Engine Controls - Theory & Operation Chevrolet Beretta I

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.

Mass Airflow (3.3L & 3.8L)

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. Mass Airflow (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. Sensor produces a frequency signal that cannot be easily measured in testing (32-150 Hertz). This varying signal is proportional to airflow.

Speed Density (Except 3.3L & 3.8L)

On models equipped with MAP and MAT sensors, the speed density method is used to compute the airflow rate. Manifold pressure and temperature are used to calculate the airflow rate to the ECM. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes.

The ECM sends a voltage signal to the MAP sensor. Manifold pressure changes result in resistance changes in the MAP sensor. By monitoring MAP sensor output voltage, the ECM determines manifold pressure. If MAP sensor fails, the ECM will supply a fixed MAP value and use the TPS to control fuel.

COMPUTERIZED ENGINE CONTROLS

The computerized engine control system monitors and controls a variety of engine/vehicle functions. 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.

The computerized engine control system consists of the following sub-systems: Electronic Control Module (ECM), input devices (sensors and switches) and output signals.

ELECTRONIC CONTROL MODULE (ECM)

Note. Some models use what is termed a Powertrain Control Module (PCM). The only difference between an ECM and PCM is that, in addition to electronic engine controls, the PCM also controls electronic transmission internals and cruise control system. Unless specifically stated, references to ECM also apply to PCM equipped vehicles.

On most vehicles, ECM is located in passenger compartment. For exact locations of ECM for a particular model, see ECM LOCATION in the G - TESTS W/ CODES article in this section, or refer to COMPONENT LOCATIONS in I - SYSTEM/COMPONENT LOCATIONS article in this 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. CPU also calculates spark timing and idle speed. The CPU commands operation of emission control, "closed loop" fuel control and diagnostic system.

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 scratch pad 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 CALPAC and, on some models, the ESC control module. If power to ECM is removed, MEM-CAL information will be retained.

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 device usage on a specific model, refer to WIRING DIAGRAMS article in this section. The available input signals include the following

A/C "On" Switch

The air conditioner "on" switch is mounted in instrument panel. This switch provides a simple "on" or "A/C request" signal which is monitored by the ECM. The ECM uses this signal to determine control of the A/C clutch relay (if equipped) 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, perform functional check of switch. Refer to the article SYSTEM/COMPONENT TESTS in this section.

A/C Pressure Sensor

Some models are equipped with an air conditioner pressure sensor which is used to inform ECM of A/C system pressure levels. Low pressure signal will cause ECM to disengage the A/C compressor to prevent system damage. High pressure levels cause ECM to energize high speed fans while A/C compressor clutch is engaged. Extremely high pressure levels will cause ECM to disengage A/C compressor clutch to prevent system damage.

A/C Pressure Switches

A/C high and low pressure switches may be used in the ECM-monitored A/C request circuit. Switches are normally closed, completing the circuit between ignition and ECM. ECM will engage or disengage A/C clutch relay based upon status of this circuit. When system freon pressure increases beyond a certain point, high side switch will open, causing A/C request line voltage to drop. If system freon level decreases, causing freon pressure to drop below normal, low side pressure switch will open, once again causing A/C request line voltage to drop. Switches may be used as normal clutch cycling devices or as safety devices which prevent compressor damage in the event of excessively high or low freon pressure.

A/C Temperature Sensors (Cadillac 4.9L)

Cadillac models (except Brougham) are equipped with air conditioner high side and low side temperature sensors which are used to inform ECM of A/C system temperature levels. Low temperature signal will cause A/C compressor to disengage. High temperature levels help ECM determine control of A/C compressor relative to cooling fans and idle speed.

Battery Voltage

Battery voltage is monitored by ECM (and BCM on Eldorado and Seville). 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 enrichen the air/fuel mixture. If voltage swings excessively high or low, ECM may set a charging system fault code and turn on the "SERVICE ENGINE SOON" light.

Brake Switch Feedback

Models equipped with cruise control systems may monitor the brake switch circuit to determine when to engage and disengage cruise control. On vehicles equipped with a Torque Converter Clutch (TCC) or Viscous Converter Clutch (VCC), one circuit of brake switch is in series with the power supply for the TCC or VCC solenoid located in the transmission/transaxle.

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 monitored 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. When coolant temperatures are high, CTS resistance is low, and a low 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, cooling fan operation, emission control devices and converter clutch application. A CTS which is out of calibration will not set a trouble code, but will cause fuel delivery and driveability problems. A coolant sensor circuit problem (open or short to ground) will swing monitored voltage high or low and should set a related trouble code.

Camshaft Position Sensor (C(3)I System)

A 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 4.9L engine uses a Hall Effect camshaft sensor located inside of the HEI distributor.

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 equipped models. A fault in the cam sensor circuit (no cam sensor signal) will result in a no-start condition (except 4.9L) and should set a related trouble code. For additional information, refer to COMPUTER CONTROLLED COIL IGNITION (C(3)I) SYSTEM and HEI-EST DISTRIBUTOR under IGNITION SYSTEMS in this article.

Crankshaft Position Sensor

Crankshaft position sensor, used on 3.3L and 3.8L engines, utilizes a Hall Effect switch mounted near vibration damper. The sensor monitors vibration damper position (crankshaft position) and sends signals to ignition module. These signals provide ECM with a TDC position reference for each piston, as well as supplying an engine speed (RPM) signal.

The 2.2L, 2.5L, 3.1L, and 3.4L Direct Ignition System (DIS) and 2.3L Integrated Direct Ignition (IDI) system crankshaft position sensor protrudes through side of engine block, to within .05" (1.3 mm) of an internally-mounted crankshaft reluctor 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 the 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.

Vehicles equipped with HEI-EST distributor systems use the RPM reference signal from the ignition module in the distributor for a crankshaft position signal. Although there is no differentiation between TDC intake and TDC exhaust, this is not necessary on non-sequential fuel injected or carbureted engines. Signal is used on fuel injected vehicles to trigger fuel injector(s). For additional information, see COMPUTER CONTROLLED COIL IGNITION (C(3)I) SYSTEM and DIRECT IGNITION SYSTEM (DIS) & INTEGRATED DIRECT IGNITION SYSTEM (IDIS) under IGNITION SYSTEMS in this article.

Fuel Pump Feedback

On some models, the fuel pump circuit between the relay and fuel pump is monitored by ECM. This enables ECM to determine when the fuel pump relay is energized and voltage is being delivered to fuel pump. Voltage monitored on this circuit is also used in calculations to determine changes in idle speed, air/fuel ratio and ignition dwell. A failure in this monitored circuit will result in the setting of a related trouble code in ECM memory.

Gear Switches

Gear switches are located inside automatic transmission. Switches may be normally open or closed and change status depending upon internal hydraulic pressures. High gear switch information is used by ECM in controlling emission components and engagement of Torque Converter Clutch (TCC), or Viscous Converter Clutch (VCC) on Cadillac (except Brougham).

Ignition/Crank Signal

The ECM looks at the initial cranking (RPM) signal on circuit No. 430 to determine when the engine is being started. This information is used for starting enrichment. If this signal is intermittent or not available, hard starting or a no-start condition may result.

