Ccc Theory/operation Chevrolet Camaro III рестайлинг

DESCRIPTION

The computerized engine control system monitors as many as 19 engine/vehicle functions. (Scheme 1) The computerized engine control system is primarily an emission control system which is designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the 3-way catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC) and carbon monoxide (CO) emissions.

MODEL IDENTIFICATION

Repair procedures in this article are identified by body type. The following table lists GM division, model name, and body type.

MODEL IDENTIFICATION

Scheme 1

SYSTEM OPERATION INFORMATION

The computerized engine control system consists of the following suBsystems: fuel control, Electronic Control Module (ECM), input signals (sensors and switches), output signals (controlled devices), emission control and diagnostic system. (Scheme 2)

Typical Schematic of CCC System (Carbureted Shown). Scheme 2

FUEL CONTROL OPERATION - CARBURETED MODELS

All carbureted vehicles are equipped with a 4-Bbl. "feedback" carburetor using an electric Mixture Control (M/C) solenoid. (Scheme 3) The M/C solenoid operates a metering rod system in the float bowl which supplements fuel supplied by idle and main systems in carburetor to vary air/fuel ratio within a pre-calibrated range. The M/C solenoid also controls air/fuel ratio through use of an idle air bleed that operates in conjunction with metering rod system.

Scheme 3

FUEL CONTROL OPERATION - FUEL INJECTED MODELS

An in-tank electric fuel pump delivers fuel to injector(s) through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. A pressure regulator mounted in fuel rail (port systems) or throttle body unit (throttle body systems) keeps fuel available to injectors at a constant pressure of 9-13 psi (.6-.9 kg/cm 2 ) at idle for throttle body injection and 30-43 psi (2.1-3.0 kg/cm 2 ) at idle for port injection systems. Excess fuel is returned to fuel tank through pressure regulator return line. For fuel system performance check, see appropriate CHART A7, FUEL SYSTEM DIAGNOSIS for that specific system.

1. Throttle Body Injection (TBI) - An electrically pulsed injector is located in intake manifold throttle body unit. The ECM controls injector "on" time (pulse width) to provide proper amount of fuel to engine. The 5.0L (VIN E) and 5.7L (VIN 7) engines use a throttle body with 2 injectors.
2. Port Fuel Injection (PFI) - Individual, electrically pulsed injectors are located in intake manifold. These injectors are next to intake valves in cylinder head. The ECM controls the "on" time (pulse width) of each injector to provide the proper amount of fuel to engine. Standard PFI systems feature simultaneously double-fired injection. On these systems, all injectors pulse one time for each engine revolution. Thus, 2 injections of fuel are mixed with incoming air to produce the fuel charge for each combustion cycle. On models with Sequential Fuel Injection (SFI), injectors are pulsed sequentially in spark plug firing order.

ELECTRONIC CONTROL MODULE (ECM) OPERATION

ECM is located in passenger compartment. For exact location of ECM, see appropriate COMPONENT LOCATION figure at end of each engine application section. The ECM consists of the Arithmetic Logic Unit (ALU), Central Processing Unit (CPU), power supply and system memories.

The ECM has a "learning" ability which allows it to make minor corrections for fuel system variations. If battery power is interrupted, a vehicle performance change may be noticed. This will correct itself and normal performance will return if vehicle is allowed to "relearn" optimum control conditions. This is accomplished by driving vehicle at normal operating temperature, under part throttle, moderate acceleration and idle conditions.

ARITHMETIC LOGIC UNIT (ALU)

This internal component of the ECM converts electrical signals, received by ECM from various engine sensors, into digital signals for use by the CPU.

CENTRAL PROCESSING UNIT (CPU)

Digital signals received by CPU are used to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. The CPU also calculates spark timing and idle speed information. The CPU commands operation of emission control, "closed loop" fuel control and diagnostic system.

INPUT/OUTPUT DEVICES

These devices are an integral part of ECM. They convert electrical signals, received by ECM from various engine sensors, into digital signals for use by the CPU.

POWER SUPPLY

Power for ECM reference output signals (5 volts) and control devices (12 volts) is received from the battery (through ignition circuit when ignition switch is in "ON" position). Keep alive memory power is received directly from the battery.

