Engine Controls - Theory & Operation Geo Storm 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.

Storm GSi (DOHC)

1. Secondary air control system controls auxiliary valves located in the ports of lower intake manifold. Auxiliary valves are opened when engine speed is greater than 5000 RPM for better engine performance. Valves are closed at less than 5000 RPM to reduce airflow and improve idle quality.
2. Valves are operated by a vacuum diaphragm which is controlled by a Secondary Air Control Vacuum Switching Valve (SEC-VSV). Vacuum switching valve is controlled by Electronic Control Module (ECM). If auxiliary valves are open at idle, idle quality and engine performance will be poor.

COMPUTERIZED ENGINE CONTROLS

Fuel injection system is Throttle Body Injection (TBI) on all Metro and Tracker models. Port Fuel Injection (PFI) is used on Prizm and Storm models. The electronic fuel injection engine control system monitors vehicle operating conditions through input signals and regulates air/fuel mixture and other engine control operations by output signals. This lowers exhaust emissions, while maintaining fuel economy and driveability.

The control systems have a fail-safe mechanism. If a fault occurs while driving, the system will substitute pre-programmed values. Driving performance will be affected, but vehicle may still be driven. These systems have a self-diagnostic feature capable of recognizing a system fault and storing a related trouble code in memory for future retrieval and diagnosis.

ELECTRONIC CONTROL MODULE (ECM)

Note. For electronic control module locations, see ECM LOCATION table.

METRO

Power for ECM is supplied through the taillight fuse. The ECM distributes power or controls ground of various sensors, switches and solenoids for engine control.

PRIZM

Power for ECM is received from stoplight fuse. The ECM distributes power or controls ground of various sensors, switches and solenoids for engine control.

STORM

Power for ECM is supplied from the ECM fuse. The ECM distributes power or controls ground of various sensors, switches and solenoids for engine control.

TRACKER

Power for ECM is received from the taillight/dome fuse. The ECM distributes power or controls ground of various sensor, switches and solenoids for engine control.

ECM LOCATION

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.

CEC 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, see appropriate wiring diagram in WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. The available input signals include the following

A/C SIGNAL

On models with A/C, a signal is sent to the ECM when A/C is operating. The ECM uses this signal with other inputs to determine idle speed.

A/T MODULE GEAR SIGNAL (METRO A/T)

On A/T applications, the ECM monitors voltage signal sent from the A/T control module when transaxle is in any gear except Park or Neutral. The ECM uses this input signal to help control of fuel injectors and idle speed control.

BATTERY VOLTAGE SIGNAL (METRO, PRIZM & TRACKER)

The ECM monitors battery voltage. A drop in battery voltage directly effects pulse width of the fuel injector. As battery voltage drops, pulse width decreases, causing a leaner air/fuel mixture. The ECM compensates by increasing pulse width to provide richer mixture.

COOLANT TEMPERATURE SENSOR

Coolant temperature sensor monitors coolant temperature. A reference voltage (supplied and monitored by ECM) is modified by sensor resistance, which changes according to temperature. High coolant temperature causes low resistance and low coolant temperature causes high resistance. The ECM uses this information to determine control fuel injectors and emission components.

Metro XFi With Electronic Spark Control, Prizm & Tracker

Engine speed signal is generated by pick-up coil/crank angle sensor and trigger wheel assembly in the distributor. This pulsing signal is sent to ECM where it is used to calculate engine speed. It is also used as input to determine control system functions.

California Prizm, Storm & Tracker

The EGR temperature sensor is located at the EGR valve. Reference voltage, supplied and monitored by ECM, is modified by sensor resistance. Sensor resistance changes with respect to exhaust gas temperature. High temperature causes low resistance. Low temperature causes high resistance. The ECM uses this information to determine if EGR is functioning.