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 from the knock sensor back to the ESC controller, or to the MEM-CAL portion of the ECM on models not equipped with a controller. The ECM will then retard ignition timing until the engine knock ceases.

For additional information on knock sensor operation, see ESC DETONATION RETARD OPERATION within IGNITION TIMING SYSTEMS under the IGNITION SYSTEMS heading in this article.

A fault in the ESC circuit may set a related trouble code. When a related trouble code is not present and the ESC system is suspected as the cause of a driveability problem, perform functional check of ESC system. Refer to SYSTEM/COMPONENT TESTS article in this section.

Manifold Absolute Pressure (MAP) Sensor (Except 3.3L & 3.8L)

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 (about 1.5 volts at idle to about 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 related trouble code. If a related trouble code is not present and MAP sensor is suspected of causing a driveability problem, perform functional check of MAP sensor. Refer to SYSTEM/COMPONENT TESTS article in this section.

Mass Airflow (MAF) Sensor (3.3L & 3.8L)

The MAF 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.

This 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 a related trouble code.

Manifold Air Temperature (MAT) Sensor

The MAT sensor (may also be referred to as an intake air temperature 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. 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 sat overnight, MAT and CTS signals (resistance and temperature) should be close to same reading. Failure in MAT sensor circuit (open or short to ground) will cause monitored voltage to swing high or low and should set a related trouble code.

Oil Temperature (Engine) Sensor

Corvette is equipped with an oil temperature sensor. If sensor indicates oil temperature is high when it should be low, or low when it should be high, a trouble Code 52 (low) or 62 (high) will set in ECM memory; however, sensor will not cause driveability problems. Sensor information is sent from ECM to be used by Central Control Module (CCM) to determine oil life expectancy. If an oil temperature sensor code is set in memory, CCM has been calculating oil life from inaccurate ECM input. Oil and filter must be changed, code must be cleared and oil life monitor must be reset.

To reset oil life monitor, turn ignition on. Depress ENG MET button on trip monitor and release. Within 5 seconds, depress and release ENG MET button again. Within 5 seconds, depress and hold the RANGE button on trip monitor. The CHANGE OIL light should flash.

Hold the RANGE button depressed until the CHANGE OIL light stops flashing and goes out. When the light goes out, the engine oil life monitor is reset. This should take about 10 seconds. If the light does not reset, turn the ignition off and repeat the procedure.

Oxygen (O2) Sensor

The O2 sensor is mounted in the exhaust system where it monitors oxygen content of exhaust gases. Two oxygen sensors are used on some Cadillac models. 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 or MAF values to determine air/fuel ratio from a table built into memory. When ECM reads a voltage signal greater than .45 volt from the O2 sensor, ECM will begin to alter commands to injector to produce either a leaner or richer mixture.

Once vehicle has entered "closed loop", a fault in the O2 circuit (cooled-down sensor or open or shorted O2 sensor circuit) is the only thing which can return it to "open loop". A problem in the O2 sensor circuit should set a related trouble code.

Park/Neutral Switch (P/N)

This switch is connected to transmission gear selector. The switch signals ECM when transmission is in Park or Neutral. Information from P/N switch is used by ECM for determining control of ignition timing, converter clutch and idle speed. To check function of P/N switch, perform functional check of switch. See the appropriate SYSTEM/COMPONENT TESTS article in this section.

Power Steering (P/S) Pressure 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 by ECM to help control idle speed and also A/C clutch on some models. To check function of P/S switch, perform functional check of switch. Refer to SYSTEM/COMPONENT TESTS article in this section.

RPM Reference Signal

The RPM is monitored by ECM through ignition module tach/pulse signals (circuit No. 430) 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 determining control of timing, fuel delivery, EGR function and idle speed.

Throttle Position Sensor (TPS)

The TPS is a variable mechanical resistor connected either directly 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 determining control of fuel, idle speed, spark timing and converter clutch. A problem in the TPS circuit may set a related trouble code.

Throttle Switch (Cadillac 4.9L)

On Cadillac models using an Idle Speed Control (ISC) motor, an idle switch is incorporated into ISC motor. This switch informs ECM when throttle lever is contacting ISC plunger. This allows ECM to determine when to control idle speed. When throttle is open sufficiently to relieve pressure from the ISC plunger, switch will open and ECM will no longer attempt to control idle speed.

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) mounted 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 converter clutch engagement.

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.

1. A/C Clutch - See MISCELLANEOUS CONTROLS.
2. Air Injection Control Solenoid - See EMISSION SYSTEMS.
3. Canister Purge Solenoid - See EMISSION SYSTEMS.
4. Computer Controlled Coil Ignition (C(3)I) - See IGNITION SYSTEM.
5. Cooling Fan Relay - See MISCELLANEOUS CONTROLS.
6. Digital EGR Valve - See EMISSION SYSTEMS.
7. Direct Ignition System (DIS) - See IGNITION SYSTEM.
8. EGR Control Solenoid - See EMISSION SYSTEMS.
9. ESC Timing Retard - See IGNITION SYSTEM.
10. EST Timing Control - See IGNITION SYSTEM.
11. Fuel Injectors - See FUEL CONTROL.
12. Fuel Pump & Fuel Pump Relay - See FUEL DELIVERY.
13. HEI-EST Ignition - See IGNITION SYSTEM.
14. HOT Light or Coolant TEMP Light - See MISCELLANEOUS CONTROLS.
15. Idle Air Control (IAC) Valve - See IDLE SPEED.
16. Idle Speed Control (ISC) Motor (4.9L Cadillac) - See IDLE SPEED.
17. Integrated Direct Ignition System (IDIS) - See IGNITION SYSTEM.
18. Self-Diagnostics - See appropriate G - TEST W/CODES article in this section.
19. Serial Data - See appropriate G - TEST W/CODES article in this section.
20. "SERVICE ENGINE SOON" Light - See appropriate G - TEST W/CODES article in this section.
21. Shift Light - See MISCELLANEOUS CONTROLS.
22. Torque Converter Clutch - See MISCELLANEOUS CONTROLS.

Fuel Pump

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. The pressure relief valve in the fuel pump, controls maximum fuel pump pressure.

A pressure regulator mounted in fuel rail (port injection systems) or on throttle body unit (throttle body injection systems) keeps fuel available to injector(s) at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line. For fuel pressure specifications, refer to the appropriate article below

1. «SPECIFICATIONS - 4-CYL»(/chevrolet/beretta/i-1987-1996/remont/specifications/#engine-controls-specifications-4-cyl)
2. «SPECIFICATIONS - V6»(/chevrolet/beretta/i-1987-1996/remont/specifications/#engine-controls-specifications-v6)
3. «SPECIFICATIONS - V8»(ref-10187)

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 continue to energize relay if the engine is running or cranking (ECM is receiving reference pulses from the ignition module). If there are no reference pulses, ECM de-energizes fuel pump relay within 2 seconds after key is turned on. For additional information, see FUEL PUMP RELAY in this article.

