DESCRIPTION
Note. Most CCC problems are result of mechanical breakdowns, poor electrical connections, or damaged vacuum hoses. Ignition high tension wires, fuel supply, electrical connections and vacuum hoses should be checked before CCC. Failure to do so will result in wasted time.
Note. This system is also covered in the FUEL SYSTEMS section. Additional information on only the fuel portion of this system can be found in the ELECTRONIC FUEL INJECTION articles in the FUEL SYSTEMS section.
Note. This article refers to models by body style. "T" bodies - Chevette and T1000; "X" bodies - Citation, Omega, Phoenix and Skylark; "A" bodies - Century, Celebrity, Ciera and 6000; "F" bodies - Camaro and Firebird; "J" bodies - Cavalier, Cimmaron, Firenza, J2000 and Skyhawk.
Schematic of Computer Command Control System. Scheme 1
The Computer Command Control (CCC) system is a system used on all General Motors gasoline engines (except Cadillac with DFI). It monitors up to 15 engine/vehicle functions. It controls engine operation and lower exhaust emissions while maintaining good fuel economy and driveability. The Electronic Control Module (ECM) is the "brain" of the CCC system. The ECM can control as many as 9 engine-related systems to constantly adjust engine operation.
The CCC system is primarily an emission control system, designed to maintain a 14.7:1 air/fuel ratio under all conditions. When an ideal ratio is maintained, the catalytic converter can control nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).
Three basic versions of the CCC system are used. The Minimum Function system controls the fuel and air management systems only. The Full Function system adds control of ignition system, torque converter clutch and emission controls. The EFI system is similar to the Full Function system except that it controls fuel injection instead of a carburetor.
SYSTEM OVERVIEW
The CCC system consists of the following sub-systems: Fuel Control, Data Sensors, Electronic Control Module (ECM), Spark Timing, Idle Speed Control (ISC) system (carbureted models), Idle Air Control (IAC) system (fuel injected models), Air Management, Emission Control, Torque Converter Clutch (TCC), Diagnostic System and Catalytic Converter.
Note. Not all sub-systems are used on all models.
FUEL CONTROL (CARBURETTED MODELS)
All models (except Skyhawk, Cimmaron, Cavalier & Firenza) are equipped with "feedback" carburetors which contain an electrically operated Mixture Control (M/C) solenoid. The M/C solenoid operates single or dual metering rods in float bowl. The rods meter fuel supplied too idle and main systems and vary air/fuel ratio within a pre-calibrated range. An idle air bleed circuit also operates in conjunction with metering rods.
Mixture Control Solenoid (Carb. Models Exc. 1000 & Chevette. Scheme 2
The ECM, responds to inputs from data sensors and constantly adjusts air/fuel mixture. The ECM controls M/C solenoid by providing a ground for solenoid. When solenoid is energized, fuel flow through carburetor is reduced, providing a leaner mixture. When solenoid is de-energized, fuel flow through carburetor is increased, providing a richer mixture. The solenoid is cycled (turned on and off) at a rate of 10 times per second.
When ECM responds to signals received from oxygen sensor, the CCC system is in closed loop operation. Under certain operating conditions, ECM may ignore sensor and use pre-programmed instructions. Operating conditions which cause ECM to ignore oxygen sensor signals result in open loop mode. For example, during cold engine starts (engine speed below 200 RPM), M/C solenoid is turned off by ECM to provide a rich mixture.
FUEL CONTROL (FUEL INJECTED MODELS)
All electronic fuel injected models are equipped with an electrically-pulsed injector located in a throttle body unit. The ECM controls the injector "on" time (pulse width) to provide the proper amount of fuel.
ELECTRONIC CONTROL MODULE (ECM) OPERATION
The ECM is located in passenger compartment and controls all CCC system functions. The ECM consists of input/output devices, Central Processing Unit (CPU), power supply and memories.