ELECTRICAL LOAD (METRO)

The ECM monitors a signal from diode module to determine electrical load caused by cooling fan, blower motor, rear defogger, stoplight or headlights. When voltage decreases below a specified value, ECM increases idle speed. As voltage increases, ECM readjusts the idle speed.

ENGINE START SIGNAL (METRO, PRIZM & TRACKER)

Engine start signal, sent from starter circuit, is used by ECM to determine when engine is cranking. Using this signal, ECM calculates fuel injection timing, idle speed control and throttle opener control (Tracker).

IGNITION REFERENCE SIGNAL (STORM)

Ignition module sends a signal to ECM for calculating spark advance and fuel delivery.

INTAKE AIR TEMPERATURE (IAT) SENSOR

Intake air temperature sensor measures intake manifold air temperature. The IAT sensor resistance changes with respect to temperature. High air temperature decreases IAT sensor resistance; low air temperature increases IAT sensor resistance. A reference voltage, supplied and monitored by ECM, is modified by sensor resistance. The ECM uses this information to control output signals to the fuel injectors.

On Storm models, IAT sensor is used to control spark timing and delay EGR operation when engine is cold. See INTAKE AIR TEMPERATURE SENSOR (IAT) LOCATION table.

INTAKE AIR TEMPERATURE SENSOR (IAT) LOCATION

KNOCK SENSOR (PRIZM GSI)

Knock sensor, located in rear of cylinder block, sends a variable AC voltage signal to ECM depending on engine detonation. The ECM uses this signal to determine if ignition timing should be retarded.

Metro, Prizm 4A-FE, Storm & Tracker

MAP sensor connects to ECM by a wiring harness and to engine by a manifold vacuum hose. The ECM supplies a 5-volt reference signal to MAP sensor. The MAP sensor voltage varies according to changes in engine load (manifold vacuum).

The ECM interprets voltage change as a change in the engine load and uses signal to help determine control of fuel injector, shift-indicator light (if equipped), idle speed control solenoid (Metro models) and EGR solenoid.

MASS AIRFLOW (MAF) SENSOR (PRIZM GSI)

Airflow sensor measures amount of air entering the engine and produces an appropriate output signal proportional to air intake. The ECM uses this information for fuel injector control.

OXYGEN (O2) SENSOR

Oxygen sensor is mounted in exhaust manifold or exhaust pipe, where it contacts exhaust gases. The O[I2] sensor generates voltage according to oxygen content of exhaust gases. Voltage will vary from .1 volt (lean condition) to as high as one volt (rich condition).

Sensor will not generate a voltage signal until it reaches operating temperature. Sensor sends voltage signal to ECM, which uses it to obtain correct emissions by adjusting air/fuel ratio. Until sensor is warmed up, ECM adjusts air/fuel mixture based upon preprogrammed tables in ECM memory.

On Prizm GSi, Storm GSi and Tracker models, a heated oxygen sensor is used. This sensor works the same as the non-heated sensor except the oxygen sensor is heated for more precise determination of exhaust gas oxygen concentration.

A/T Models Except Metro

On A/T applications, the ECM monitors switch position and receives a voltage signal when transaxle/transmission is in any forward or reverse gear. The ECM uses this input signal to help control of fuel injectors, idle speed control and EGR valve (Storm and Storm GSi models).

Note. On Metro A/T models, gear position signal is received from the A/T module. See A/T MODULE GEAR SIGNAL (METRO A/T MODELS) under INPUT DEVICES.

POWER STEERING PRESSURE SWITCH (STORM & TRACKER)

Power steering pressure switch indicates to the ECM when power steering pressure is applied. The ECM uses this signal to alter engine speed. On Storm models, the A/C will also be shut off when high pressure is noted.

SUB-OXYGEN (O2) SENSOR (CALIFORNIA PRIZM GSI)

Sub-oxygen sensor is mounted in exhaust system, where it contacts exhaust gases after the catalytic converter. The sub-oxygen sensor rechecks emission level after the catalytic converter and sends reference signal to the ECM. This will provide a more precise air/fuel ratio.