MEMORIES

The 5 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM), Programmable Read Only Memory (PROM), fuel system CALPAC and Memory Calibration unit (MEM-CAL).

1. Read Only Memory (ROM) - ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
2. Random Access Memory (RAM) - RAM is the scratchpad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from ECM, all information stored in RAM is lost.
3. Programmable Read Only Memory (PROM) - PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from ECM. If battery voltage is removed, PROM information will be retained.
4. CALPAC - Some fuel injected models use a PROM and a device called a CALPAC. The CALPAC provides fuel delivery back-up so engine will run in case of a PROM or ECM failure. Any time ECM is replaced, PROM and CALPAC must both be installed into replacement ECM. If battery voltage is removed, CALPAC information will be retained.
5. MEM-CAL - Vehicles with fuel injection may also use another type of ECM containing a Memory Calibration unit (MEM-CAL). This assembly contains functions of PROM and CALPAK and on some models, the ESC control module. If power to ECM is removed, MEM-CAL information will be retained.

INPUT SIGNALS OPERATION

Note. Not all sensors and input signals are used on all models.

Each sensor or switch furnishes electronic (voltage) signals to ECM. The ECM uses these input signals to compute spark timing, air/fuel ratio and idle speed for proper driveability and emission control.

AIR CONDITIONER "ON" SWITCH

The air conditioner "on" switch is mounted in instrument panel. This switch provides a simple "on" or "A/C request" signal to ECM. The ECM uses this signal to determine control of the A/C clutch relay and to adjust idle speed when air conditioner compressor clutch is engaged. On some models, ECM may also activate radiator cooling fan when this signal is present. If this signal is not present on A/C equipped vehicles, vehicle may idle rough when A/C compressor cycles. To check function of the A/C switch, see appropriate component CHART C10 for that system.

BAROMETRIC PRESSURE (BARO) SENSOR

This sensor is mounted above right kickpad. This sensor measures ambient or barometric pressures and signals ECM of pressure changes due to altitude and/or weather. A single BARO sample is taken by ECM whenever ignition is turned on or wide open throttle is achieved. This sensor may be used on engines equipped with MAP sensor. BARO sensor circuit problems may set Code 32.

BATTERY VOLTAGE

Battery voltage is monitored by ECM on ignition input terminal. If battery voltage swings low, a weak spark or improper fuel control may result. To compensate for low battery voltage, ECM may increase idle speed, advance ignition timing, increase ignition dwell or enriched the air/fuel mixture. If voltage swings high, ECM may set a charging system fault code and turn on "SERVICE ENGINE SOON" light. If voltage signal swings excessively low (less than 9 volts) or excessively high (16 volts, most models) ECM will shut down for as long as condition exists. If condition is short-term, vehicle may stumble and "SERVICE ENGINE SOON" light will flicker. If condition lasts long enough, EFI-equipped vehicles will die. Carbureted (full function) vehicles will lose computer control function, but will continue to run.

COOLANT TEMPERATURE SENSOR (CTS)

The CTS is a thermistor (temperature sensitive resistor) located in an engine coolant passage. The ECM supplies and monitors a 5-volt signal to CTS. This 5-volt signal is then reduced by resistance of the CTS. When coolant temperatures are low, CTS resistance is high and a high monitored voltage signal is seen by the ECM. See COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES table in CODE 14 or 15 chart. When coolant temperatures are high, CTS resistance is low and a lower monitored voltage is seen by the ECM. When fully warmed, CTS should reflect a temperature of at least 185°F (85°C).

Coolant temperature input is used in the control of fuel delivery, ignition timing, idle speed, emission control devices and Torque Converter Clutch (TCC). A CTS which is out of calibration will not set a trouble code, but can cause fuel delivery and driveability problems. A coolant sensor "circuit" problem should set Code 14 or Code 15.

