See Figure 1
The Computerized Emission Control (CEC) system is used on all 4.2 L engines covered by this guide. The CEC system is a carbureted fuel feedback system, meaning that the carburetor receives electronic control signals to help regulate the air/fuel mixture based on "feedback'' from an exhaust gas oxygen sensor. The 4.2L engine is normally equipped with a model BBD two-venturi, feedback carburetor.
There are two primary modes of operation for the CEC feedback system; open loop and closed loop. The system operates in the open loop mode of operation (or a variation of it) whenever the engine operating conditions do not meet the programmed criteria for closed loop operation. In open loop operation, the oxygen sensor data is not accepted by the system, instead, the air/fuel mixture is maintained at a programmed ratio that is dependent upon the type of engine operation involved. The following conditions involve open loop operation:
When all input data meets the programmed criteria for closed loop operation, the exhaust gas oxygen content signal from the oxygen sensor is accepted by the computer. This results in an air/fuel mixture that is determined and regulated for optimum engine operating conditions and also will correct any pre-existing mixture condition which is too lean or too rich.
A high oxygen content in the exhaust gas indicates a lean air/fuel mixture, while a low oxygen content indicates a rich mixture. The optimum air/fuel mixture ratio is 14.7:1.
See Figures 2 through 15Micro Computer Unit (MCU)
The Micro Computer Unit (MCU) is the heart of the CEC system. The MCU receives signals from various engine sensors to constantly monitor the engine operating conditions. It then makes adjustments, based on this information, in order to achieve the optimum performance and economy with a minimum of engine emissions. The MCU monitors the oxygen sensor voltage and, based upon the mode of operation, generates an output control signal for the carburetor stepper motor or mixture control solenoid. If the system is in the closed loop mode of operation, the air/fuel mixture will vary according to the oxygen content in the exhaust gas and engine operating conditions. If the system is in the open loop mode of operation, the air/fuel mixture will be based on a predetermined ratio that is dependent on engine rpm. In addition, the MCU generates output signals to control ignition timing and engine idle speed, PCV flow and Pulse Air System operation.Idle Relay and Solenoid
The idle relay is energized by the MCU (by providing a ground for the relay) in order to control the vacuum actuator portion of the Sole-Vac throttle positioner. The relay energizes the idle solenoid, which allows vacuum to operate the Sole-Vac vacuum actuator. This, in turn, opens the throttle and increases engine speed. The idle solenoid is located on a bracket on the left front inner fender panel and can be identified by the red connecting wires.Sole-Vac Throttle Positioner
The Sole-Vac throttle positioner is attached to the carburetor. The unit consists of a closed throttle switch, a holding solenoid and a vacuum actuator. The holding solenoid maintains the throttle position, while the vacuum actuator provides additional engine idle speed when accessories such as the air conditioner or rear window defogger are in use. The vacuum actuator is also activated during deceleration, or any time the steering wheel is turned to the full stop position on vehicles equipped with power steering.Upstream and Downstream Air Switch Solenoids
The upstream and downstream solenoids of the pulse air system distribute air to the exhaust pipe and catalytic converter. Both solenoids are energized by the MCU to route air into the exhaust pipe at a point after the oxygen sensor. When energized, the downstream solenoid routes air into the second bed of the dual-bed catalytic converter. This additional air reacts with the exhaust gases to further reduce engine emissions.