THROTTLE POSITION SENSOR (EXCEPT METRO M/T & PRIZM)

The ECM supplies throttle position sensor with a 5-volt reference signal. Throttle position sensor contains a variable resistor and idle switch. Throttle position sensor sends an output signal to ECM corresponding to opening of throttle valve and an idle switch signal (only when throttle is in idle position).

The ECM uses these signals to control air/fuel ratio during acceleration, deceleration and idling. These signals are also used to determine idle speed. On Metro A/T models, the ECM converts the signal to a on/off signal and sends it to the A/T module for controlling the transmission.

THROTTLE SWITCH (METRO M/T & PRIZM)

The throttle switch sends a signal to ECM to indicate if throttle valve is in idle position or wide open throttle by the use of an idle switch and wide open throttle switch (internal components of throttle switch). Throttle switch is open except when throttle is closed or wide open. These on/off signals are monitored by ECM and used to control fuel injection, idle speed control, shift indicator light (if equipped) and EGR vacuum switching valve (if equipped).

VEHICLE SPEED SENSOR

Vehicle speed sensor consists of a lead switch and magnet built into speedometer head. As magnet rotates with speedometer cable, its magnetic force causes lead switch to turn on and off. This on/off pulse is sent to ECM and interpreted as vehicle speed.

CEC 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 Vacuum Switching Valve. See IDLE SPEED.
2. Check Engine Light. See SELF DIAGNOSTIC SYSTEM.
3. Circuit Opening Relay. See FUEL DELIVERY.
4. Canister Purge Vacuum Switching Valve. See EMISSION SYSTEMS.
5. Distributor Ignitor. See IGNITION SYSTEM.
6. EFI Main Relay. See FUEL DELIVERY.
7. EGR Vacuum Switching Valve. See EMISSION SYSTEMS.
8. Fuel-Cut System. See FUEL CONTROL.
9. Fuel Injector. See FUEL CONTROL.
10. Fuel Pressure Regulator Vacuum Switching Valve. See FUEL DELIVERY.
11. Fuel Pump Relay. See FUEL DELIVERY.
12. Idle Air Control Valve. See IDLE SPEED.
13. Idle Speed Control Solenoid. See IDLE SPEED.
14. Idle-Up Vacuum Switching Valve. See IDLE SPEED.
15. Secondary Air Vacuum Switching Valve. See AIR INDUCTION SYSTEM.
16. Shift Indicator Light. See TRANSMISSION CONTROL.
17. Throttle Opener Vacuum Switching Valve. See IDLE SPEED.
18. Throttle Position Sensor Output Signal. See MISCELLANEOUS CONTROLS.
19. Transmission Torque Converter Clutch Relay. See MISCELLANEOUS CONTROLS.

CIRCUIT OPENING RELAY (PRIZM)

Circuit opening relay is located behind center console of instrument panel. Relay is mounted on bracket attached to the ECM. During cranking, a signal from ignition switch closes the contacts in circuit opening relay to energize fuel pump. Once engine is started, ECM controls circuit opening relay and fuel pump remains on. Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If no ignition references are received, ECM will shut off the fuel pump.

CIRCUIT OPENING RELAY (PRIZM GSI)

Circuit opening relay is located behind center console of instrument panel. Relay is mounted on bracket attached to the ECM. During cranking, a signal from ignition switch closes the contacts in circuit opening relay to energize fuel pump. Once engine is started and cranking signal is removed from circuit opening relay, fuel control switch in Mass Airflow Sensor (MAF) continues to operate fuel pump.

Note. The EFI main relay may also be referred to as the main relay.

EFI MAIN RELAY (METRO)

The EFI main relay is located in fuse/relay block on left side of engine compartment, near the battery. The EFI main relay supplies voltage to fuel pump relay when ignition is turned on. The ECM grounds fuel pump relay and power is then supplied to the fuel pump. Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If no ignition references are received, ECM will shut off the fuel pump.

