Engine Control Module: Description and Operation

EFC Closed Loop System:

PURPOSE
The Electronic Fuel Controlled (EFC) Carburetor Control Unit is a digital computer with three main functions:
^ Judging the rich or lean state of the air/fuel ratio according to the oxygen sensor output voltage.
^ Sending control signals to the carburetor duty solenoid valve to make mixture adjustments according to oxygen sensor output voltage and information input from various other sensors (feedback control).
^ Fixing control signals under specific driving conditions such as cold starting.

LOCATION
The ECU is located behind the left side of the dash board.

OPERATION
When the EFC Control Unit is sending fixed signals without regard to oxygen sensor output voltage, the system is said to be in "open loop." Conversely, when the control unit is computing signals based on oxygen sensor voltage, the system is said to be in "closed loop."

The EFC Control Unit has built-in preprogrammed values that it will default to if system malfunctions are detected. This usually permits the vehicle to be driven until repairs are made, although performance will be affected.

The EFC Control Unit has a self-diagnostic function that stores trouble codes in its memory. A "Check Engine" light alerts the driver to seek service. A technician can then extract trouble codes from memory which indicate where system problems have occurred and make necessary corrections.