The solenoids are located on a bracket attached to the left inner front fender panel. The idle solenoid is also located on this same bracket.PCV Shutoff Solenoid
The positive crankcase ventilation shutoff solenoid is installed in the PCV valve hose and is energized by the MCU to turn off the crankcase ventilation system when the engine is at idle speed. An anti-diesel system, consisting of an anti-diesel relay and a delay relay, prevents engine run-on by preventing air from entering below the throttle plate whenever the ignition is switched OFF . This is accomplished by momentarily energizing the PCV valve solenoid whenever the ignition is switched to the OFF position.Intake Manifold Heater Switch
The intake manifold heater switch is located in the manifold and is controlled by engine coolant temperature. Below 160ºF (71ºC) the switch activates the intake manifold heater to improve fuel vaporization. The switch is not controlled by the MCU and does not provide input information to it. The switch does however help reduce exhaust emissions during engine warm-up through the improved fuel vaporization.Oxygen Sensor
This component of the system provides a variable voltage signal (at a level of millivolts) for the Micro Computer Unit (MCU) that is proportional to the oxygen content in the exhaust gas. Various other data sensors and inputs are used along with the oxygen sensor signal to supply the MCU with the necessary engine operation information.Knock Sensor
The knock sensor is a tuned piezoelectric crystal transducer that is threaded into the intake manifold. The knock sensor provides the MCU with an electrical signal that is created by vibrations which correspond to its center frequency (5550 Hz). Vibrations from engine knock (detonation) cause the crystal inside the sensor to vibrate and produce an electrical signal that is sent to the MCU. The control unit then retards the ignition timing to eliminate the knock condition.Vacuum Switches
Two vacuum-operated electrical switches (ported and manifold) are used to detect and send throttle position data to the MCU for idle (closed), partial and wide open throttle (WOT). These switches are located together in a bracket attached to the dash panel in the engine compartment. The 4 in. Hg (14 kPa) vacuum switch can be identified by its natural (beige) color, while the 10 in. Hg (34 kPa) vacuum switch is green. The 4 in. Hg (14 kPa) switch is controlled by ported vacuum and its electrical contact is normally in the open position. When the vacuum exceeds 4 in. Hg (14 kPa), the switch closes. The 4 in. Hg (14 kPa) vacuum switch tells the MCU when either a closed or wide open throttle condition exists.
The 10 in. Hg (34 kPa) vacuum switch is controlled by manifold vacuum. Its electrical contact is normally closed; if the vacuum level exceeds 10 in. Hg (34 kPa), the switch opens. This switch tells the MCU that either a partial or medium throttle condition exists.Engine RPM Voltage Terminal
A voltage is supplied from a terminal on the distributor to the MCU. Until a voltage equivalent to a predetermined rpm is received by the MCU, the fuel system remains in the open loop mode of operation. The result is a fixed rich air/fuel mixture for starting purposes.Coolant Temperature Switch
The temperature switch supplies engine coolant temperature data to the MCU. Until the engine is sufficiently warmed (above 135ºF/57ºC), the system remains in the open loop mode of operation (i.e., a fixed air/fuel mixture based upon engine rpm).Thermal Electric Switch
The thermal electric switch is located inside the air cleaner and is used to sense the incoming air temperature. The signal then indicates a cold weather start-up condition to the MCU when the air temperature is below 50ºF (10ºC). Above 65ºF (18ºC), the switch opens to indicate a normal engine start-up condition to the MCU.Wide Open Throttle (WOT) Switch
The wide open throttle switch is attached to the base of the carburetor by a mounting bracket. It is a mechanically operated electrical switch that is controlled by the physical position of the throttle. When the throttle is placed in the wide open position, a cam on the throttle shaft actuates the switch about 15º before the wide open position is reached. This tells the MCU that a full-throttle demand is underway.Altitude Jumper Wire Connector
The altitude jumper wire connector is located next to the MCU. A jumper wire is installed in the connector to provide the MCU with an indication that the vehicle is being operated above a 4000 ft. elevation (high altitude operation). Unless operating the vehicle in a high altitude area, the connector should have no jumper wire installed. If the vehicle is to be operated in a designated high altitude area, a jumper wire must be installed to assure proper engine and emission performance.
CEC SYSTEM OPERATION
The open loop mode of operation occurs during any of the following conditions:
- Starting the engine, engine is cold or temperature of the air inside the air cleaner is cold.
- Engine is at idle speed.
- Carburetor is either at or near wide open throttle (WOT).
Should any of these conditions occur, the carburetor fuel metering pins are driven to a predetermined (programmed) position for each condition. This is known as open loop operation because the positions are predetermined and no feedback results are used. There are five open loop operating conditions which are characterized by the metering pins being driven to a position where they are stopped and remain stationary.