EFI MAIN RELAY (PRIZM)

The EFI main relay is located in fuse/relay block on left front side of engine compartment. When ignition is turned on, EFI main relay supplies voltage to circuit opening relay. The ECM grounds the circuit opening relay and operates the fuel pump.

EFI MAIN RELAY (PRIZM GSI)

The EFI main relay is located in fuse/relay block on left front side of engine compartment. When ignition is turned on, EFI main relay supplies voltage to circuit opening relay. Once engine is started and cranking signal is removed from circuit opening relay, fuel control switch in the Mass Airflow Sensor (MAF) controls circuit opening relay ground circuit for fuel pump operation.

EFI MAIN RELAY (STORM & STORM GSI)

The EFI main relay is located in fuse/relay block on left side of engine compartment, near the battery. When ignition is turned on, an input signal is sent from EFI main relay to the ECM. The ECM uses this input signal to determine when to apply battery voltage to the fuel pump relay.

EFI MAIN RELAY (TRACKER)

The EFI main relay is mounted on the ECM, located under left side of instrument panel, near kick panel. When ignition is turned on, the ECM grounds the EFI main relay and provides voltage to the fuel pump relay.

FUEL PUMP

Electric fuel pump is located in the fuel tank. Fuel pump delivers fuel to the fuel injectors where system pressure is controlled by fuel pressure regulator. Fuel pump contains an internal check valve to maintain pressure in fuel lines after fuel pump is turned off.

FUEL PUMP RELAY (METRO)

Fuel pump relay is located in fuse/relay box on left side of engine compartment, near the battery. When ignition is turned on, fuel pump relay receives voltage from EFI main relay. The ECM grounds fuel pump relay and power is then supplied to the fuel pump.

Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If no ignition references are received, ECM will shut off the fuel pump.

FUEL PUMP RELAY (STORM)

Fuel pump relay is located in fuse/relay box on left side of engine compartment, near the battery. When ignition is turned on, an input signal is sent from EFI main relay to the ECM.

The ECM uses this input signal to determine when to apply battery voltage to fuel pump relay. The ECM supplies power to the fuel pump relay. Fuel pump relay contacts close and power is supplied to fuel pump. Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If no ignition references are received, ECM will shut off the fuel pump.

FUEL PUMP RELAY (TRACKER)

Fuel pump relay is mounted on the ECM, located under left side of instrument panel, near kick panel. When ignition is turned on, the ECM grounds the EFI main relay and provides voltage to the fuel pump relay. The ECM will ground the fuel pump relay and provide power to the fuel pump.

Fuel pump will operate as long as engine is cranking or running and ECM is receiving ignition reference pulses. If no ignition references are received, ECM will shut off the fuel pump.

FUEL PRESSURE REGULATOR

Fuel pressure regulator is a spring/vacuum-operated, diaphragm-type relief valve which maintains a regulated fuel pressure under all conditions. When manifold vacuum is high (low fuel requirements), diaphragm is drawn in, counteracting spring pressure. In this condition, excess fuel is routed back to fuel tank. When manifold vacuum drops (engine load), spring pressure overcomes vacuum, closing off fuel tank return line. This maintains pressure and volume to fuel injectors.

FUEL PRESSURE REGULATOR VACUUM SWITCHING VALVE (PRIZM GSI)

Fuel pressure regulator vacuum switching valve controls vacuum supply to the fuel pressure regulator. The ECM controls vacuum switching valve ground by monitoring engine coolant temperature, intake air temperature and starting signal.

The ECM monitors battery voltage. A drop in battery voltage directly affects fuel injector pulse width. As battery voltage drops, pulse width decreases, causing a leaner air/fuel mixture. ECM compensates by increasing pulse width to provide richer mixture.

