Open Loop Mode
OPERATIONThe engine is in this mode of operation when:
1. Starting the engine and the engine is cold or air cleaner air is cold.
2. Engine is at idle speed.
3. Carburetor is either 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.
Modes Of Operation
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.
NOTE: 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.
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 [1][2]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 [1][2]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 "up- stream". 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 pre-determined level. During this operation, the [1][2]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 pre-determined [1][2]stepper motor positions are chosen, depending on whether the altitude (lean limit) jumper wire is installed.