Fuel Pressure Regulator (TBI)

On TBI systems, a constant fuel pressure is maintained by a factory preset, nonadjustable, spring loaded diaphragm contained within the throttle body. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (PFI)

Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.

During periods of high manifold vacuum, regulator-to-fuel tank return orifice is fully open, keeping fuel pressure on the low side of its regulated range. As throttle valve opens, vacuum to regulator diaphragm decreases, allowing spring tension to gradually close off return passage. At wide open throttle when vacuum is at its lowest, return orifice is restricted, providing maximum fuel volume and maintaining constant fuel pressure to injectors.

Fuel Pump Relay

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 relay energized 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.

As a back-up system to fuel pump relay, fuel pump is also activated by the oil pressure switch. The oil pressure switch is normally open until oil pressure reaches approximately 4 psi (.28 kg/cm 2 ). If fuel pump relay fails, the oil pressure switch closes when oil pressure is obtained, operating the fuel pump. An inoperative fuel pump relay may result in extended cranking times due to the time required to build up oil pressure. Oil pressure switch may be combined into a single unit with an oil pressure gauge sender or sensor.

For additional information on fuel pump activation, refer to the following articles in this section

1. «BASIC TESTING»(/chevrolet/beretta/i-1987-1996/remont/testing-diagnostics/#engine-controls-basic-testing) And
2. «SYSTEM/COMPONENT TESTS»(/chevrolet/beretta/i-1987-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests)

FUEL CONTROL

The ECM, using input signals, determines adjustments to the air/fuel mixture in order to provide the optimum ratio for proper combustion under all operating conditions. One of 2 types of fuel control systems are used: throttle body injection or port fuel injection. These systems can operate in the "open loop" or "closed loop" mode. Description of these modes is as follows: 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) or Mass Airflow (MAF) sensor readings. Engine will remain 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 oxygen sensor has reached operating temperature, coolant temperature has reached 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 mixture as possible. If oxygen sensor cools off (due to excessive idling) or a fault occurs in the oxygen sensor circuit, vehicle will once again enter "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.

Fuel Cut-Off

Injectors are de-energized when ignition is turned off so that dieseling is prevented. Injectors will not be energized if RPM reference pulses are not received by the ECM, even with ignition on. This prevents flooding before starting. Fuel cut-off will also occur at high engine RPM to prevent internal damage to engine. Some fuel injected models may also cut off fuel injector signals during periods of high speed, closed throttle deceleration (when fuel is not needed).

Throttle Body Injection (TBI)

Injector is located in throttle body unit. Dual injectors are used on 5.0L (VIN E) and 5.7L (VIN 7) engines. Battery voltage is supplied to the injector when the ignition is on. ECM energizes solenoid by providing a ground path through its internal circuitry. By regulating the injector ground circuit, ECM controls injector "on" time (pulse width) to provide proper amount of fuel to engine.

Pressure to injector is maintained at a constant level by the pressure regulator. Excess fuel passes through pressure regulator and is returned to fuel tank.

In the "run" mode, ECM uses tach (RPM) signal to determine when to pulse injector. Fuel injectors are pulsed once for each engine revolution, each spray providing 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle. On models equipped with dual injectors in the throttle body, injectors are pulsed alternately.

During starting, clear flood mode, deceleration and heavy acceleration, fuel delivery is controlled by internal ECM calibration.

1. Starting During engine starts, ECM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. Air/fuel ratio is determined by ECM when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 201°F (94°C). At lower coolant temperatures, injector pulse width is longer (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes shorter (leaner air/fuel ratio).
2. Clear Flood If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, ECM adjusts injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio will be maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 80 percent open and/or engine speed exceeds 600 RPM, ECM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).
3. Heavy Acceleration Fuel enrichment during heavy acceleration is controlled by the ECM. Sudden opening of throttle valve causes rapid increase in MAP signal. Pulse width is directly related to the MAP, throttle position and coolant temperature. Higher MAP and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are non-synchronized (not in proportion to distributor reference signals). Any reduction in throttle angle cancels fuel enrichment.
4. Deceleration During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut-off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.

Port Fuel Injection (PFI)

Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in cylinder head.

Standard PFI systems feature simultaneous double-fire injection. Fuel injectors are pulsed once for each engine revolution, each spray providing 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle.

The 3.8L and 4.9L models use Sequential Fuel Injection (SFI). Injectors on these models are pulsed sequentially in spark plug firing order. Main differences between sequential and simultaneous systems are injectors, wiring and the ECM.

In all systems, constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by the time that injector stays open (pulse width). Various sensors provide information to the ECM to control pulse width.

IDLE SPEED

Engine idle speed is controlled by the ECM depending upon engine operating conditions. The ECM senses engine operating conditions and determines the best idle speed.

Idle Air Control Valve (Fuel Injection Except 4.9L Cadillac)

The Idle Air Control (IAC) valve controls engine idle speed during engine load changes to prevent stalling. The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC valve moves its pintle in and out in steps referred to as "counts" (0 counts-fully seated, 255 counts-fully retracted) to control engine idle speed. Counts can be measured using a "Scan" tester plugged into the Assembly Line Data Link (ALDL).

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 positioning 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 related code.

The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valve stuck closed allows insufficient airflow, resulting in low idle speed. For calibration purposes, several different design IAC valves are used. Ensure proper design valve is used during replacement.

Idle Speed Control (ISC) Motor (Fuel Injection 4.9L Cadillac)

The ISC, mounted to the throttle body, is an electrically driven actuator which changes throttle angle according to ECM demands. An internal idle switch by-passes this function when throttle is opened enough to allow TPS to move from idle position. The ISC motor is factory calibrated and should not be disassembled. Replace as complete assembly only.

IGNITION SYSTEM

All vehicles are equipped with a high energy ignition system capable of producing in excess of 50,000 volts. Vehicles except those using a C(3)I system (3.3L and 3.8L), IDIS (2.3L) or DIS (2.2L, 2.5L, 3.4L and some 3.1L) are equipped with a High Energy Ignition Electronic Spark Timing (HEI-EST) distributor.

HEI-EST DISTRIBUTOR

The Delco-Remy High Energy Ignition Electronic Spark Timing (HEI-EST) system consists of distributor housing, rotor, cap, 7 or 8-terminal ignition module, magnetic pick-up, pole piece, pick-up coil, connecting harness and the EST portion of the ECM. The distributor is connected to the EST system by means of a 4-wire connector, leading to Electronic Control Module (ECM).

On some models, the ignition coil is contained within the distributor cap, while others have an externally mounted coil. A capacitor is installed in the distributor for radio noise suppression.

No vacuum or centrifugal advance mechanisms are used. All spark timing changes are controlled by the Electronic Control Module (ECM) based upon monitored input signals. Some models use an additional Electronic Spark Control (ESC) ignition retard system in the event of engine detonation (knock). Most models are equipped with sealed ignition coil and ignition module connectors.