Each system operation condition (except closed loop) has a specific metering pin position and because more than one of the operation selection conditions can be present at one time, the MCU is programmed with a priority ranking for the operations. The MCU complies with conditions that pertain to the operation having the highest priority. The priorities are as described as follows:Cold Weather Engine Start-Up and Operation
If the air temperature inside the air cleaner is below the calibrated value of the Thermal Electric Switch (TES), the stepper motor is positioned a predetermined number of steps richer than the initialization position and the air injection is diverted upstream. Lean air/fuel mixtures are not permitted for a preset period following a cold weather start-up.Open Loop 1 (OL1)
Open Loop 1 will be selected if the air temperature inside the air cleaner is above a calibrated value, open loop 2, 3, or 4 is not selected, and if the engine coolant temperature is below the calibrated value. The OL1 mode operates in lieu of normal closed loop operation during a cold engine operating condition. If OL1 operation is selected, one of two predetermined stepper motor positions are chosen, dependent upon whether the altitude circuit (lean limit) jumper wire is installed. With each engine start-up, a start-up timer is activated. During this interval, if the engine operating conditions would otherwise trigger normal closed loop operation, OL1 operation is selected.Open Loop 2 (OL2)/Wide Open Throttle (WOT)
Open Loop 2 is selected whenever the air temperature inside the air cleaner is above the calibrated value of the Thermal Electric Switch (TES) and the WOT switch has been engaged. When the Open Loop 2 mode is selected, the stepper motor is driven to a calibrated number of steps richer than initialization and the air control valve switches air downstream. However, if the "lean limit'' circuit (with altitude jumper wire) is being used, the air is instead directed upstream. The WOT timer is activated whenever OL2 is selected and remains active for a preset period of time. The WOT timer remains inoperative if the "lean limit'' circuit is being used.Open Loop 3 (OL3)
Open Loop 3 is selected when the ignition advance vacuum falls below a predetermined level. When the OL3 mode is selected, the engine rpm is also determined. If the rpm (tach) voltage is greater than the calibrated value, an engine deceleration condition is assumed to exist. If the rpm (tach) voltage is less than the calibrated value, an engine idle speed condition is assumed to exist.Open Loop 4 (OL4)
Open Loop 4 is selected whenever manifold vacuum falls below a predetermined level. During OL4 operation, the stepper motor is positioned at the initialization position. Air injection is switched upstream during OL4 operation. However, air is switch downstream if the extended OL4 timer is activated and if the "lean limit'' circuit is not being used (without altitude jumper wire). Air is also switch downstream if the WOT timer is activated.Closed Loop
Closed loop operation is selected after either OL1, OL2, OL3 or OL4 modes have been selected and the start-up timer has timed out. Air injection is routed downstream during closed loop operation. The predetermined lean air/fuel mixture ceiling is selected for a preset length of time at the onset of closed loop operation.High Altitude Adjustment
An additional function of the MCU is to correct for a change in ambient conditions (e.g., high altitude). During closed loop operation the MCU stores the number of steps and direction that the metering pins are driven to correct the oxygen content of the exhaust. If the movements are consistently to the same position, the MCU will vary all open loop operation predetermined metering pin positions a corresponding amount. This function allows the open loop air/fuel mixture ratios to be "tailored'' to the existing ambient conditions during each uninterrupted use of the system. This optimizes emission control and engine performance.Closed Loop Operation
The CEC system controls the air/fuel ratio with movable air metering pins, visible from the top of the carburetor air horn, that are driven by the stepper motor. The stepper motor moves the metering pins in increments or small steps via electrical impulses generated by the MCU. The content of oxygen in the exhaust gas indicates the completeness of the combustion process, therefore, the MCU causes the stepper motor to drive the metering pins to a richer or leaner position in reaction to the voltage input from the oxygen sensor.
Because the oxygen sensor only reacts to oxygen, any air leak or malfunction between the carburetor and sensor may cause problems with the voltage output. This could be caused by a manifold air leak or malfunctioning secondary air check value. The engine operation characteristics never quite permit the MCU to compute a single metering pin position that constantly provides the optimum air/fuel mixture. Therefore, closed loop operation is characterized by continued movement of the metering pins because the MCU is forced constantly to make small corrections in the air/fuel mixture in an attempt to create an optimum air/fuel mixture ratio.