CAMSHAFT POSITION SENSOR

A separate Hall Effect camshaft position sensor is used on 3.8L C(3)I-equipped models, while 3.3L C(3)I-equipped models use a combination cam and crank Hall Effect sensor. The cam sensor provides ECM with a TDC No. 1 signal used to compute the exact position of valves. This allows ECM to properly time ignition and fuel injection operation on PFI and SFI-equipped models. A fault in the cam sensor circuit (no cam sensor signal) will result in a no-start condition and should set a Code 41.

CRANKSHAFT POSITION SENSOR

The C(3)I (3.3L and 3.8L) crankshaft position sensor utilizes a Hall Effect switch mounted near vibration damper. The sensor monitors vibration damper position (crankshaft position) and sends a signal to ignition module. This signal indicates TDC position of each piston as well as supplying an engine speed (RPM) signal.

The 2.0L, 2.5L, 2.8L and 3.1L Direct Ignition System (DIS) and 2.3L Integrated Direct Ignition (IDI) system crankshaft position sensor protrudes into side of engine block, to within .05" (1.3 mm) of an internally-mounted crankshaft ring. The reluctor ring is a special trigger wheel cast into the crankshaft. As crankshaft rotates, 7 notches in the reluctor ring change the magnetic field at the tip of position sensor. This creates an induced AC voltage signal in the sensor windings, resulting in reference signals which are sent to ECM by ignition module. This allows ECM to compute crankshaft position and RPM, and fire appropriate ignition coil at the proper time.

HIGH GEAR SWITCH

The high gear switch is located inside automatic transmission. This switch is open in high gear (3rd or 4th) and closes when transmission shifts into any other gear. High gear switch information is used by ECM in controlling emission components and Torque Converter Clutch (TCC) engagement. For additional information on the high gear switch, see appropriate CHART C8, TORQUE CONVERTER CLUTCH DIAGNOSIS.

IGNITION/CRANK SIGNAL

The ECM looks at the initial cranking (RPM) signal to determine when the engine is being started. This information is used for starting enrichment. If this signal is not available, EFI vehicles may be hard to start.

KNOCK SENSOR

The knock sensor is a piezoelectric device which detects abnormal engine vibrations (spark knock) in the engine. This vibration results in the production of a very low AC signal which is sent back to the ECM controller or to the MEM-CAL portion of the ECM. The ECM will then retard ignition timing until the engine knock ceases. A fault in the ESC circuit should set a Code 43. When Code 43 is not present and the ESC system is suspected as the cause of a driveability problem, see appropriate CHART C5 for that system for additional diagnostic information.

MASS AIRFLOW (MAF) SENSOR

The MAF sensor is used on PFI systems. Sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening and air volume), similar to the relationship of engine load to MAP or vacuum sensor signal. MAF signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. The ECM uses this information to control fuel delivery.

Two different types of MAF sensor are used, depending upon engine application. The hot wire type sensor uses a 12-volt supply to maintain a calibrated temperature on the hot wire. As airflow increases or decreases across the wire, the current required to maintain the calibrated temperature will vary. As this current varies, internal circuitry causes a monitored voltage signal from the ECM (5 volts) to vary from about .4 volt (low airflow) to about 5.0 volts (high airflow).

The frequency generator type MAF sensor produces a frequency signal that cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow. A fault in the MAF sensor circuit should set Code 33 or 34.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

The MAP sensor measures changes in manifold pressure. Changes in manifold pressure result from engine load and speed changes. The MAP sensor converts these changes in manifold pressure into a voltage output signal to ECM (1.5 volts at idle to 4.5 volts at WOT). The ECM can monitor these signals and adjust air/fuel ratio and ignition timing under various operating conditions. If MAP sensor fails, the ECM will substitute a fixed MAP value and will use the TPS to control fuel delivery. A fault in the MAP circuit should set a Code 33 or 34. If Code 33 or 34 is not present and MAP sensor is suspected of causing a driveability problem, see appropriate CHART C1D for that system.

MANIFOLD AIR TEMPERATURE (MAT) SENSOR

The MAT sensor is a thermistor (temperature sensitive resistor) mounted in the intake manifold. Low intake air temperature produces high internal sensor resistance, while high temperature causes low internal sensor resistance. See MAT SENSOR TEMPERATURE TO RESISTANCE VALUES table in CODE 23 or 25 chart. The ECM supplies and monitors a 5-volt signal to sensor through a resistor in ECM. By monitoring this voltage, ECM determines manifold air temperature. After a vehicle has set overnight, MAT and CTS signals (resistance and temperature) should be close to same reading. Failure in MAT sensor circuit should set either a Code 23 or 25.

