Engine Controls - Theory & Operation: Other

COMPUTERIZED ENGINE CONTROLS

Fuel injection system is Throttle Body Injection (TBI). 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.

ENGINE CONTROL MODULE (ECM)

Note. ECM is also known as Powertrain Control Module (PCM). All references to ECM include PCM unless specified otherwise.

Note. For ECM locations, see ECM LOCATION table.

Power for ECM is supplied through the TAIL fuse, located in fuse/relay block. The ECM distributes power or controls ground of various sensors, switches and solenoids for engine control.

Application	Location
Metro	On Left Of Steering Column

ECM LOCATION

Note. Components are grouped into 2 categories. The first category is INPUT DEVICES, which are components that control or produce voltage signals monitored by the ECM. The second category is OUTPUT SIGNALS, which are components controlled by the ECM.

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 this 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 and fuel mixture.

Camshaft Position (CMP) Sensor

Engine speed signal is generated by pick-up coil/camshaft position sensor and signal rotor 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 the energizing of fuel injector.

Crank Signal

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 Air Control (IAC) valve.

Electrical Load Signal

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

Engine Coolant Temperature (ECT) Sensor

Engine Coolant Temperature (ECT) 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 control fuel injectors and emission components.

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.

Sensor resistance modifies a reference voltage that is supplied and monitored by ECM. The ECM uses this information to control output signals to the fuel injectors.

For IAT sensor locations, see INTAKE AIR TEMPERATURE SENSOR (IAT) LOCATION table.

Application	Location
Metro	Inside Air Cleaner Assembly

INTAKE AIR TEMPERATURE SENSOR (IAT) LOCATION

Manifold Absolute Pressure (MAP) Sensor

MAP sensor is connected 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.

Oxygen (O2) Sensor

O2 sensor is mounted in exhaust manifold or exhaust pipe, where it contacts exhaust gases. O2 sensor generates voltage according to oxygen content in 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 ECM memory.

Note. On A/T, gear position signal is received from the TCM. See TCM GEAR SIGNAL (A/T) below.

System Voltage

The fuel injector is driven by its solenoid coil based upon the ECM output signal. There is a delay between the ECM signal and valve action when no fuel is provided to the injector. This delay, known as ineffective injection time, depends on system voltage. The ECM takes voltage information to compensate for fuel injection time.

TCM Gear Signal (A/T)

On A/T applications, the ECM monitors voltage signal sent from the TCM when transaxle is in any gear except Park or Neutral. The ECM uses this input signal to help control fuel injector and Idle Air Control (IAC) valve.

Throttle Position Sensor

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 idle switch signal (only when throttle is in idle position).

ECM uses these signals to control air/fuel ratio during acceleration, deceleration and idling. These signals are also used to determine idle speed. On A/T models, the ECM converts signal to an on-off signal and sends it to Transaxle Control Module (TCM) for transmission control.

Vehicle Speed Sensor

Vehicle Speed Sensor (VSS) consists of a reed switch built into speedometer head. ECM supplies VSS with a 5-volt reference signal. As speedometer cable rotates, the reed switch closes the circuit to ground 4 times per revolution. As vehicle speed increases, frequency of ground pulses increase. This pulse is sent to ECM and interpreted as vehicle speed.

OUTPUT SIGNALS

Note. Vehicles are equipped with various combinations of computer-controlled components. Not all components listed are used on every vehicle. For theory and operation on each output component, refer to the system indicated after component.