DESCRIPTION
The Computerized Emission Control system (CEC) is used on all 6-cylinder models. It is an electronically controlled system that manages the air/fuel ratio and controls the AIR injection, idle speed control, and ignition systems. The primary objective of the CEC system is to maintain an ideal air/fuel ratio of 14.7:1 under all operating conditions. When the ideal ratio is maintained, the catalytic converter can effectively control NOx, HC and CO emissions.
CEC System for 6-Cylinder Models Carburetor (not shown) is controlled by this system. Scheme 36
OPERATION
The CEC system consists of 7 sub-systems: fuel control, data sensors, Microcomputer Control Unit (MCU), catalytic converter, idle speed control, air injection control, and ignition advance control.
FUEL CONTROL
All models are equipped with feedback carburetors which contain an electronically operated stepper motor. The stepper motor controls metering pins which vary the size of idle and main air bleed orifices in carburetor body.
The stepper motor moves the pins in and out of the orifices in steps, in response to signals received from MCU. The motor has a range of 100 steps, but normally operates in the middle of its range.
When the metering pins are stepped into the orifices, the air/fuel mixture becomes richer. When the pins are stepped out of the orifices, the mixture becomes leaner.
Oxygen Sensor
The oxygen sensor is located in the exhaust manifold to measure oxygen content of exhaust gases. As more oxygen is sensed (lean mixture indication), the electrical signal generated by the sensor drops in voltage. A lower oxygen content (rich mixture indication) causes an increase in voltage signal output.
Thermal Electric Switch (TES)
This switch is attached inside air cleaner to provide either a ground circuit for MCU to indicate cold weather engine start-up (air temperature below calibrated value) or an open circuit to indicate normal start-up (air temperature above calibrated value).
Coolant Temperature Switch
This switch is an integral component of the coolant temperature control switch. This switch is controlled by coolant temperature and is normally closed. When closed, the switch indicates engine is cold (less than 135°F/57°C).
4 in. Hg Vacuum Switch
This switch is mounted on a bracket attached to the inner fender. This switch is controlled by carburetor ported vacuum and has a normally open electrical switch (indicating a closed throttle position). The electrical switch is closed with 4 in. Hg of carburetor ported vacuum.
10 in. Hg Vacuum Switch
This switch is located on the same bracket as the 4 in. Hg switch. The 10 in. Hg switch is a manifold vacuum operated switch that, when open, signals the computer of a throttle position that is above partial throttle, but below wide open throttle. This switch is normally closed.
Wide Open Throttle Switch (WOT)
This mechanically operated electrical switch is located on carburetor and is controlled by throttle position to indicate a wide open throttle condition. This switch is normally open.
Engine RPM Voltage
This voltage is supplied from the tach terminal on the distributor. Until a voltage equal to a predetermined RPM is received by the MCU, the system remains in open loop mode of operation. The result is a fixed rich air/fuel mixture for engine starting.
Timer
The timer is activated whenever system is operating in open loop 2 mode (wide open throttle). This timer remains active for a preset period of time. If a "lean limit" condition (altitude jumper wire installed) occurs, the timer becomes inoperative. The timer has multi-function abilities; in addition to OL2 mode, it is used as a WOT timer and start-up timer.
MICROCOMPUTER CONTROL UNIT
The MCU is located in passenger compartment, behind right-hand kick panel. The MCU monitors the CEC system data sensors and, based upon mode of operation, generates an output control signal to the stepper motor mounted in carburetor. The MCU allows the following 3 modes of operation
Initialization
This function occurs when ignition switch is turned on. This sets initial air bleed metering rod position by signaling the stepper motor to drive them first to a full rich position (fully toward front of vehicle) and then, by a pre-programmed number of steps, in lean direction (toward rear of vehicle). This serves as a starting point of mixture control operation.
Open Loop
In this mode, the MCU determines the air/fuel mixture based upon engine operation rather than oxygen sensor input signals. There are 5 open loop modes of operation and each has a specific metering pin position.
However, because more than one condition may exist at any time, the MCU is programmed with a priority ranking for each operation. The MCU complies with the highest priority. The open loop priorities (listed from highest to lowest) are as follows: Cold Weather Start-Up, Open Loop 2 (Wide Open Throttle), Open Loop 4 (Low Manifold Vacuum), Open Loop 3 (Low Ported Vacuum), Open Loop 1 (Cold Engine Operation).
Note. With each engine start-up, a start-up timer is activated. During this interval, if engine operating condition would otherwise trigger normal closed loop operation, OL1 mode is selected.
Closed Loop
When all input data and engine operation meet programmed criteria (when OL1, OL2, OL3, OL4 and cold start modes are not selected and start-up timer has deactivated), the CEC system goes into closed loop operation. In this mode, oxygen sensor input signals are accepted by MCU to determine proper air/fuel mixture based upon oxygen content of exhaust gases. Air injection is routed "downstream" during this mode for partial or wide open throttle conditions and both "upstream" and "downstream" for all other throttle positions.
Note. Closed loop operation is characterized by constant movement of the metering pins. The MCU is constantly making small corrections in air/fuel ratio in an attempt to create the ideal air/fuel ratio.