NOSE SWITCH

This switch (also referred to as an idle switch) is an integral part of the Idle Speed Control (ISC) motor mounted on carburetor linkage. Switch tells ECM when it should control idle speed. It is a part of ISC motor. Switch should be "on" in the idle control range.

OXYGEN (O2) SENSOR

The O2 sensor is mounted in the exhaust system where it monitors oxygen content of exhaust gases. The oxygen content causes the Zirconia/Platinum-tipped O2 sensor to produce a voltage signal which is proportional to exhaust gas oxygen concentration (0-3%) compared to outside oxygen (20-21%). This voltage signal is low (about .1 volt) when a lean mixture is present and high (about 1.0 volt) when a rich mixture is present. As ECM compensates for a lean or rich condition, this voltage signal constantly fluctuates between high and low, crossing a .45-volt reference voltage supplied by ECM on the O2 signal line. This is referred to as "cross counts".

The O2 sensor will not function properly (produce voltage) until its temperature reaches 600°F (316°C). At temperatures less than the normal operating range of the sensor, vehicle will function in "open loop" mode and ECM will not make air/fuel adjustments based upon O2 sensor signals but will use TPS and MAP values to determine air/fuel ratio from a table built into memory. When ECM reads a voltage signal of more than .45 volts from the O2 sensor, ECM will begin to alter commands to injector or M/C solenoid to produce either a leaner or richer mixture. Once vehicle has entered "closed loop", a fault in the O2 circuit (cooled-down, open or shorted O2 sensor) is the only thing which can return it to "open loop". A problem in the O2 sensor circuit should set Code 13, 44 or 45.

PARK/NEUTRAL SWITCH (P/N)

This switch is connected to transmission gear selector. The switch indicates when transmission is in Park or Neutral. Information from P/N switch is used for determining control of ignition timing, Torque Converter Clutch (TCC) and Idle Air Control (IAC) valve. To check function of P/N switch, see appropriate component CHART C1A for that system.

POWER STEERING (P/S) SWITCH

This switch informs ECM of engine load conditions which exist when steering wheel is turned from center to full lock position. Information is used to help control idle speed. To check function of P/S switch, see appropriate component CHART C1E for that system.

PRESSURE SENSOR

This sensor is a combination MAP and BARO sensor used on 5.0L (VIN Y) engines. When ignition is first turned on, ECM records sensor input as a barometric pressure reading. After vehicle starts, signal is processed as a MAP sensor input. ECM then internally converts the MAP sensor signal to a vacuum signal by comparing current input to the BARO input stored in memory. Thus, if a "scan" tester is used to measure pressure sensor voltage, reading will differ from that obtained with a DVOM at the sensor. This input is used primarily as an indication of engine load and is utilized by ECM for timing and fuel control calculations. Pressure sensor circuit problems should set Code 34. If Code 34 is not present and pressure sensor is suspected of being the cause of driveability problems, see 5.0L FULL FUNCTION CHART C1D for further diagnosis.

RPM REFERENCE SIGNAL

The RPM is monitored by ECM through tach/pulse signals produced by either the HEI module (tach reference line of 4-wire EST connector) or crankshaft position sensor signal (Hall Effect signal on C(3)I, PM generator signal on DIS and IDI). These signals are used by ECM for controlling timing, fuel delivery, EGR function and idle speed.

THROTTLE POSITION SENSOR (TPS)

The TPS is a variable mechanical resistor connected either directly (EFI) or indirectly (carbureted) to the throttle shaft linkage. The TPS has 3 wires connected to it. One is connected to a 5 volt reference voltage supply from ECM. The second is connected to ECM ground and the third is the signal return which is monitored by ECM. The voltage signal from the TPS varies from closed throttle (.5-1.0 volt) to wide open throttle (4.5-5 volts). This signal is used by ECM for control of fuel, idle speed, spark timing and torque converter clutch. A problem in the TPS circuit may set a Code 21 or 22.

