYSTEMS.
EGR VALVE VACUUM SOLENOID
See EXHAUST GAS RECIRCULATION (EGR) CONTROL (for appropriate engine size) under EMISSION SYSTEM.
FUEL INJECTOR
See FUEL CONTROL under FUEL SYSTEM.
FUEL PUMP CONTROL RELAY
See FUEL DELIVERY under FUEL SYSTEM.
IDLE SPEED ACTUATOR (ISA) MOTOR
See IDLE SPEED under FUEL SYSTEM.
IGNITION TIMING CONTROL
See IGNITION TIMING CONTROL SYSTEM under IGNITION SYSTEM.
OXYGEN SENSOR HEATER RELAY
See FUEL CONTROL under FUEL SYSTEM.
SHIFT INDICATOR LAMP
See TRANSMISSION under MISCELLANEOUS CONTROLS.
FUEL PRESSURE REGULATOR
Each injection system (TBI & MPI) uses a different Fuel Pressure Regulator.
TBI regulator is an overflow type attached to the throttle body. TBI regulator is adjustable, and is not controlled by ECU. See SYSTEM/COMP TESTS article for specifications and testing procedures.
MPI regulator is vacuum balanced and non-adjustable. Regulator is mounted to output end of fuel rail and is connected to intake manifold vacuum. MPI regulator is not controlled by ECU. See SYSTEM/COMP TESTS article for specifications and testing procedures.
Fuel Pump
There are three types of fuel pumps: a mechanical pump driven by a lobe on the cam shaft for carbureted vehicles, and two different types electric pump for injected engines. TBI engines and MPI engines have separate pumps mounted in the fuel tank. Pump are not interchangeable. Both electric pumps have an internal check valve to prevent fuel backflow when pump is not operating. See BASIC TESTING and SYSTEM/COMP TESTS articles for specifications and testing procedures.
Located at right front inner fender panel or right strut tower. Battery voltage is applied to relay via the ignition switch. Relay is energized when ground is provided by ECU. When relay is energized, voltage is applied to fuel pump.
Fuel Return
Fuel in excess of engine needs (determined by regulated pressure) is bypassed to the return hose and sent back to fuel tank where it empties into tank reservoir.
B+ LATCH RELAY (CARBURETED ENGINES)
Relay is located on right inner fender panel and is initially energized during engine start-up. It remains energized for 3-5 seconds after engine is stopped. The relay supplies current to ISA motor, enabling ECU to extend idle speed actuator (ISA) for the next start-up.
FUEL INJECTOR (TBI)
ECU calculates base injection pulse width by processing signals from crankshaft angle sensor and other inputs.
FUEL INJECTOR (MPI)
There is one injector per cylinder. Injectors are attached to fuel rail and aimed at intake valve ports in cylinder head. ECU calculates injector pulse width based on various inputs.
In order for oxygen sensor to reach operating temperature earlier, an internal oxygen sensor heating element is used on some models. This heating element allows vehicle to enter closed loop operation earlier and maintain closed loop even during extended idle periods.
ECU controls engine idle speed according to engine operating conditions. ECU senses engine operating conditions and determines the best idle speed.
IGNITION TIMING CONTROL (2.5L & 4.0L)
The distributors used in 2.5L and 4.0L engines do not have built-in centrifugal or vacuum-assisted advance. Ignition timing is controlled by ECU. Distributors are locked in place by ear on distributor housing which a hold-down bolt passed through.
IGNITION TIMING CONTROL (4.2L)
The distributor used in 4.2L engines is equipped with centrifugal and vacuum-assisted advance systems. Ignition timing is controlled through the engine Microprocessor Control Unit (MCU).
IGNITION TIMING CONTROL (5.9L)
The distributor used in 5.9L engines is equipped with centrifugal and vacuum-assisted advance systems. Ignition timing is advanced to the correct number of degrees during engine operation by these advance systems.
DETONATION RETARD OPERATION (4.0L)
Knock sensor is located on left side of the engine block, above the oil pan. Sensor sends input signal to ECU when it detects detonation (knocking) during engine operation. To eliminate detonation condition, ECU retards the ignition advance.
EMISSION SYSTEMS
Note. For specific emission system and component application, see VACUUM DIAGRAMS article.
AIR INJECTION SYSTEM
Air injection system is designed to send air to exhaust manifold to reduce exhaust emissions (CO, HC and NOx). System may utilize a belt driven pump, or pulse injection type system.
Exhaust Gas Recirculation (EGR) Control (2.5L & 4.0L Engines)
To lower oxides of nitrogen (NOx) exhaust gas emissions, a computer controlled exhaust gas recirculation system is used. Coolant temperature and other inputs are monitored by ECU to determine EGR flow. ECU regulates vacuum to EGR valve through the EGR valve vacuum solenoid. Some models utilize a combined EGR/evaporative canister purge solenoid.
EXHAUST GAS RECIRCULATION (EGR) CONTROL (5.9L ENGINES)
Valve is controlled by an EGR valve vacuum transducer (exhaust backpressure), coolant temperature override valve (CTO), thermal vacuum switch and connecting hoses.
EGR/COOLANT TEMP OVERRIDE (EGR/CTO) CONTROL (4.2L ENGINES)
The EGR/CTO valve is a dual function valve used to serve distributor vacuum advance control. Valve is controlled by coolant temperature and manifold vacuum.
EVAPORATIVE EMISSIONS SYSTEM
System utilizes a charcoal canister storage system. Fumes are taken from tank and carburetor bowl (carbureted), and stored in canister while engine is not running. Fumes are purged to intake manifold when engine is started. On some models, purge vacuum may be controlled by ECU through an evaporative canister purge solenoid, or a combined EGR/evaporative canister purge solenoid.
THERMOSTATIC AIR CLEANER
Intake air temperature control system provides heated air for fuel intake system during engine warm-up. It consists of a heat stove on the exhaust manifold, hot air pipe, regulator door, and thermostatic and vacuum controls.
ON-BOARD DIAGNOSTIC CAPABILITY
To properly interface with Jeep/Eagle vehicles, an adapter must be attached to the DRB-II. The adapter is required, as diagnostics on Jeep/Eagle products are performed Off-Board. The adapter measures signals at the diagnostic connector. Signals are converted by the adapter into a recognizable form for DRB-II to use during tests. The adapter is a computer and MUST be used with the DRB-II to diagnose Jeep/Eagle vehicles. See SELF-DIAGNOSTICS article.
MISCELLANEOUS CONTROLS
Note. Although not considered "true" engine performance related systems, some of the following controlled devices may affect driveability if they malfunction.
Clutch may cause high idle if signal is present without clutch engagement. If clutch is engaged with no signal to ECU, it may cause engine to stall.
COOLING FAN MOTOR
May cause detonation due to overheating if it is not operating.
COOLING FAN CONTROL RELAY
If relay is defective, cooling fan may not come on and an overheating and detonation conditions may result.
TORQUE CONVERTER CLUTCH
May cause surge or detonation due to overheating if clutch does not lock-up.
LOCK-UP RELAY
Defective relay could cause lock-up malfunction.
SHIFT INDICATOR LIGHT
A shift indicator light is used on vehicles equipped with manual transmission. Light indicates proper shift points in order to maintain maximum fuel economy and power. Failure in shift light circuit should not cause driveability problems; however, a noticeable change in fuel economy may result from varying driving habits.