ECM monitors a signal from diode module to determine electrical load caused by cooling fan, blower motor, rear defogger, stoplight or headlights. When voltage decreases below a specified value, ECM increases idle speed. As voltage increases, ECM readjusts the idle speed.

FUEL-CUT SYSTEM (METRO & TRACKER)

The fuel-cut system will stop fuel injection during deceleration to prevent unburned gases from being exhausted. Fuel-cut system will also deactivate injectors when engine speed exceeds 7000 RPM (Metro) or 7200 RPM (Tracker). This prevents engine damage due to excessive engine speed. As engine speed drops to less than 6800 RPM, injection will once again occur.

FUEL-CUT SYSTEM (PRIZM & STORM)

The fuel-cut system will stop fuel injection during deceleration to prevent unburned gases from being exhausted. On Storm models, fuel injection will be shut off if engine exceeds 6800 RPM.

FUEL INJECTOR

When fuel injector solenoid coil is energized by the ECM (on time), coil becomes an electromagnet. This opens the fuel injector, allowing fuel to be injected into the intake manifold or cylinder. The air/fuel mixtures are controlled by the fuel injector pulse width (on time). The ECM determines proper pulse width based upon input signals from various sensors and switches.

Fuel injection timing is determined by ECM based upon RPM signals received from either the distributor or ignition coil. Metro and Tracker models have Throttle Body Injection (TBI), which incorporates a single fuel injector in the throttle body unit. All other models use Port Fuel Injection (PFI), which incorporates fuel injectors mounted in a fuel rail assembly.

A/C VACUUM SWITCHING VALVE (A/C VSV) (METRO)

When A/C is operated, ECM receives a signal and controls A/C VSV operation. The A/C VSV supplies a certain amount of by-pass air to increase engine idle speed.

AIR VALVE (METRO, PRIZM & TRACKER)

Air valve is used to increase idle speed when engine coolant is less 176°F (80°F) on Prizm GSi models or 140°F (60°F) on all others. The coolant temperature alters the thermowax pellets to increase or decrease air into the intake manifold.

When coolant temperature is less than specified, valve opens by spring pressure, allowing air into intake manifold. This increased airflow increases the idle speed. As coolant temperature increases, valve closes and shuts off the airflow, causing idle speed to decrease. When engine coolant temperature is greater than 176°F (80°F) on Prizm GSi models or 140°F (60°F) on all others, valve fully closes and normal idle speed is obtained.

IDLE AIR CONTROL (IAC) VALVE (STORM)

Engine idle speed is controlled by IAC valve mounted on side of throttle body. The ECM delivers voltage pulses to IAC motor to retract or extend pintle to maintain proper engine speed. As pintle moves, air flows through passage around throttle valves to control idle speed. The ECM sends pulses to IAC whenever idle speed drops to less than preprogrammed RPM due to engine load (i.e. electrical, A/C, P/S, A/T in Drive, etc.).

If battery is disconnected, it may be necessary to partially depress accelerator when starting until ECM relearns idle control. ECM normally resets IAC pintle once during each cycle of the ignition switch and when vehicle is driven above 30 MPH on moderate acceleration.

IDLE SPEED CONTROL (ISC) SOLENOID (METRO & TRACKER)

The ISC solenoid, located below left side of air cleaner on Metro or on right side of throttle body on Tracker. The ISC solenoid by-passes air around throttle valve directly into the intake manifold. Air is allowed to pass through ISC solenoid when it is energized by the ECM. Solenoid is energized whenever idle speed drops to less than desired RPM due to engine load (i.e. electrical, P/S, A/T in Drive, etc.). The ISC solenoid is also energized each time engine is started, or during periods of deceleration to compensate for rich mixtures caused by a fully closed throttle.

Note. The idle-up vacuum switching valve may be referred to as the Idle Speed Control (ISC) solenoid.