VACUUM SENSOR

Vehicles not equipped with a MAF or MAP sensor may be equipped with a vacuum sensor. The vacuum sensor measures the difference between atmospheric pressure (outside air) and manifold vacuum and converts this to a voltage signal (4.5 volts at idle to 1.5 volts at WOT) which is used to determine engine load. This voltage signal is opposite of that produced by MAP sensor.

VEHICLE SPEED SENSOR (VSS)

Depending upon vehicle application, VSS is either a Permanent Magnet (PM) generator mounted in transmission or a Light Emitting Diode (LED) in instrument panel cluster, behind speedometer. The VSS sends a pulsing signal to ECM, which ECM converts into miles per hour (MPH). This sensor input is used by ECM in controlling Torque Converter Clutch (TCC). The VSS circuit may be diagnosed by using chart for Code 24.

AIR/FUEL CONTROL

Three types of fuel control systems are used on General Motors vehicles. Descriptions of each system is as follows

1. Mixture Control Solenoid - Solenoid is spring loaded in the normally open (full fuel) delivery mode. When ignition is turned on, current is supplied to solenoid. The ECM controls air/fuel ratio by supplying a ground for the M/C solenoid. This energizing process occurs 10 times per second. The air/fuel ratio is varied by controlling the length of time solenoid is energized during each of the 10 on-off cycles. More "on" time (high dwell) will produce a leaner mixture, while less "on" time (low dwell) will deliver a richer mixture. In "open loop" mode, dwell will be fixed. Solenoid "on" time may be measured in degrees using a dwell meter set on the 6-cylinder scale.
2. Throttle Body Injector - Injector is an electrical solenoid located in throttle body (dual injectors are used on 4.3L, 5.0L and 5.7L engines) which receives current when ignition is on. The ECM controls air/fuel mixture by regulating "on" time (pulse width) of injector. The ECM accomplishes this by pulsing the ground circuit for injector. ECM uses tach (RPM) signal to determine when to pulse injector (one time for each distributor reference pulse, alternately on dual injector systems) except during cranking, "clear flood" mode, deceleration and heavy acceleration, when fuel delivery is controlled by internal ECM calibration.
3. Port Fuel Injectors - Injectors are electrical solenoids (located in intake manifold fuel rails) which supply fuel to individual intake valves. Injectors receive current from ignition switch and are energized by ECM when it provides a ground. There are 2 types of port fuel injection systems: simultaneous double-fire injection and sequential injection. Simultaneously double-fire injection sprays injectors once for each crankshaft rotation. Injectors deliver 1/2 the required fuel during each injector pulse. This provides the required fuel for each combustion cycle (every 2 crankshaft rotations). Sequential fuel injection fires injectors in engine spark plug firing order, using individual ground circuits which are controlled by ECM.

FUEL PUMP

An electric fuel pump is used on fuel injected vehicles. When the ignition switch is turned to the "ON" position, ECM will turn on the electric fuel pump by energizing the fuel pump relay. The ECM will keep the pump on if the engine is running or cranking (ECM is receiving reference pulses from the ignition module). If there are no reference pulses, ECM turns pump off within 2 seconds after key on.

The fuel pump is capable of producing fuel pressure in excess of engine requirements. Excess fuel passes through the pressure regulator and is returned to the tank via the fuel return line. For additional information on fuel pump activation, see appropriate CHART A5, FUEL PUMP RELAY CIRCUIT. For information on fuel system pressure testing, see appropriate CHART A7, FUEL SYSTEM DIAGNOSIS for that system.

IGNITION TIMING

Several types of ignition timing control systems are used on 1989 models, conventional HEI/EST system and 3 types of distributor less ignition systems. When engine speed reaches 400 RPM or more (about 5-15 seconds after starting), ECM senses this on the RPM reference wire from the ignition module. When this RPM signal is sensed, ECM transmits a constant 5-volt signal to the ignition module on the ignition by-pass wire. This changes the position of the by-pass switch in ignition module. When this occurs, ignition module no longer controls firing of ignition coil. Instead, timing is controlled by ECM on EST wire of ignition module.

