Modes of Operation

OPEN LOOP MODE (6-258 ENGINE)

The engine is in this mode of operation when:
a. Starting the engine and the engine is cold or air cleaner air is cold.
b. Engine is at idle speed.
c. Carburetor is at or near wide open throttle (WOT).

Whenever any of the above conditions exist, the metering pins in the carburetor are driven to a pre-determined (programmed) position for each condition. Because the positions are pre-determined, no feedback signals relative to the results are accepted. The five open loop operations are characteristic of the metering pins being driven to a position where they stop and remain stationary.

Each operation (except closed loop) has a specific air/fuel ratio and because more than one of the engine operational selection conditions can be present at one time, the MCU is programmed with a priority ranking for the operations that comply with the operation having the highest priority. These priorities are described below.

Cold Weather Engine Start-Up And Operation
During this mode, if the air cleaner temperature is below the calibrated value of the thermal electric switch (TES), the stepper motor is positioned a predetermined number of steps rich of the initialization position and air injection is diverted upstream. Lean air/fuel mixtures are not programmed into the system for a preset amount of time after a cold-weather start up.

Open Loop 2 (OL2) Wide Open Throttle (WOT)
The engine is in this mode of operation when the air cleaner air temperature is above the calibrated value of the thermal electric switch (TES) and the WOT switch has been activated. When this mode of operation is selected, the stepper motor is driven to a calibrated number of steps rich of initialization and the air control valve switches the air injection "downstream". However, if the lean "lean limit" circuit (with altitude jumper wire) is being used, the air is directed "upstream". The WOT timer is activated whenever OL2 is selected and it remains active a preset amount of time. The WOT timer remains inoperative if the "lean limit" circuit is being used.

Open Loop 4 (OL4)
The engine is in this mode of operation whenever manifold vacuum falls below a predetermined level. During this operation, the stepper motor is positioned at the initialization position and air injection is switched "upstream". If the extended OL4 timer is activated and the "lean limit" circuit is not being used (without altitude jumper wire), air injection is switched "downstream". Air is also switched "downstream" if the WOT timer is activated.

Open Loop 3 (OL3)
The engine is in this mode of operation when the ignition advance vacuum level falls below a predetermined amount. When this mode is selected, the engine rpm (tach) voltage is also used for reference. If the tach voltage is greater than the calibrated value, an engine deceleration condition exists. If the tach voltage is less than the calibrated value, an engine idle speed condition exists.

Open Loop 1 (OL1)
The engine is in this mode of operation when the air cleaner temperature is above a calibrated value and OL2 ,3 or 4 is not selected and if the engine coolant temperature is below the calibrated value. When this mode of operation is selected, one of two predetermined stepper motor positions are chosen, depending on whether the altitude (lean limit) jumper wire is installed. Every time the engine is started, a start-up timer is activated. During this interval, if the engine operating condition would otherwise trigger normal closed loop operation, OL1 operation is selected.

CLOSED LOOP MODE (6-258 ENGINE)

The engine is in this mode of operation when none of the open loop engine operating conditions exist. The MCU causes the MC solenoid to vary the air/fuel mixture in reaction to the voltage input from the oxygen content of the exhaust gases, Since the oxygen content of the exhaust gases indicates the completeness of the combustion process, it is a reliable indicator of the air/fuel mixture entering the combustion chamber.

During operation, the MCU causes the stepper motor to drive the metering pins to a leaner or richer position in reaction to the voltage input from the oxygen sensor. The stepper motor drives two moveable air metering pins in the carburetor to control the air/fuel ratio.

The engine operating characteristics almost never allow the MCU to compute a single metering pin position that constantly provides the optimum air/fuel mixture. Therefore, the closed loop operation is characterized by the constant movement of the metering pins because the MCU is constantly making corrections to the air/fuel mixture in an attempt to create an optimum air/fuel mixture ratio.

Fig. 3 Idle speed control system diagram. 6-258:

INTERRELATED SYSTEMS (6-258 ENGINE)

The Idle Speed Control System, Fig. 3, Electronic Ignition Retard function of the MCU and Air Injection system are all interrelated systems of the CEC and must be diagnosed along with the CEC diagnosis.