Electronic Control Module (ECM)

The ECM is located in the passenger compartment (behind glove compartment) and controls all functions of the Closed Loop Emission Control system. The ECM sends an electrical signal to the vacuum control solenoid, which controls the air/fuel mixture by vacuum signals. This control signal is constantly cycling the solenoid between "on" and "off" time (duty cycle) as a function of the input voltages from the data sensors. The control signal generated by the ECM is selected from 4 operational modes. These modes include: Inhibit Mode, Enrichment Mode, Open Loop Mode and Closed Loop Mode. A brief description of each mode follows:

• Inhibit Mode 
• No electrical signals are sent to the vacuum control solenoid by the ECM in this mode.
• Enrichment Mode 

  In this mode a fixed, pre-programmed duty cycle electrical signal is sent to the vacuum control solenoid by the ECM. This signal is sent to the solenoid when fuel enrichment is necessary for cold engine starts or sudden acceleration.

• Open Loop Mode 

  In this mode the ECM sends electrical signals to the vacuum control solenoid based upon stored information within the ECM. This information has been calculated and is used by the ECM to operate the engine at optimum efficiency for that particular operating condition of the engine, without any input from the data sensors. Open loop mode is used when the engine has not reached operating temperature.

• Closed Loop Mode 

  In this mode the ECM sends an electrical signal to the vacuum control solenoid based upon input from the oxygen sensor and other data sensors. In closed loop, the air/fuel mixture is controlled directly by the ECM in response to oxygen sensor signals.

  During any operating mode, the ECM maintains the current duty cycle being used within its memory; for either idle or off-idle operation. When the ECM receives a change in idle position, as signaled by the Idle Position Switch or WOT Switch, the ECM retrieves data from its memory to operate the engine at the duty cycle last recorded for optimum operation. After the initial change in idle position, the ECM then controls the system in one of the 4 operational modes.

  The ECM also controls operation of the slow cut solenoid valve incorporated within the carburetor. When the ECM senses a coasting condition (based upon signals received from the Transmission Gear Position Switch, Clutch Pedal Position Switch and Idle Position Switch), it opens the circuit to the slow cut solenoid valve (engine speed above a predetermined value). The circuit to the slow cut solenoid valve is cut off only when the following 4 conditions exist at the same time:

• Transmission is not in Neutral (man. trans.) or in "N" or "P" (auto. trans.).
• Clutch is engaged on manual transmission models.
• Throttle is at idle position.
• Engine speed exceeds predetermined speed.