The Programmable Read Only Memory (PROM) portion of ECM has a basic spark advance curve based on engine speed. Spark timing is calculated by ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by ECM to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see appropriate component CHART C4 for that system.

An Electronic Spark Control (ESC) system is also used on some models. There are 4 basic components to ESC system: a detonation (knock) sensor, a high energy ignition system, an ESC controller (some models), and the ECM.

When detonation (engine knock) occurs, detonation sensor produces a low voltage AC signal. This signal goes to the ESC controller or directly to the MEM-CAL unit inside the ECM, depending upon ignition application. On models using an ESC controller, controller supplies the ECM with a 12-volt signal. When detonation occurs, controller grounds the 12-volt signal to the ECM, pulling the signal down to zero volts. The ECM interprets this as a need to retard timing.

The ECM then retards spark timing until the ESC controller returns the 12-volt signal, or until the MEM-CAL no longer receives knock sensor signals. A malfunction in the ESC circuit should set Code 43. If Code 43 is not present and ESC system is suspected as the cause of driveability problems, see appropriate component CHART C5 for that system.

1. HEI-EST - Vehicles not using a C(3)I (3.3L and 3.8L), IDI (2.3L) or DIS (some 2.0L, 2.5L, 2.8L and 3.1L) EST system are equipped with a Delco-Remy High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains a 7 or 8-terminal HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to Electronic Control Module (ECM). Connectors on the 8-terminal ignition module are sealed at distributor and ignition coil.
2. Computer Controlled Coil Ignition (C(3)I) - C(3)I is a distributor less system used on 3.3L and 3.8L models. It consists of ECM, ignition module, 3 ignition coils (type III uses a 3-coil single unit, type II uses 3 individual coils in one unit), crankshaft position sensor, camshaft sensor (3.3L models use a combination cam and crank sensor) and connecting wires. Ignition module uses a sealed 14-pin connector cable which goes directly to the ECM. Input signals from the Hall Effect crankshaft (RPM and crank position) and camshaft (TDC No. 1) position sensors are used by ignition module to determine engine speed and piston position. This information is passed to ECM which determines when to fire spark plugs. Using camshaft sensor signal, ignition module then selects and sequentially triggers each of 3 interconnected coils, causing spark plugs to fire at the proper time. Each cylinder is paired with the cylinder opposite it in the firing order. These pairs are 1-4, 2-5 and 3-6. Both cylinders are fired at the same time; the cylinder on compression and the cylinder on exhaust. Since the cylinder on exhaust requires little available voltage to arc (low compression=low resistance), bulk of the voltage produced is used to fire the spark plug of the cylinder on the compression stroke. A failure in the C(3)I system should set a Code 41 or 42.
3. Direct Ignition System (DIS) - DIS is a distributorless system used on 2.0L (VIN 1), 2.5L, 2.8L (VIN W) & 3.1L models. The 2.3L system is referred to as the Integrated Direct Ignition (IDI) system. The operation of both systems is quite similar to that of C(3)I system. It consists of 2 or 3 ignition coils (4-cylinder or V6), spark plug wires, ignition module (located under coil pack), a crankshaft position sensor and necessary wiring. On 2.3L models, coils, module and spark plug connectors are all combined in one unit which plugs directly onto spark plugs. Rather than a crankshaft position sensor mounted at crankshaft pulley (such as C(3)I), spark is timed by a signal sent from a crankshaft sensor mounted on side of block. This signal is received by ECM (through ignition module) and is used to trigger each coil at the proper time. See CRANKSHAFT POSITION SENSOR in INPUT SIGNALS section of this article. As with the C(3)I system, each cylinder is fired consecutively with the cylinder opposite it in the firing order. On the 2.8L and 3.1L, cylinders No. 1-4, 3-6 and 2-5 are paired. On 2.0L, 2.3L and 2.5L engines, cylinders No. 1-4 and 2-3 are paired. Each pair of cylinders is fired by its own ignition coil. Idle Air Control (IAC) Valve (Fuel Injected Models) The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC motor moves its pintle in and out in steps (0-fully seated, 255-fully retracted) to control engine idle speed. If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM. Normal counts on an idling engine should be 4-60. When engine is idling, ECM determines proper position of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM. If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. On other models it may be necessary to drive vehicle (at normal operating temperature) over 35 MPH with circuit properly connected. Problems in IAC circuit should set a Code 35. To check function of IAC system, see Code 35 chart and appropriate component CHART C2 for that system.

