Computers and Control Systems: Description and Operation

General description
The Engine Control Module (ECM) operates the fuel system. It regulates ignition timing, air fuel ration, emission control devices, charging system, idle speed control, air conditioning clutch engagement, etc. The ECM has the ability to adapt to changing operating conditions.

The ECM receives input signals from various switches and sensors. Based on these inputs, the ECM regulates various engine and vehicle operations through different system components also known as ECM Outputs. On the other hand, the sensors and switches that provide inputs to the ECM are ECM Inputs. For example, the ECM adjusts ignition timing based upon inputs it receives from sensors that react to: engine RPM, engine coolant temperature, throttle position, transaxle gear, vehicle speed, etc. The ECM adjusts idle speed based on inputs it receives from sensors that react to: throttle position, vehicle speed, transaxle gear, etc.

Mass air flow sensor
The Mass Air Flow (MAF) Sensor is the most direct method of measuring engine load because it measures the mass of air intake. This is also known as a "hot- wire" sensor because it depends on the measurement of current flowing through heated wires to measure air flow. The hot wire is heated to specific temperature differential above incoming air. Two wires inside the MAF Sensors are exposed to a portion of the airflow entering the engine:

The ambient temperature wire or "cold wire" isn't heated so this wire is the temperature of the surrounding air and serves as the reference temperature.

The hot wire is heated by the MAF to be a certain amount above the ambient air. As soon as air flows over the wire, both wires are cooled. The control circuits then apply more voltage to keep the hot wire at the original temperature differential. This creates a voltage signal monitored by the ECM. The greater the air flow and wire cooling, the greater the signal.

Engine coolant temperature sensor
The Engine Coolant Temperature (ECT) Sensor provides an input voltage to the ECM relating to coolant temperature. The ECM uses this input signal along with inputs from other sensors to determine injector pulse width and ignition timing. As coolant temperature varies the ECT Sensor's resistance changes. The change in resistance results in a different input voltage to the ECM. When the engine is cold, the ECM will operate in Open Loop cycle. It will demand slightly richer air/fuel mixtures and higher idle speeds until normal temperatures are reached.

Throttle position sensor
The Throttle Position (TP) Sensor is a variable resistor that provides the ECM with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TP Sensor changes. The ECM supplies a 5 V reference voltage to the TP Sensor. The TP Sensor output voltage (input signal to the ECM) represents the throttle blade position. This will usually vary from around 0.25 V at minimum throttle opening, to around 4.7 V at wide open throttle.

Crankshaft position sensor
The Crankshaft Position (CKP) Sensor is used to signal the ECM when a spark and/or fuel injection event required. The output from this sensor, in conjunction with the Camshaft Position Sensor signal, is used to determine whether spark or fuel is required in a particular cylinder. The engine will not start without a CKP Sensor signal to the ECM.

Camshaft position sensor
The Camshaft Position (CMP) Sensor works with the CKP Sensor to provide inputs to the ECM to establish and maintain correct injector firing order.

Knock sensor
The Knock Sensor is a feedback signal used to control of ignition timing. When knocking occurs, the Knock Sensor sends the signal to the ECM. The ECM then retards the ignition timing to reduce or eliminate the knock.

Intake air temperature sensor
This sensor measures the temperature of the intake air and converts its charge into a resistance through a thermistor similar to the ECT Sensor. By monitoring the IAT Sensor, the ECM detects the intake air temperature and use it as one of the signals to control fuel injectors and ignition timing.

HO2 (Oxygen) sensor
Two heated O2 Sensors are used in the vehicle. The sensors produce voltages from 0 to 1 volt, depending upon the oxygen content of the exhaust has in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air/fuel mixture), the sensors produce a low voltage. When there is a lesser amount (rich air/fuel mixture) the sensors produce a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensors act as a rich-lean switch.

Both HO2 Sensors are equipped with a heating element that keeps the sensors at proper operating temperature during all modes of operation. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation faster and allows the system to remain in closed loop during periods of extended idle.

Front HO2 sensor
The front (upstream) HO2 Sensor is located in the exhaust downpipe after the TWC and provides an input voltage to the ECM. The input tells the ECM the oxygen content of the exhaust gas. The ECM uses this information to fine tune the air/fuel ratio by adjusting injector pulse width.

Rear HO2 sensor
The rear (downstream) HO2 Sensor is located near the outlet end of the catalytic convertor. The rear HO2 Sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the rear HO2 Sensor input to the input from the front sensor, the ECM calculates catalytic convertor efficiency.