DATA SENSORS
Each sensor furnishes electronic impulses to ECM. The ECM computes spark timing and fuel mixture ratio necessary for proper engine operation. Minimum Function systems use only coolant temperature, oxygen and throttle position sensors. Full Function and EFI systems can use any or all of the sensors. Operation of each sensor is as follows
Oxygen Sensor
mounted in engine exhaust, it supplies a low voltage (under 1/2 volt) when fuel mixture is lean (too much oxygen) and a higher voltage (up to 1 volt) when fuel mixture is rich (not enough oxygen). Oxygen sensor must be hot (over 600°F/316°C) to function properly. On some models, oxygen sensor may cool off during idle, causing CCC system to go into open loop mode. Running the engine at fast idle will warm up oxygen sensor.
Note. No attempt should be made to measure oxygen sensor voltage output. Current drain of voltmeter could damage sensor. Do not connect any wiring or test equipment to sensor.
Coolant Temperature Sensor (CTS)
The CTS is located in the engine coolant stream to supply coolant temperature information to ECM. This information affects the following engine systems: Air/fuel ratio control (as engine coolant temperature varies with time during a cold start), switching functions for emission controls and torque converter clutch, spark timing, and engine temperature lamp operation.
Manifold Absolute Pressure Sensor (MAP)
This sensor is mounted in engine compartment or under the instrument panel. It measures changes in manifold pressure (vacuum). The MAP sensor sends electrical signals to ECM, reflecting need for adjustment in air/fuel mixture and spark timing under various operating conditions.
Note. The MAP sensor is not used on all engine applications. Some engines may use a special MAP sensor that also detects altitude changes.
Vacuum Sensor
Vehicles not equipped with MAP sensors may be equipped with vacuum sensors which are located on right fender panel. The vacuum sensor performs the same function as the MAP Sensor. The vacuum sensor may be called a pressure differential sensor because it works on the difference between atmospheric and manifold pressure.
Barometric Pressure Sensor (BARO)
This sensor is mounted on MAP sensor bracket. This sensor measures ambient or barometric pressures and signals ECM of pressure changes due to altitude and/or weather. This sensor is used only on engines equipped with MAP sensor.
Throttle Position Sensor (TPS)
This sensor is mounted on the carburetor or throttle body unit. The sensor, a variable resistor similar to fuel tank sending unit, signals ECM of changes in throttle blade position from closed to wide open throttle.
Note. Some engines may use a vacuum sensor or WOT switch for fuel enrichment.
Vehicle Speed Sensor (VSS)
This sensor is mounted behind the speedometer in instrument cluster. It provides a series of 8 volt pulses to ECM to indicate vehicle speed. The VSS is not used on all vehicles.
High Gear Switch
The high gear switch is mounted on automatic transmissions. This switch opens when transmission has shifted into high gear (3rd or 4th) and closes under all other conditions. High gear switch information is used for emission control.
Park/Neutral Switch (P/N Switch)
This switch is connected to transmission gear selector. It is closed when selector is in "P" or "N" positions and open when selector is in gear. This switch is used for ISC and TCC operations.
Air Conditioner "On" Switch (A/C "On")
This switch is mounted in air conditioner compressor of some vehicles to signal ECM that air conditioner is operating. This switch supplies 12 volts if compressor is engaged and 0 volts if disengaged.
The ECM is located in passenger compartment and controls all CCC system functions. The ECM consists of input/output devices, Central Processing Unit (CPU), power supply and memories. A brief description and operation of each component is as follows
INPUT/OUTPUT DEVICES
These integral devices of ECM convert electrical signals received by data sensors and switches to digital signals for use by the CPU.
Central Processing Unit (CPU)
Digital signals received by CPU are used to perform all mathematical computations and logic functions necessary to deliver proper air/fuel mixture. The CPU also calculates spark timing and idle speed information. The CPU commands operation of emission control, closed loop fuel control and diagnostic system.
Power Supply
Main source of power for the ECM is from the battery, through the No. 1 ignition circuit.