IDLE AIR CONTROL (IAC) VALVE (FUEL INJECTED MODELS)

The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC motor moves its pintle in and out in steps (0-fully seated, 255-fully retracted) to control engine idle speed. If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM. Normal counts on an idling engine should be 4-60.

When engine is idling, ECM determines proper position of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM. If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. On other models it may be necessary to drive vehicle (at normal operating temperature) over 35 MPH with circuit properly connected. Problems in IAC circuit should set a Code 35. To check function of IAC system, see CODE 35 flow chart and appropriate component CHART C2 for that system.

IDLE LOAD COMPENSATOR (CARBURETED MODELS)

ILC system controls idle speed during long deceleration modes using a vacuum motor which is regulated by an ECM-controlled vacuum solenoid. To check function of ILC system, see 5.0L (VIN Y) FULL FUNCTION CHART C2.

IDLE SPEED CONTROL (ISC) (CARBURETED MODELS)

The ISC is a motor which opens or closes the throttle (in idle position) according to commands from ECM. The ISC maintains low idle speeds while preventing stalling due to engine load. The base idle speed is programmed into ECM memory and is not adjustable.

When engine is cold, ISC holds throttle valve open for a longer period of time to provide faster warm-up. This function is by-passed when throttle is opened enough to bring TPS off its idle circuit (nose switch "off").

Note. Not all carbureted engines are equipped with ISC system. Some may use an Idle Stop Solenoid (ISS) or an Idle Load Compensator (ILC). To control engine idle speed when A/C is on, ISS controls throttle angle using a solenoid which is energized by an ECM controlled relay. The ILC system controls idle speed during long deceleration modes using a vacuum motor which is regulated by an ECM-controlled vacuum solenoid. To check function of ISC, ISS and ILC system, see appropriate component CHART C-2 for that system.

TORQUE CONVERTER CLUTCH (TCC)

The purpose of the transmission/transaxle Torque Converter Clutch (TCC) feature is to eliminate power loss of torque converter stage when vehicle is in a cruise condition. This allows convenience of automatic transmission/transaxle and fuel economy of a manual transmission. Fused battery ignition is supplied to TCC solenoid through a brake switch, and transmission 3rd gear apply switch.

TCC will engage when vehicle speed is greater than 30 MPH, engine at normal operating temperature (greater than 158°F/70°C), throttle position sensor output not changing (indicating a steady road speed), transmission 3rd gear switch closed, and brake switch closed.

When vehicle speed is great enough (about 20-25 MPH as indicated by the vehicle speed sensor), ECM energizes TCC solenoid mounted in transmission. This allows torque converter to directly connect engine to the transmission. When operating conditions indicate that transmission should operate as normal, TCC solenoid is de-energized. This allows transmission to return to normal automatic operation. The transmission will also return to normal automatic operation when brake pedal is depressed. To check function of TCC system, see appropriate component CHART C8 for that system.

EMISSION CONTROL OPERATION

The ECM electrically controls the following emission control systems: AIR management, Exhaust Gas Recirculation (EGR), catalytic converter and Evaporative Emission Control (EEC).

AIR INJECTION REACTION (AIR) MANAGEMENT SYSTEM

This system helps reduce hydrocarbon (HC) and carbon monoxide (CO) exhaust emissions. It also helps warm up catalyst and oxygen sensor quickly. This is accomplished by injecting air into the exhaust manifold during engine warm-up.

When ECM energizes the air control valve on a cold vehicle, air is allowed to flow to the air switching valve. The air switching valve then directs this air to the exhaust port. During warm engine operation (closed loop), ECM de-energizes the air control valve. This causes air switching valve to direct air to the catalytic converter.