CATALYTIC CONVERTER
Proper emission control is accomplished with the special catalytic converter used with the CEC system. All models use a dual bed monolithic-type converter with "downstream" air injection. The injection of air between the 2 beds allows more complete oxidation of HC and CO in the closed loop mode. In order for these converters to be effective, precise control of the oxygen content of exhaust gases entering the converter is necessary; thus the need for the oxygen sensor, MCU and feedback carburetor.
IDLE SPEED CONTROL
The idle speed control system is operated by vacuum signals and the MCU. The idle speed system raises and/or maintains the engine idle whenever high electrical loads or air conditioning compressor loads are present. The idle speed control system consists of a sole-vac (throttle positioner), an idle vacuum switching solenoid, and an idle speed relay.
AIR INJECTION CONTROL
The air injection system is switched from upstream to downstream injection (or both) by the MCU. Two electrically operated vacuum valves supply operating vacuum to the upstream air injection valve and the downstream air injection valve. This allows the MCU to control catalyst operation and thereby reduce exhaust emissions.
IGNITION ADVANCE CONTROL
A vacuum operated electrical switch is used to electronically retard the ignition timing advance during certain phases of engine operation.
COMPONENT TESTING
The steps listed in the following charts will provide a systematic evaluation of each component that could cause the malfunction. After completing a repair, repeat the test to ensure the malfunction has been eliminated.
Before performing any of the tests, make sure that the following related systems are operating properly
- Basic carburetor adjustments.
- Mechanical engine operation (plugs, valves, rings, etc.).
- Ignition system.
- Intake manifold, carburetor or base plate gaskets.
- Loose vacuum hoses or fittings.
TEST EQUIPMENT
- The equipment required for testing includes: tachometer, hand vacuum pump, digital volt-ohmmeter (minimum 10 megohm impedance) and a jumper wire.
- Before beginning any of the tests, a clear air cleaner cover must be fabricated from clear plastic at least.25" thick. This is secured with air cleaner wing nut after top of air cleaner has been removed to observe operation and position of metering pins. (Scheme 37)
Note. The metering pins operate in tandem. Only the upper pin is visible.
Air Cleaner Cover Dimensions Fabricate cover to allow observation of metering pins. Scheme 37
| Chart No. | Test |
|---|---|
| Test No. 1 | Operational Test |
| Test No. 3 | Open Loop Switch Test |
| Test No. 4 | Closed Loop Operational Test |
| Test No. 5 | Electronic Ignition Retard Test |
| Test No. 6 | Oxygen Sensor & Closed Loop Test |
| Test No. 7 | Downstream Solenoid Test |
| Test No. 8 | Upstream Solenoid Test |
| Test No. 9 | Idle Speed Control System Test |
| Test No. 10 | Sole-Vac Vacuum Switching Solenoid Test |
| Test No. 11 | Sole-Vac Idle Speed Relay Test |
| Test No. 12 | Basic Engine Test |
COMPONENT-TO-TEST MENU
Test No. 1, Operational Test (1 of 5). Scheme 38
Test No. 1, Operational Test (2 of 5). Scheme 39
Test No. 1, Operational Test (3 of 5). Scheme 40
Test No. 1, Operational Test (4 of 5). Scheme 41
Test No. 1, Operational Test (5 of 5). Scheme 42
Test No. 3, Open Loop Switch Test (1 of 5). Scheme 43
Test No. 3, Open Loop Switch Test (2 of 5). Scheme 44
Test No. 3, Open Loop Switch Test (3 of 5). Scheme 45
Test No. 3, Open Loop Switch Test (4 of 5). Scheme 46
Test No. 3, Open Loop Switch Test (5 of 5). Scheme 47
Test No. 4, Closed Loop Operational Test. Scheme 48
Test No. 5, Electronic Ignition Retard Test. Scheme 49
Test No. 6, Oxygen Sensor & Closed Loop Test (1 of 2). Scheme 50
Test No. 6, Oxygen Sensor & Closed Loop Test (2 of 2). Scheme 51
Test No. 7, Downstream Solenoid Test (1 of 2). Scheme 52
Test No. 7, Downstream Solenoid Test (2 of 2). Scheme 53
Test No. 8, Upstream Solenoid Test (1 of 2). Scheme 54
Test No. 8, Upstream Solenoid Test (2 of 2). Scheme 55
Test No. 9, Idle Speed Control System Test. Scheme 56
Test No. 10, Sole-Vac Vacuum Switching Solenoid Test. Scheme 57
Test No. 11, Sole-Vac Idle Speed Relay Test. Scheme 58
Test No. 12, Basic Engine Test. Scheme 59
Removal & Installation
Remove MCU attaching bolts. Disconnect electrical plug connector. To install MCU, reverse removal procedure and ensure terminal ends are not forced out of position when connecting plug.
STEPPER MOTOR
| CAUTION | Do not drop metering pins and spring when removing stepper motor. |
Remove air cleaner and disconnect motor connector. Remove retaining screw and unit from carburetor. To install, reverse removal procedure and tighten screw to 25 INCH Lbs. (2.8 N.m).
Stepper Motor Connector Terminal Identification. Scheme 60
Disconnect electrical lead and remove sensor from manifold. Clean threads of manifold. To install, coat threads of new oxygen sensor with anti-seize compound and carefully install sensor. Tighten sensor to 31 ft. lbs. (42 N.m). Reconnect electrical lead.
Note. Do not push rubber boot down on sensor body more than 1/2" above base. Also, oxygen sensor pigtail wires cannot be spliced or soldered. If broken, replace sensor.