When the external teeth on the timing core approach, align with, and pass the pick-up coil windings, an alternating current is produced in the pick-up coil windings. In the cranking mode, this alternating current signals switching transistors in the HEI module to make or break the ignition coil primary ground circuit. Once the engine has started, ECM takes control of primary ground circuit (EST mode).

When the primary ground circuit is removed, the magnetic field created by the flow of current in the primary windings collapses across the primary and secondary windings of the coil. This induces a high-voltage surge in the secondary windings of the coil. Secondary voltage is then discharged to the rotor, which distributes it to the appropriate spark plug terminal. The distributor module may have either a 7-terminal ignition module or an 8-terminal ignition module (sealed connector module), depending on application.

The 4.9L HEI-EST system is also equipped with a Hall Effect switch inside of the distributor. The Hall Effect switch produces a camshaft signal which is used by the ECM to determine the proper firing sequence for the injectors on the sequential fuel injection system. Loss of the camshaft signal will result in the fuel injection operating in a non-sequential mode and the setting of a related trouble code.

COMPUTER CONTROLLED COIL IGNITION (C(3)I)

The Computer Controlled Coil Ignition (C(3)I) system, used on 3.3L and 3.8L PFI engines, eliminates the need for a mechanical distributor. The C(3)I ignition system consists of a coil pack (3 coils), ignition module, camshaft and crankshaft (3.8L) or combination (3.3L) sensor, wiring harness, and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM).

In the C(3)I system, each cylinder is paired with the cylinder that is opposite it in the firing order. Cylinders No. 1/4, 5/2, and 3/6 are paired. Spark occurs simultaneously in the cylinder coming up on the compression stroke and in the cylinder coming up on the exhaust stroke. The cylinder on the exhaust stroke requires less voltage for the spark plug to fire. This leaves the bulk of the available voltage to fire the spark plug for the cylinder on the compression stroke. The process is repeated when the cylinders reverse roles. Each cylinder pair is fired by its own ignition coil.

Input from the Hall Effect combination sensor (3.3L) or cam and crank sensors (3.8L) is used by the ignition module to determine when to trigger the appropriate coil pack. On 3.8L models, module passes on camshaft sync-pulse signal to the ECM so that sequential fuel injector timing can be initialized.

Type II Ignition Coil Pack (3.3L)

On type II ignition coil pack, 3 separate twin tower coils are independently mounted over the C(3)I ignition module. Each coil provides the spark for 2 simultaneously paired spark plugs. Each coil can be replaced separately.

Type III Ignition Coil Pack (3.8L)

On type III ignition coil pack, 3 twin tower coils are combined into a single coil pack. Coil pack is mounted directly over the C(3)I ignition module. Each coil provides the spark for 2 simultaneously paired spark plugs. All 3 coils must be replaced as a unit. Although old-style type I coil pack will physically fit on ignition module, the No. 1/4 coil pack is in a different location in relation to module connector.

Combination Sensor (3.3L)

The combination cam/crank sensor actually consists of 2 Hall Effect sensors mounted, in a single unit, near the harmonic balancer. Since the 3.3L engine uses a double-fire simultaneous injection system rather than a sequential fuel injection system, it does not require a distinctive (TDC No. 1 piston compression) camshaft signal. Instead, each engine revolution, camshaft portion of the combination sensor generates TDC signal for the No. 1/4 cylinder pair. Each engine revolution, the second sensor (crankshaft) generates RPM information and signals for the following cylinder pairs: 1/4, 2/5 and 3/6.

Camshaft Position Sensor (3.8L)

The 3.8L camshaft sensor is located on the timing cover, behind and below water pump. The ECM uses camshaft "sync-pulse" signals (passed to ECM by the ignition module) to determine the exact position of the No. 1 piston. Signal is used by ECM to properly initialize fuel injector firing. If camshaft sensor signal is lost, Code 41 (E041 on some models) will be set. The engine can be restarted and will run in sequential mode; however, without the camshaft signal, there is a 1 in 6 chance of injectors spraying correctly. This provides "walk home" protection against cam sensor failure.

Combination 3X & 18X Sensors (3.8L)

In addition to the camshaft sensor, the 3.8L engine contains sensors which are similar to the combination sensor used on the 3.3L engine; however, the interrupter rings on the back side of the balancer differ in configuration and purpose. The outside ring contains 18 evenly spaced interrupters, producing 18 pulses per crankshaft revolution. The inner ring has 3 interrupters spaced at irregular intervals (10 degrees, 20 degrees and 30 degrees apart).

The ignition module monitors signals generated by the 2 interrupter rings. The 18X ring will change state once during the 10 degree gap of the 3X ring, twice during the 20 degree gap, and 3 times during the 30 degree gap. The changing relationship between the 2 rings allows the ignition module to identify the correct ignition coil to fire within the first 120 degrees of crankshaft rotation. This system provides for a faster start and a more accurate measurement of the crankshaft sensor signals.

If the 3X signal to ignition module is lost while the engine is running, the fuel injection system will continue to run in sequential mode; however, loss of the 3X, or the 18X signal, will not allow the vehicle to restart.

Fuel Control Signal (3.8L)

In addition to the RPM reference (18X) signal and fuel sync (camshaft) signals generated by the ignition module on 3.8L models, a fuel control reference signal must also be passed on to the ECM in order to inform ECM that proper signals are being generated to the ignition module. The fuel control signal is generated by the C(3)I module from calculations involving signals from the 18X and the 3X pulse rings.

DIRECT IGNITION SYSTEM (DIS) & INTEGRATED DIRECT IGNITION SYSTEM (IDIS)

DIS is a distributorless system used on 2.2L, 2.5L, 3.4L and some 3.1L models. On the 2.3L, a similar system is referred to as the Integrated Direct Ignition System (IDIS). The operation of both DIS and IDI is quite similar to that of C(3)I system. Systems consist of 2 or 3 ignition coils (4-cylinder or V6), spark plug wires, ignition module (located under coil pack), a crankshaft position sensor, necessary wiring and the Electronic Spark Timing (EST) portion of the Electronic Control Module (ECM). On 2.3L models, coils, module and spark plug connectors are all combined into 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 through side of engine 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 under INPUT SIGNALS. As with the C(3)I system, each cylinder is fired consecutively with the cylinder opposite it in the firing order. On V6 engines, cylinders No. 1/4, 3/6 and 2/5 are paired. On 4-cylinder engines, cylinders No. 1/4 and 2/3 are paired. Each pair of cylinders is fired by its own ignition coil.

The crankshaft position sensor is mounted on the bottom of the DIS ignition module or near the ignition module. The sensor protrudes through the side of engine block to within .05" (1.3 mm) of an internally-mounted crankshaft reluctor ring. Sensor position is not adjustable.

The reluctor is a special piece of metal, cast with the crankshaft. It has 7 slots machined into it, 6 of which are equally spaced (60 degrees apart). A seventh slot is spaced about 10 degrees from one of the other slots and generates a synchronization pulse signal. As crankshaft rotates, 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 RPM reference signals which are sent to ECM by ignition module. This allows ECM to compute crankshaft position and RPM.