IDLE-UP VACUUM SWITCHING VALVE (IVSV) (PRIZM)

The IVSV is located on end of intake manifold on Prizm (below mass airflow sensor on Prizm GSi). The IVSV by-passes air into intake manifold (around the throttle valve of mass airflow sensor on Prizm GSi) when it is activated by the ECM. The IVSV is energized whenever idle speed drops to less than desired RPM due to engine load (i.e. electrical, P/S, A/T in Drive, etc.).

THROTTLE OPENER CONTROL SYSTEM (TRACKER)

When vehicle is first started, ECM activates a throttle opener Vacuum Switching Valve (VSV) which supplies vacuum to the throttle opener. The ECM controls VSV according to ignition signal, starter signal and signal from coolant temperature sensor. The VSV is activated when engine is cranking or engine speed is less than 4000 RPM for 2-35 seconds after engine starts.

Energizing the VSV opens a vacuum passage between VSV filter and throttle opener and closes the manifold vacuum passage. When vacuum passage is closed, diaphragm spring pushes on throttle linkage, increasing throttle opening. Once vehicle starts, ECM de-energizes the VSV, allowing manifold vacuum to pass through VSV to throttle opener diaphragm. Diaphragm will retract, allowing throttle linkage to return to normal base idle position.

METRO XFI (WITH ELECTRONIC SPARK CONTROL)

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce reference signals to the ECM.

Ignition coil power is provided through 15-amp fuse located in fuel block below left side of instrument panel. Fuse receives power when ignition is turned on. Ground circuit for ignition coil is regulated by the ECM. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to ECM. ECM uses this signal to determine when to ground and open primary ignition circuit. When ECM opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs.

METRO & METRO LSI (WITHOUT ELECTRONIC SPARK CONTROL)

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce ignition pulses through the ignitor. Ignitor is mounted inside the distributor.

Ignition coil power is provided through 20-amp fuse, located in fuse block below left side of instrument panel. Fuse receives power when ignition is turned on. Ground circuit for ignition coil is regulated by the ignitor. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, an alternating current is produced. Ignitor uses this alternating signal to determine when to ground and open primary ignition coil circuit.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs.

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce reference signals to the ECM. Ignition coil is mounted on the side of the distributor. Ignitor is mounted inside the distributor.

Power for ignition coil is provided through the ignition switch. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to the ECM. The ECM uses this signal to determine when to signal the ignitor to open ground circuit for primary ignition.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs. Ignitor sends a signal back to ECM to confirm primary ignition circuit operation was completed.

PRIZM GSI

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce reference signals to the ECM. External ignitor is mounted on right side of firewall in engine compartment.

Power for ignition coil is provided through the ignition switch. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to the ECM and ignitor. The ignitor sends a RPM reference signal to the ECM. The ECM uses reference signals to determine when to signal the ignitor to open ground circuit for primary ignition.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs. Ignitor sends a signal back to ECM to confirm primary ignition circuit operation was completed.

Ignition system consists of a distributor which uses a signal rotor, pick-up coil, ignition module and external coil. Power for ignition coil is provided through the ignition switch. As rotating signal rotor passes pole piece of pick-up coil, a reference signal is sent from the ignition module to the ECM. The ECM uses reference signal to determine when to signal ignition module to open ground circuit for primary ignition.

When ignition module opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs.

Ignition system consists of a distributor which uses a signal rotor and pick-up coil to produce ignition pulses through the externally mounted ignitor. Ignitor is mounted on right side of firewall in engine compartment. Power for ignition coil and ignitor is provided through 15-amp coil fuse, located in fuel block below left side of steering column. Fuse receives power when ignition is turned on. Ground circuit for ignition coil is regulated by the ECM. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to the ECM. The ECM uses this signal to determine when to signal the ignitor to open ground circuit for primary ignition.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge used to operate the spark plugs.

Metro & Metro LSI

Distributor utilizes a centrifugal advance and vacuum advance ignition timing relative to engine speed and load.

