Modes of Operation
MICRO-COMPUTOR UNIT (MCU)Modes of Operation
As input signals to the MCU change, the MCU adjusts its response that it sends to the output devices. For example, the MCU must calculate a different stepper motor dwell and ignition timing setting for idle than it does for wide open throttle (WOT). There are four modes of operation that determine how the MCU responds to the various input signals. Modes of operation are separated into two different types, OPEN LOOP and CLOSED LOOP. During OPEN LOOP modes the MCU receives input signals and responds only to preset MCU programming. Input from the oxygen sensor is not monitored during OPEN LOOP modes. During CLOSED LOOP modes the MCU monitors the oxygen sensor input. This input indicates to the MCU whether or not the calculated injector pulse width results in the ideal air/fuel ratio of 14.7 parts of air to 1 part of fuel. By monitoring the exhaust oxygen content, using the oxygen sensor, the MCU can "fine tune" the (MC) solenoid or stepper motor dwell to achieve optimum fuel economy and performance combined with low emission levels. The Fuel Feedback system has the following modes of operation:
^ Key-on
^ Start-up
^ Warm-up
^ Cruise
The key-on, start-up, and warm-up modes are OPEN LOOP modes. The cruise mode is a CLOSED LOOP mode.
Key-ON
When the MCU is in this mode it operates in the following order:
^ Ignition switch ON
^ MCU is energized
^ Sole-Vac holding solenoid is energized
^ Idle relay activated (voltage supplied; no ground)
^ PCV valve shut-off solenoid energized by the MCU (acts as an anti-dieseling device)
Start-Up
When the MCU is in this mode it operates in the following order:
^ Key to the START position
^ Starter motor solenoid is energized
^ Choke relay and intake manifold heater relay (if the intake manifold heater switch is closed) are energized by the oil pressure switch
^ Intake manifold heater is activated (if the coolant is cold)
^ Start signal provided to the MCU
^ Engine RPM (tach) voltage from the distributor provided to the MCU
^ MCU controls the ignition timing
Warm-Up
When the MCU is in this mode it operates in the following order:
^ System is in open loop operation
^ MCU receives certain inputs
^ 4-inch Hg vacuum switch is open because the vacuum is less than 4-inch Hg
^ 10-inch Hg vacuum switch closed below 10-inch Hg and open at 10-inch Hg vacuum level or greater
^ Coolant temperature switch closed at 57~ C (135~ F)
^ Wide-open throttle switch open
^ Thermal electric switch opens at 18~ C(65~ F)
^ Manifold heater switch opens at 71~ C (160~ F) (the manifold heater is off)
^ Mixture control (MC) solenoid or stepper motor controlled by the MCU to provide a pre-programmed amount of air for the air/fuel mixture
^ Upstream and downstream solenoids controlled by the MCU to distribute air to either the exhaust or the catalytic converter
^ Sole-Vac holding solenoid is activated if the air conditioner is in operation or if the vehicle is decelerating.
Cruise
When the MCU is in this mode it operates in the following order:
^ System is in closed loop operation
^ MCU produces outputs
^ Components are controlled as in the warm-up mode
^ Oxygen sensor input accepted by the MCU
^ MCU controls the mixture control (MC) solenoid or stepper motor according to the inputs
^ MCU may receive input from the knock sensor to advance or retard the ignition timing
^ Upstream and downstream solenoids route air to the exhaust pipe or catalytic converter
^ Wide-open throttle switch indicates the throttle position