Ignition Timing Advance

At engine speeds less than 400 RPM, the ignition module controls spark advance by triggering coil(s) at a predetermined interval based on engine speed only. At engine speeds greater than 400 RPM (EST mode), the ECM takes over control of the ignition timing. On 3.8L engines, when in EST mode, ECM also changes fuel injection timing to a sequential mode.

Ignition timing is controlled by the ECM based upon input signals from the engine RPM reference line (ignition module), coolant temperature sensor, manifold air temperature sensor, throttle position sensor, knock sensor, vehicle speed sensor, gear position switch, and the MAF or MAP sensor.

The PROM/MEM-CAL portion of the ECM has a programmed 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/MEM-CAL information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, go to the following articles in this section

1. «BASIC TESTING»(/chevrolet/beretta/i-1987-1996/remont/testing-diagnostics/#engine-controls-basic-testing) And
2. «SYSTEM/COMPONENT TESTS»(/chevrolet/beretta/i-1987-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests)

Although several types of ignition systems are used, all ignition systems use the same 4 basic ignition circuits. Models may use a conventional HEI/EST distributor system or one of 3 types of distributorless ignition systems. The C(3)I uses the same ignition module-to-ECM circuits that IDI, DIS and distributor type ignition systems use with the addition of fuel control and fuel sync (camshaft) signals on 3.8L engines. For description of fuel control and sync signals, see IGNITION SYSTEMS in this article.

The ignition module is connected to ECM by 4 EST circuits. Circuits perform the following functions

1. Reference (RPM) Alternating current signals from the pick-up coil (HEI distributor), PM generator (DIS and IDI) or Hall Effect sensors (C(3)I and Cadillac 4.9L) are converted by the ignition module converter to digital signals for use by the ECM. This supplies RPM data and crankshaft position reference to the ECM. Since the signal on this circuit is used as an injector trigger reference on fuel injected vehicles, if circuit is open or grounded, engine will not run.
2. By-Pass When an engine speed signal of approximately 400 RPM is received by the ECM, ECM considers engine to be running and applies 5 volts to the ignition module on the by-pass wire. This causes ignition module to switch timing control over to the variable timing control circuit in the ECM. On some models, this by-pass wire contains a connector located between the 4-wire connector and the ECM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit will set a related trouble code in ECM memory. The engine will run at base timing plus a small amount of advance built into the HEI module.
3. EST When 5 volts is present on the by-pass circuit and ignition module has turned control of engine timing over to ECM, the ECM advances or retards spark on this circuit based on calculations involving the reference signal and other sensor input signals. If base timing is incorrectly set, entire advance curve will be incorrect.
4. Ground This is the reference ground circuit. It is grounded at distributor and ECM, ensuring there is no voltage drop in the EST circuit which could affect ignition operation.

ESC Detonation Retard Operation

In conjunction with the HEI-EST system, an Electronic Spark Control (ESC) retard system is used on some models. System consists of the following: a detonation (knock) sensor, a high energy ignition system, an ESC controller (some models), and the ECM. On some models, the function of the ESC controller is built into the Memory Calibration (MEM-CAL) unit of 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 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 near 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. If signal wire were to become open or grounded on models utilizing ESC controller, ECM would continuously provide full ignition timing retard.

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 will pull 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 back to the ECM. The voltage and frequency of this signal depend upon knock signals received by the sensor. The ECM will retard spark timing until signals from detonation sensor cease.

A malfunction in the ESC circuit should set a related trouble code. If a code is not present and ESC system is suspected as the cause of driveability problems, perform functional check of ESC system. Refer to SYSTEM/COMPONENT TESTS article in this section.

EMISSION SYSTEMS

Note. To determine emission systems usage, go to appropriate EMISSION APPLICATION article in this section.

AIR INJECTION SYSTEM

This system helps reduce hydrocarbon (HC) and carbon monoxide (CO) exhaust emissions by injecting air into the exhaust system. The induction of additional air promotes further oxidation (combustion) of unburned and partially burned exhaust gases. During cold engine operation air is injected into exhaust manifold. This quickly warms up catalytic converter and oxygen sensor. When vehicle warms up, air is diverted to atmosphere, or on models with a TWC/OC, to the catalytic converter. See CATALYTIC CONVERTER in this article.

Air Pump (Except 3.4L "W" Body Man. Trans.)

The air pump is a belt driven, positive displacement vane-type pump. Air drawn into pump is purged of dirt and contaminates by a centrifugal filter mounted behind the pulley. The air pump is permanently lubricated and requires no periodic service.

Air Pump (3.4L "W" Body Man. Trans.)

Air pump is an electric-motor type located in the right front corner of the engine compartment. Pump is energized by an ECM-controlled relay which is activated when fuel system is functioning in open loop mode and/or less than 80 seconds has passed since relay was energized.

Note. Always cover centrifugal filter fan before cleaning engine to prevent liquid from entering air pump. DO NOT oil air pump.

Check Valve

The check valve prevents the backflow of exhaust gases into the air injection system. The check valve closes when exhaust gas pressure in exhaust manifold exceeds pressure delivered by pump. This occurs when air pump by-passes at high speeds, air delivery is switched to catalytic converter, air is diverted to atmosphere or air cleaner, or when air pump malfunctions.

Air Injection Reaction Management System

When ECM energizes the air control (divert) and air switching valves on a cold vehicle, air is allowed to flow through the control valve 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 switching valve. This causes air switching valve to direct air to the catalytic converter.

If air control (divert) valve detects a rapid increase in manifold vacuum (deceleration condition), or if high RPM operation causes pump output pressure to exceed normal operating range, air is mechanically diverted to the air cleaner by the air control (divert) valve. If ECM detects any failure in the computerized engine control system, air control (divert) valve will be de-energized, also causing air to be diverted to the air cleaner or atmosphere. To check function of AIR system, perform functional check of system and refer to article SYSTEM/COMPONENT TESTS in this section.

Note. Air control (divert) valve and air switching valve may be separate or combined into a single assembly.

Electric Air Divert/Electric Air Switching Valves

Electric divert and electric switching valves are used on Federal vehicles (except 3.1L and 3.4L with manual transaxle). System may combine both divert function and air switching function into one integral component.

The valves are electrically controlled by the ECM and operated by air pump pressure. The operation of the valves is not dependent on intake manifold vacuum.

For cold engine (open loop) operation, the divert solenoid is energized and air flows to exhaust ports. In warm engine (closed loop) operation, the divert solenoid is de-energized and switching solenoid is energized. This forces airflow to the converter. In the divert mode, both solenoids are de-energized and airflow is allowed to vent to atmosphere.

Divert will occur during rich operating condition, when the ECM recognizes a problem and turns on the SERVICE ENGINE SOON light, during deceleration (high vacuum) and during heavy acceleration when air pressure exceeds the setting of the relief valve in the air divert valve.

Electric Air Divert Valve (EADV)

The Electric Air Divert Valve (EADV) is used on California vehicles and 3.1L and 3.4L Cutlass Supreme, Grand Prix and Lumina with manual transaxle. Valve performs normal diverter valve operation and may provide air divert to the air cleaner for catalytic converter protection during wide open throttle and high temperature conditions.