Metro XFi With ESC, Prizm & Tracker

Ignition timing is controlled by ECM based upon various sensor input signals.

Electronic Spark Timing (EST) is controlled by the ECM. Ignition module sends a reference signal on the Yellow/Red wire to the ECM when engine is cranking. With engine speed less than 400 RPM, ignition module will control ignition timing.

When engine speed is greater than 400 RPM, ECM applies 5 volts on the by-pass line (Yellow/Green wire) to switch timing control to ECM (EST mode). In the EST mode, ECM controls the ignition timing.

EXHAUST GAS RECIRCULATION

To lower oxides of nitrogen (NOx) exhaust gas emissions, an Exhaust Gas Recirculation (EGR) system is used. The EGR system introduces exhaust gases into intake system. Exhaust gases are noncombustible gases which, when combined with the incoming air/fuel mixture, lower peak combustion chamber temperatures.

The EGR valve receives ported operating vacuum from an ECM regulated EGR Vacuum Switching Valve (VSV). Various inputs to ECM are used to determine EGR operation. On California Prizm, Storm and Tracker models, EGR valve operation is monitored by ECM through signal from EGR temperature sensor. The EGR temperature sensor monitors exhaust gas temperature. If abnormal temperature exists, CHECK ENGINE light will be activated. On all models, vacuum signal to EGR valve is further controlled by a vacuum modulator located in the vacuum line between the EGR VSV and ported vacuum supply. Under low driving speeds and light load conditions, vacuum modulator diaphragm is pushed downward and opens vacuum modulator. This allows air to enter modulator from the outside, reducing vacuum supply to EGR valve. The EGR valve closes slightly, reducing amount of recirculated exhaust gases. Under high driving speeds and heavy load conditions, vacuum modulator diaphragm is pushed upward, closing vacuum modulator. This increases vacuum supply to EGR valve and valve opens slightly, increasing amount of recirculated exhaust gases. Under following conditions, ECM does not allow EGR operation

1. Coolant temperature is low
2. Engine is operating under heavy load
3. Engine speed exceeds 6000 RPM (Tracker)
4. Low intake manifold pressure
5. Mass airflow sensor reading is low (Prizm GSi)
6. Throttle valve is at idle position
7. A/T in lock-up condition (Federal Tracker Models)

EGR Temperature Sensor (California Prizm, Storm & Tracker)

The EGR temperature sensor changes resistance with respect to exhaust gas temperature. High exhaust gas temperature decreases sensor resistance; low exhaust gas temperature increases sensor resistance. A reference voltage, supplied and monitored by ECM, is modified by sensor resistance. ECM uses this information to determine EGR operation.

Fuel tank vapors flow through an in-line 2-way check valve from fuel tank to charcoal canister. Check valve maintains constant pressure in the fuel tank. When pressure exceeds specification, fuel tank vapors flow to charcoal canister. Charcoal canister retains vapors until canister purge valve opens in accordance with engine conditions, allowing fuel vapors to enter intake manifold.

Bimetallic Vacuum Switching Valve (BVSV) is mounted in engine coolant passage. When engine coolant reaches specified temperature, BVSV opens, allowing air to flow through the valve and into the intake manifold. This airflow opens canister purge valve and allows fuel vapors to flow from carbon canister into intake manifold. Canister purge valve will only be opened with engine running, engine at normal operating temperature and throttle valve is above idle position.

Fuel tank vapors are stored in a charcoal canister. When engine is running and manifold vacuum is supplied to top of canister purge valve, canister purge valve is opened. This allows fuel vapors to flow from carbon canister into intake manifold. Lower tube on canister purge valve is connected to ported vacuum above throttle valve. Rate of canister purge is controlled by the throttle position. A vapor restrictor is used in the line between fuel tank and charcoal canister.

