Engine Control Module: Description and Operation
Computerized Engine Control System:
The PGM-CARB Unit is a dedicated microcomputer which reads the signals from the various input devices and, based on the program contained in ROM, determines current driving conditions. Once driving conditions are known, the output devices are used to optimize engine efficiency thereby providing maximum attainable fuel economy and minimum achievable emissions output.
Some of the Control Unit functions are:
Air Injection Function:
This system makes use of vacuum pulses in the exhaust manifold to draw air from the air cleaner to the exhaust manifold to promote oxidation of hydrocarbons.
Evaporative Function
The Evaporative Control System minimizes the amount of fuel vapor until it can be purged from the canister into the engine and burned.
EGR Function
The EGR System is designed to control of oxides of nitrogen emissions by recirculating a part of the exhaust gas into the intake manifold through the EGR valve mounted on the intake manifold.
Feedback function:
The feedback system has four functions:
1. Air-Fuel Ratio Control
2. Shot Air Control
3. Deceleration Air Supply
4. Hot Engine Start Control
1. AIR-FUEL RATIO CONTROL:
The system is designed to achieve a stoichiometric air-fuel mixture ratio. The carburetor air-fuel mixture is basically calibrated on the richer side of the stoichiometric ratio, and the air supply through the EACV dilutes the mixture for controlling the mixture close to the stoichiometric.
The system performs feedback functions in most of the driving conditions based on the output from the oxygen sensor. However, the system interrupts the feedback function when the engine needs richer or leaner mixture for the operating condition, such as when the vehicle is in power mode or when the engine is warming up.
2. SHOT AIR CONTROL:
The control unit receives signals of vehicle speed, engine coolant temperature, intake manifold vacuum and engine speed. Shot air is induced from the EACV when the manifold vacuum increases suddenly except when the vehicle is moving at a very low speed with the engine coolant temperature below the normal operating level. This improves emissions performance and prevents afterburning due to the over-rich mixture during short deceleration.
3. DECELERATION AIR SUPPLY:
This system is designed to improve emissions performance by supplying air into the intake manifold during deceleration at relatively high engine speeds. The control unit receives signals from the MAP sensor, TW sensor, speed sensor, vacuum switch, gear position switch and ignition coil, and identifies driving conditions for deceleration air supply.
The control unit transmits the electric current to the EACV which opens and supplies air into the intake manifold.
4. HOT ENGINE START CONTROL:
The control unit receives the signal of engine coolant temperature. When it is higher than the normal temperature, the EACV is activated to supply air into the intake manifold before the vehicle speed exceeds the set speed.
Fuel Cut-off:
The fuel cut-off relay applies voltage to the fuel pump whenever ignition pulses are applied to the fuel cut-off relay.
Idle Function:
Idle speed is controlled through the use of the EACV and the A/C idle boost system.
Back-up Functions:
1. Fail-Safe Function
When an abnormality occurs in the signal from a sensor, the control unit ignores that signal and assumes a preprogrammed value that allows the engine to continue to run.
2. Back-Up Function
When an abnormality occurs in the control unit itself, a back-up circuit independent of the system permits minimal driving.
3. Self-Diagnosis Function (Check Engine Warning light and LED indicator)
When an abnormality occurs in a signal from a sensor, the control unit lights the Check Engine warning light, stores the failure code in erasable memory and indicates the code with a LED on the control unit anytime the ignition is on. When the ignition is initially turned on, the control unit supplies ground for the Check Engine warning light for about 2 seconds.