The ECM de-energizes EADV solenoid (located in EADV), preventing manifold vacuum from entering the chamber during the above conditions. Spring tension against the lower diaphragm pushes the diaphragm up, diverting air to air cleaner. Air from the air pump is always shut off from the engine unless ECM grounds EADV circuit (solenoid energized).

Electric Air Pump Relay (3.4L Man. Trans.)

On 3.4L with manual transmission, an electric air pump relay is used. When vehicle is cold (open loop mode), ECM provides a ground for the EADV solenoid and relay. When relay is energized, power is supplied to the electric air pump. When fuel system goes into "closed loop", or electric air pump has been on for more than 80 seconds, the ECM opens the ground circuit. When solenoid is de-energized, air is diverted to the atmosphere until air pump stops swimming.

CATALYTIC CONVERTER

A 3-way catalytic (TWC) converter is used on all vehicles to reduce exhaust emissions. This type of converter reduces hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) levels.

TWC (Except Camaro, Corvette & Firebird)

Converter contains a reducing agent (Rhodium and Platinum) to reduce NOx and an oxidizing agent (Paladium and Platinum) to oxidize HC and CO. This causes HC and CO to oxidize (break down with the addition of oxygen and heat) into the harmless base elements water (H2O) and carbon dioxide (CO2). Oxygen is removed from NOx causing it to reduce to the harmless base elements nitrogen (N) and oxygen (O2).

TWC/OC (Camaro, Corvette & Firebird)

In addition to the standard TWC features, this converter contains a second Oxidizing Catalyst (OC) bed which continues to oxidize carbon monoxide and hydrocarbons. An air tube from the air injection system injects additional air between the 2 beds. This allows the second converter bed to oxidize any remaining HC and CO to efficiently reduce exhaust emissions.

EXHAUST GAS RECIRCULATION (EGR)

The Exhaust Gas Recirculation (EGR) system is designed to reduce oxides of nitrogen (NOx) emissions by lowering combustion temperatures. This is accomplished when a metered amount of exhaust gas is recirculated into the intake manifold and mixed with the air/fuel mixture.

There are 3 types of EGR systems used: pulse width modulated backpressure (positive and negative) EGR using an EGR solenoid (V8), backpressure EGR (positive and negative) without EGR solenoid control (4-cylinder TBI), and digital EGR (2.3L, 3.1L & 3.4L).

On computer-controlled EGR systems using a solenoid, ECM controls ported vacuum to EGR valve through solenoid valve. Solenoid may be normally open or normally closed, depending upon application.

ECM uses coolant temperature, throttle position and manifold pressure signals to determine vacuum solenoid operation. During cold engine operation and idle, EGR is not desired; ECM causes solenoid to block vacuum to EGR valve. During warm engine operation and at speeds greater than idle, vacuum is allowed through solenoid, opening EGR valve. To check function of EGR system, perform functional check of system. See the SYSTEM/COMPONENT TESTS article in this section.

Pulse Width Modulated (PWM) EGR System

This type EGR system is controlled entirely by the ECM. ECM regulates EGR vacuum signal by controlling an electrical signal to a solenoid vacuum valve. The ECM controlled vacuum solenoid valve is located in series between vacuum source and EGR valve. The solenoid is pulsed at a rate of up to 32 times per second. The ECM uses a ported vacuum signal to determine the flow rate signal to the solenoid. PWM systems also use a backpressure EGR valve to prevent EGR function until engine loads are present. See EXHAUST BACKPRESSURE EGR SYSTEM.

Digital EGR System (2.3L)

The digital EGR valve is designed to accurately supply EGR to engine, independent of intake manifold vacuum. The valve controls EGR flow from exhaust to intake manifold through 2 internally-mounted solenoids. When each solenoid is energized, a pintle is lifted to allow exhaust gas to flow through valve. Solenoids are energized individually or together in different combinations to tailor EGR flow to specific engine requirements.

Digital EGR System (3.1L & 3.4L)

The digital EGR valve is designed to accurately supply EGR to engine, independent of intake manifold vacuum. The valve controls EGR flow from exhaust to intake manifold through 3 internally-mounted solenoids. When each solenoid is energized, a pintle is lifted to allow exhaust gas to flow through valve. Solenoids are energized individually, in pairs, or together to provide 7 different EGR flow ratios. This enables ECM to tailor EGR flow to specific engine requirements.

Exhaust Backpressure EGR System

Two types of backpressure EGR valves are used: positive and negative backpressure valves. These valves may be identified by the letter in the last position of part number. Letter "P" designates a positive backpressure valve; letter "N", a negative backpressure valve. Backpressure EGR may also use an ECM-controlled solenoid to regulate vacuum signal to EGR valve.

1. Positive Backpressure EGR Valve A control valve, located in the EGR valve, acts as a vacuum regulator valve. Control valve regulates amount of vacuum to EGR diaphragm chamber by bleeding vacuum to atmosphere during certain operating conditions. When the control valve receives a backpressure signal, through the hollow shaft of EGR valve, pressure on bottom of control valve closes control valve. When control valve closes, maximum vacuum signal is applied directly to EGR valve allowing exhaust gas recirculation.
2. Negative Backpressure EGR Valve Vacuum is applied to upper EGR diaphragm via a hose connected intake manifold vacuum. Manifold vacuum is also applied to lower EGR diaphragm (through intake port at base of EGR valve). When manifold vacuum in lower chamber is insufficient to overcome spring tension on lower diaphragm, bleed valve will be closed, allowing vacuum in upper chamber to open EGR valve. With engine at idle, or under light load, high manifold vacuum applied to lower chamber opens air bleed valve in lower diaphragm. This bleeds off vacuum in upper chamber, keeping the EGR valve closed.

EVAPORATIVE EMISSION CONTROL

Carbon canister storage is used for evaporative fuel control on all vehicles. The function of evaporative emission control system is to store gasoline fumes from fuel tank in a carbon canister until fumes can be drawn into engine for burning during combustion process.

There are 3 basic components used in evaporative emission system. These are as follows

1. Activated carbon canister (may be sealed or open at top or bottom for fresh air intake).
2. ECM-controlled solenoid (mounted on canister or remotely).
3. Tank pressure control valve (mounted internal or external of the fuel tank).

For specific component application see the appropriate EMISSION APPLICATION S article in this section.

For vacuum hose routing, see VACUUM DIAGRAMS article in this section.

Carbon Canister

Evaporative fumes from the fuel tank are vented through hose(s) into a canister containing activated carbon. The activated carbon absorbs and holds fuel vapors when the engine is not operating. When the engine is started and engine speed is greater than idle (purge at idle would cause too rich a mixture), engine vacuum draws fuel vapors from the canister into the engine. Regulation of vapors through this purge line may be controlled by a vacuum canister purge valve, an ECM-controlled solenoid, or both.

Carbon canisters are either open or closed in design. When the engine is started on open canister models, engine vacuum draws outside air into canister either through the top or through a filter in bottom of canister. This helps to purge vapors from the activated carbon.