Fuel tank vapors are stored in charcoal canister. A check valve in fuel tank cap maintains constant pressure in the fuel tank. When fuel tank pressure exceeds specified pressure, fuel tank vapors flow to charcoal canister. Charcoal canister retains vapors until canister purge valve opens in accordance with engine conditions, allowing fuel vapors to enter intake manifold.

Bimetallic Vacuum Switching Valve (BVSV) is mounted in engine coolant passage. When engine coolant reaches specified temperature, BVSV opens, allowing air to flow through the valve and into intake manifold. This airflow opens canister purge valve and allows fuel vapors to flow from carbon canister into the intake manifold. Canister purge valve will only be opened with engine running, engine at normal operating temperature and throttle valve is above idle position.

Fuel tank vapors flow through an in-line 2-way check valve from fuel tank to charcoal canister. Check valve maintains constant pressure in the fuel tank. When pressure exceeds specified pressure, fuel tank vapors flow to charcoal canister.

The main relay provides voltage to canister purge Vacuum Switching Valve (VSV) when ignition is on. The ECM will ground the canister purge VSV when engine speed exceeds 1500 RPM and engine is at normal operating temperature. Canister purge VSV will open, allowing fuel vapors to be drawn from charcoal canister into the intake manifold.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system circulates crankcase blow-by gases (hydrocarbons) into the air induction system rather than allowing them to escape to the atmosphere. Crankcase gases are mixed with air/fuel mixture. Crankcase ventilation system uses a PCV valve which prevents hydrocarbon fumes from collecting in intake manifold when engine is not running. When engine is above idle, manifold vacuum is high, PCV allows crankcase fumes to be drawn into intake manifold. The PCV valve is a metered orifice type on Prizm GSi and Storm GSi models. On Storm models, a diaphragm type valve is used. On all other models, a check valve type is used.

CHECK ENGINE LIGHT

All vehicles are equipped with a CHECK ENGINE light, located on the instrument panel. Light will illuminate when ignition is turned on and engine is not running. Light should go out when engine is started. When CHECK ENGINE engine light remains on or flashes, with engine running, the self-diagnostic system has detected a problem. If problem goes away, light will go out after 10 seconds, but a trouble code will remain stored in the ECM memory. For additional information, see TESTS W/CODES article in the ENGINE PERFORMANCE Section.

Torque Converter Clutch (TCC) Relay (Tracker A/T)

The TCC relay is located in right corner of engine compartment on fusible link box. Battery voltage is supplied to TCC relay on Blue/Black wire from the EFI main relay. The EFI main relay is mounted on the ECM, located under left side of instrument panel, near kick panel.

Note. The EFI main relay may be referred to as the main relay.

Based on signals from throttle position sensor, ignition coil and coolant temperature sensor, the ECM will ground the TCC relay. If brake switch is closed, voltage will be provided through contacts of TCC relay and to oil pressure switch on the transmission. When oil pressure switch closes, voltage will be applied to TCC solenoid to allow for torque converter lock-up.

Shift Light Indicator (Metro M/T)

Shift light indicator is controlled by ECM to indicate shift point to obtain maximum fuel economy based on engine speed and load. The ECM will activate shift light when following conditions are present

1. Both idle switch and wide open throttle switches are off
2. Engine speed is greater than 1500 RPM
3. Vehicle speed is greater than 3 MPH

Note. Engine speed for shift indicator light operation may vary with intake manifold pressure and coolant temperature.

Shift Light Indicator (Storm M/T)

Shift light indicator is controlled by ECM to indicate shift point to obtain maximum fuel economy based on engine speed and load. The ECM uses coolant temperature, throttle position, vehicle speed and engine RPM to determine when to activate shift light.

Throttle Position Sensor Output Signal (Storm GSi & Prizm GSi A/T Models)

The ECM delivers a throttle position sensor output signal to the Electronic Control Unit (ECU) for the transaxle. Signal is used by ECU to determine throttle position and for determining transaxle shift points.