If air control valve detects a rapid increase in manifold vacuum (deceleration condition), or ECM detects any failure in the computerized engine control system, air is diverted to the air cleaner or is dumped to atmosphere. To check function of AIR system, see appropriate component CHART C6 for that system.

EXHAUST GAS RECIRCULATION (EGR)

On computer-controlled EGR systems, ECM controls ported vacuum to EGR valve through use of a solenoid valve. The ECM uses coolant temperature, throttle position and manifold pressure signals to compute vacuum solenoid operation. During cold engine operation and idle, solenoid valve is grounded by ECM. This blocks vacuum to EGR valve. During warm engine operation and at speeds greater than idle, solenoid is not grounded and vacuum is allowed through to open EGR valve.

Some models use an integrated electronic control EGR valve. This valve incorporates control solenoid and EGR valve position sensor. This sensor is monitored by ECM (similar to a TPS; .3 volt fully closed, 5.0 volts fully open). The ECM controls EGR flow on this valve by pulsing the signal to the EGR solenoid. This allows a more regulated EGR flow than conventional ported EGR valves. To check function of EGR system, see appropriate component CHART C7 for that system.

Note. Some vehicles may use integral EGR/ILC/RVB, TCC/EGR, or EGR/CP solenoid valves.

Early Fuel Evaporation (EFE)

The ECM controls the EFE system during warm-up using one of the following 2 methods: a vacuum operated valve and actuator, or a ceramic heater grid located underneath carburetor primary bore. The vacuum operated valve and actuator is operated by a control solenoid mounted on valve cover. This solenoid controls vacuum to EFE valve using electric signals received from ECM.

The ceramic heater grid is part of the carburetor insulator. When ignition is turned on and coolant temperature is low, voltage is applied to EFE relay through ECM, energizing EFE heater. When coolant temperature increases above 185°F (85°C), ECM de-energizes EFE relay, which shuts off voltage to EFE heater. To check function of EFE system, see appropriate component CHART C9 for that system.

Note. EFE may not be used on all vehicles. Some vehicles may incorporate EFE control through EGR or AIR systems.

EVAPORATIVE EMISSION CONTROL (EEC)

This system controls purging of the vapor canister. The ECM controls vacuum to purge valve with a solenoid. When engine is cold (open loop), solenoid valve is energized. This blocks vacuum to purge valve.

When engine is warm (closed loop), and engine or vehicle speed is greater than a preset level, solenoid valve is de-energized. This allows vacuum to be applied to purge valve. Fuel vapors are then drawn into the intake manifold for burning. To check function of evaporative emission system, see appropriate component CHART C3 for that system.

CATALYTIC CONVERTER

The 3-way catalytic converter with dual bed is used to reduce exhaust emissions. This type of converter can reduce hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).

The upstream section of the converter contains a reducing/oxidizing bed to reduce NOx while at the same time oxidizing HC and CO. An air supply pipe from the AIR system injects air between the beds of the converter. This is so the second converter bed can oxidize any remaining HC and CO to efficiently reduce exhaust emissions.

DIAGNOSTIC SYSTEM OPERATION

Note. On carbureted models, a "SERVICE ENGINE SOON" light driver module is installed in wiring harness from ECM to "SERVICE ENGINE SOON" light. This driver turns on light when ignition is turned on. When vehicle starts, ECM turns light off. If ECM malfunctions or senses a malfunction, light will turn back on. On fuel injected models, driver is an integral part of ECM and is not serviceable.

The ECM is equipped with a self-diagnostic system which detects system failures or abnormalities. As a bulb and system check, "SERVICE ENGINE SOON" light will glow when ignition switch is turned to "ON" position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in the computerized engine control system or "SERVICE ENGINE SOON" light circuit is faulty.

When a malfunction occurs, ECM will illuminate the "SERVICE ENGINE SOON" light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. Malfunctions are recorded as "hard failures" or as "intermittent failures".

"HARD FAILURES"

Hard failures cause "SERVICE ENGINE SOON" light to glow and remain on until the malfunction is repaired. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but loss of good driveability will most likely be encountered.

"INTERMITTENT FAILURES"

Intermittent failures cause "SERVICE ENGINE SOON" light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, related touble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENT in TROUBLE SHOOTING section of this article.