Purge Solenoid Valve

Purge solenoid valve is controlled by the Electronic Control Module (ECM). Current is supplied to solenoid when the ignition is on. Solenoid is energized when ECM provides a ground circuit for solenoid. Solenoid may be normally closed or normally open. When solenoid valve is open, charcoal canister is purged using manifold or ported vacuum. When solenoid valve is closed, purge vacuum to canister is blocked.

The ECM will allow vacuum to pass through solenoid when engine has been running for more than one minute, coolant temperature is above 176°F (80°C), vehicle speed is greater than 5 MPH and throttle is off idle. This solenoid (if used) is located in the purge line between the charcoal canister and the vacuum purge port or on top of the canister.

Fuel Tank Pressure Control Valve

Fuel tank pressure control valve is a vacuum regulated/pressure control valve located in the fuel tank, or in the vapor delivery hose between fuel tank and carbon canister. When engine is not running and tank pressure is less than .9 psi (.06 kg/cm 2 ), internal spring pressure holds valve in the closed position.

This causes fuel tank low-pressure vapors to be vented through a restriction in valve. This restriction will retain most of fuel tank vapors in fuel tank. When tank pressure rises and overrides spring tension, fumes are vented to the carbon canister. When engine is running, vacuum is applied to upper port of valve, opening passage between fuel tank and carbon canister, which is purged by engine vacuum.

Note. Models without fuel tank pressure control valves may utilize a special pressure/vacuum relief fuel tank filler cap or other external relief device.

Except 2.3L

The PCV system is used to provide more effective elimination of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase where it is mixed with blow-by gases and passed through a PCV valve into the intake manifold. This mixture is then passed into the combustion chamber and burned.

The PCV valve provides primary control in this system by metering the flow of the blow-by vapors, according to manifold vacuum. When manifold vacuum is high (at idle) the PCV restricts the flow to maintain a smooth idle condition.

Under conditions where abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow the excess gases to flow back through crankcase vent hose into the air inlet and be consumed during normal combustion.

2.3L

Unlike conventional crankcase ventilation systems, the 2.3L does not have a fresh air inlet to the crankcase. All blow-by gases are drawn from the crankcase through an oil/air separator. Flow is limited by a .060" (1.52 mm) orifice in the manifold intake nipple. Oil suspended in the blow-by gases is trapped in the separator and returned to the crankcase.

THERMOSTATIC AIR CLEANER (TAC)

Many models are equipped with a system for preheating the air entering the throttle body during cold engine operation.

This system maintains incoming air temperature to a point where the fuel injection system can maintain lean air/fuel ratios to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions. There are 2 types of TAC systems used: vacuum controlled and wax pellet controlled.

Vacuum Motor Controlled (Brougham, Camaro & Firebird TBI)

This system consists of an air cleaner assembly with integral air control door, vacuum control temperature sensor, vacuum motor, heat shroud (on exhaust manifold), heated air tube and vacuum hoses.

1. Vacuum Control Temperature Sensor The vacuum control temperature sensor controls the operation of the air control door. During initial start-up situations, this valve directs engine vacuum to the air control vacuum motor. The motor closes the air intake door, allowing the intake of heated manifold air. When the intake air temperature reaches a precalibrated value, this valve opens, allowing the intake of cooler outside air.
2. Air Control Door The air control door temperature sensor closes when the temperature of air entering the air cleaner is less than the calibrated temperature of the temperature sensor. This allows engine vacuum to operate the air control door vacuum motor, and warm manifold air to be routed to the throttle body.
3. Vacuum Motor When engine vacuum is applied to the vacuum motor, the air control door closes off the intake of outside air. Air is then drawn into the air cleaner from around the exhaust manifold. As air inside the air cleaner warms, the temperature sensor begins to open, bleeding off vacuum to the vacuum motor. As vacuum to vacuum motor decreases, the air control door begins to open. As air control door opens, outside air is allowed to enter air cleaner assembly. When air entering air cleaner reaches a predetermined temperature, the air control door opens completely, and closes off the intake of heated air.

Wax Pellet Controlled (Caprice, Custom Cruiser & Roadmaster)

The air regulator damper (hot/cold air delivery door) is controlled by means of a self-contained, wax pellet actuated assembly mounted in the air cleaner. When incoming air is cold, wax material sealed in the actuator is in a solid contracted state. As incoming air warms, wax material expands by changing to a liquid state. This forces piston outward, repositioning air regulator damper and allowing cold and hot air to mix, or all cold air to enter engine.

SELF-DIAGNOSTICS

The ECM is equipped with a self-diagnostic system which detects system failures or abnormalities. 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 designated as either "hard failures" or as "intermittent failures". To retrieve stored codes, see the appropriate G - TEST W/ CODES article in this section.

HARD FAILURES

Hard failures cause the "SERVICE ENGINE SOON" light to glow and remain on until the malfunction is repaired. On models using digital display on dash to indicate codes, when recalled, codes may be accompanied by a "current" or "history" indication for intermittent and hard codes. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts located in G - TEST W/ CODES in this section. 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 the "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. On models using digital display on dash to indicate codes, when recalled, codes may be accompanied by a "current" or "history" indication for intermittent and hard codes. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See the appropriate TESTS W/O CODES article in this section.

SERVICE ENGINE SOON LIGHT

As a bulb and system check, the "SERVICE ENGINE SOON" light will glow when the ignition switch is turned to the 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. Light may be used on some models to display stored trouble codes. To access codes using "Scan" or "Non-Scan" methods, go to appropriate G - TEST W/ CODES article in this section.

SERIAL DATA

ECM is equipped with a serial data line. Serial data is a stream of electrical impulses which can be interpreted by special testers of other control modules. On some models, serial data must be accessed using special Scan testers connected to the Assembly Line Data Link (ALDL). Update intervals and information contained within the data stream vary with model application.

On models utilizing an ECM and Body Control Module (BCM), serial data may be accessed using the Driver Information Center (DIC) and Climate Control Panel (CCP). On these models, serial data may be shared with BCM, A/C controller, supplemental restraint controller, anti-lock brake controller, or even cruise control unit.

MISCELLANEOUS CONTROLS

Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.

A/C CLUTCH

On many models ECM regulates operation of the A/C clutch through an ECM controlled relay. This allows the ECM to disengage the A/C compressor when compressor load on engine may cause driveability problems (i.e. during hot restart, idle, low speed steering maneuvers, and wide open throttle operation), or if A/C freon pressure drops below or rises above normal operating levels.

Freon pressure sensing may be accomplished through the monitoring of high and low pressure switches or a pressure sensor which will register either high or low pressure levels. Power steering load is monitored through a power steering pressure switch. Hot restart is monitored through the coolant temperature sensor. For component application and related wiring, see wiring schematics under MISCELLANEOUS ECM CONTROLS in SYSTEM/COMPONENT TESTS article in this section.

Some models are equipped with an air conditioner pressure sensor which is used to inform ECM of A/C system pressure levels. Low pressure signal will cause A/C compressor to disengage to prevent system damage. High pressure levels cause ECM to engage high speed fans while A/C compressor clutch is engaged. Extremely high pressure levels will cause ECM to disengage A/C compressor clutch to prevent system damage.

A/C high and low pressure switches may be used in the ECM-monitored A/C request circuit. Switches are normally closed, completing the circuit between ignition and ECM. ECM will engage or disengage A/C clutch relay based upon status of this circuit. When system freon pressure increases beyond a certain point, high side switch will open, causing A/C request line voltage to drop.

If system freon level decreases, causing freon pressure to drop below normal, low side pressure switch will open, once again causing A/C request line voltage to drop. Switches may be used as normal clutch cycling devices or as safety devices which prevent compressor damage in the event of excessively high or low freon pressure.

COOLING FAN

On many models, ECM regulates operation of the electric cooling fan through an ECM controlled relay which controls the ground circuit or power circuit for the cooling fan. This allows the ECM to operate the cooling fan based upon engine temperature.

Most systems will engage the electric cooling fan whenever the A/C clutch is engaged, regardless of engine temperature. As a back-up system, many models utilize a coolant override switch which will also engage the cooling fan in the event that the ECM fails to energize the cooling fan relay, or the cooling fan relay malfunctions. A malfunction of the cooling fan will cause engine overheating and possible detonation.

Some models utilize more than one cooling fan. The second fan may function as an auxiliary cooling device when A/C is engaged, or (on models utilizing freon temperature sensors or high pressure switches) during periods of engine overheating or high A/C freon pressures.

For component application and related wiring, see wiring schematics under MISCELLANEOUS ECM CONTROLS in the article SYSTEM/COMPONENT TESTS in this section.

HOT LIGHT OR COOLANT TEMPERATURE LIGHT

When engine coolant temperature sensor input indicates temperature exceeds prespecified range, the ECM will turn on the TEMP or HOT light by providing a ground for the light circuit. As a bulb check, the ECM also supplies a ground to turn on light when the ignition is first turned on.

Torque Converter Clutch (ECM Type)

The purpose of the transmission/transaxle converter clutch 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 converter solenoid through a brake switch. On some models, 2nd, 3rd and 4th gear hydraulic apply switches (located within the transmission) may also be in series with solenoid power or ground circuit. On other models, switch status may only be monitored by the ECM, without sharing power or ground with the converter solenoid. For wiring reference, refer to appropriate wiring schematics under MISCELLANEOUS ECM CONTROLS in article SYSTEM/COMPONENT TESTS in this section.

Converter clutch will engage when vehicle is moving greater than a precalibrated speed, engine is at normal operating temperature, throttle position sensor output is not changing (indicating a steady road speed), transmission 3rd gear or high gear switch is closed (if equipped), and brake switch is closed.

When vehicle speed is great enough (about 20-45 MPH as indicated by the vehicle speed sensor), ECM energizes converter clutch 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, converter clutch solenoid is de-energized.

This allows transmission to return to normal automatic operation. Since power for the converter solenoid is delivered through the brake switch, transmission will also return to normal automatic operation when brake pedal is depressed. To check function of converter clutch system, perform functional check of system. See MISCELLANEOUS ECM CONTROLS in the SYSTEM/COMPONENT TESTS article in this section.

Torque Converter Clutch (PCM Type w/4T60E Transaxle)

The PCM type torque converter clutch functions similar to the ECM type except that instead of a single internal solenoid, the PCM type uses 2 solenoids. A standard TCC solenoid is used in conjunction with a Pulse Width Modulated (PWM) solenoid which regulates hydraulic pressure to make locking and unlocking of the TCC smoother.

Electronic Transmission (4L80-E)

On vehicles equipped with the 4L80-E transmission, transmission is controlled by the Powertrain Control Module (PCM) on gasoline vehicles. PCM controls other vehicle functions as well as the transmission. Electronic transmission is controlled by the Transmission Control Module (TCM) on diesel vehicles. TCM controls no other components. The PCM/TCM monitors a number of engine/vehicle functions and uses the data to control shift solenoid "A", shift solenoid "B", TCC and the Force Motor to regulate TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

1. Shift Solenoid "A" Shift solenoid "A" is attached to the valve body and is a normally open exhaust valve. PCM/TCM activates solenoid by grounding it through an internal quad-driver. Solenoid "A" is on in 1st and 4th gear, but off in 2nd and 3rd gears. When on, solenoid redirects fluid to act on the shift valves. Solenoid "A" is Blue. Code 82 is associated with solenoid "A".
2. Shift Solenoid "B" Shift solenoid "B" is attached to the valve body and is a normally open exhaust valve. PCM/TCM activates solenoid by grounding it through an internal quad-driver. Solenoid "B" is on in 3rd and 4th gear, but off in 1st and 2nd gears. When on, solenoid redirects fluid to act on the shift valves. Solenoid "B" is Red. Codes 81, 86 and 87 are associated with solenoid "B".
3. Force Motor Force motor is attached to the valve body and controls line pressure by moving a pressure regulator valve against spring pressure. Force motor takes the place of the throttle valve or vacuum modulator used on past model transmissions. PCM/TCM varies line pressure based upon engine load. Engine load is calculated from various inputs, especially the TPS. Line pressure is actually varied by changing the amperage applied to the force motor from zero (high pressure) to 1.1 amps (low pressure). The force motor is periodically pulsed to prevent fluid contamination from sticking the pressure regulator valve.

Shift Light (Except Corvette)

The shift light is used on vehicles equipped with manual transmission. Light indicates the best transmission shift point for maximum fuel economy. Power for light is supplied through the GAUGES fuse. Light is illuminated when the ECM supplies a ground circuit for the bulb. For wiring reference, refer to MISCELLANEOUS ECM CONTROLS in the SYSTEM/COMPONENT TESTS article in this section.

1-4 Shift Light (Corvette)

The shift light is used on vehicles equipped with manual transmission. Light indicates when driver should shift transmission from first gear to fourth gear for maximum fuel economy. Power for light is supplied through the 10-amp AIR BAG fuse. Light is illuminated when the ECM supplies a ground circuit for the bulb. For wiring reference, see MISCELLANEOUS ECM CONTROLS in the SYSTEM/COMPONENT TESTS article in this section.

1-4 Shift Light Relay (Corvette)

Power for the relay winding is supplied by the GAUGES fuse. When ECM determines that driver should shift transmission from first gear to fourth gear for maximum fuel economy, ECM will provide a ground for the 1-4 upshift relay. When relay is energized, voltage supplied by the TURN/BACK-UP fuse will pass through relay and energize the 1-4 upshift solenoid mounted in the transmission. When solenoid is energized, transmission is locked out from shifting from first gear into any gear other than fourth. For wiring reference, see MISCELLANEOUS ECM CONTROLS in the SYSTEM/COMPONENT TESTS article in this section.

See also:
• I - SYSTEM/COMPONENT LOCATIONS
• WIRING DIAGRAMS
• SYSTEM/COMPONENT TESTS
• SPECIFICATIONS - 4-CYL
• SPECIFICATIONS - V6
• BASIC TESTING