Memories
The 3 types of memories in the ECM are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM). Function of each memory is as follows
- Read Only Memory (ROM) - The ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
- Random Access Memory (RAM) - This memory is the decision making center for the CPU. Information can be read into or out of RAM memory; similar to a calculator. Data sensor information, diagnostic codes and results of calculations are temporarily stored in RAM memory. If battery voltage is removed (ignition turned off on "T" body cars), all information stored in this memory is lost.
- Programmable Read Only Memory (PROM) - This memory is factory-programmed information containing engine calibration data for each engine, transmission, body and rear axle ratio application. The PROM is easily removed from ECM. If battery voltage is removed, PROM information will be retained.
SPARK TIMING
Spark timing control is possible only on Full Function and EFI systems. Spark timing is controlled by 1 of 2 different systems: Electronic Spark Timing (EST) and Electronic Spark Timing with Electronic Spark Control (EST/ESC). Operation and application of each system is as follows
Electronic Spark Timing (EST)
The EST system consists of ECM and modified HEI distributor with 7 terminal HEI module. The EST distributor contains no vacuum or centrifugal advance. The HEI distributor communicates to ECM through a 4-terminal connector which contains 4 circuits: Distributor reference circuit, by-pass circuit, EST circuit and ground circuit.
Whenever the pick-up coil signals HEI module to open the primary circuit, it also sends spark timing signals to ECM through the reference line. When voltage on HEI by-pass line is 0 volts (engine cranking), HEI module switches to by-pass circuit.
In by-pass circuit, HEI module provides spark advance at base timing and disregards spark advance signal from ECM. When voltage on HEI by-pass circuit is 5 volts (engine running), HEI module accepts spark timing signal provided by ECM.
The ECM monitors engine speed through HEI reference line and monitors engine operating conditions through data sensors and switches. From these parameters, ECM calculates proper spark advance and supplies signal to HEI distributor through EST line.
Electronic Spark Timing W/ Electronic Spark Control (EST/ESC)
This is a closed loop system that controls engine detonation by adjusting spark timing. This system consists of EST system, detonation sensor, modified electronic control module in HEI distributor and a controller.
The detonation sensor is mounted in engine block (behind intake manifold) and it detects presence (or absence) and intensity of detonation by vibration characteristics of engine. Sensor sends electrical signal to controller. If sensor fails, no retard will occur. The controller constantly processes sensor signal into command signal to distributor to adjust spark timing. Controller failure would cause either no ignition, no retard or full retard.
The distributor receives commands from controller, and spark firing is delayed while detonation is occurring, thus providing required retard. The amount of retard is a function of degree of detonation.
IDLE SPEED CONTROL (ISC) (CARBURETTED MODELS)
The ISC is an electrically driven actuator which opens or closes throttle (in idle position), according to commands from ECM. The ISC maintains low idle speeds while preventing stalls due to engine load changes.
The ECM monitors engine load to determine proper idle speed. To prevent stalling, the ECM monitors A/C "ON" switch, Park-Neutral switch and ISC throttle switch. With this information, the ECM will control idle speed by operating the ISC motor.
When engine is cold, ECM holds throttle valve open for longer period of time to provide faster warm-up. This function is by-passed when throttle is opened enough to bring TPS off its idle circuit. The ISC is located on side of carburetor.
Note. Not all engines are equipped with ISC system. Some may use an Idle Speed Solenoid (ISS) or an Idle Load Compensator (ILC) to control engine idle speed without ECM commands.
IDLE AIR CONTROL (IAC) (FUEL INJECTED MODELS)
The IAC valve is an electrically-driven actuator which changes the idle air flow around the throttle plate according to commands from the ECM. The IAC controls air flow in a similar manner as the ISC to maintain low idle speeds while preventing stalls due to engine load changes.
EMISSION CONTROL
The ECM electrically controls the following emission systems: AIR Management (AIR), Exhaust Gas Recirculation (EGR) and Evaporative Emission Control System (EECS). A brief description of each system follows
Air Management System
This system helps to reduce HC and CO content in exhaust gases and to quickly heat up catalytic converter and oxygen sensor during cold engine operation. This is accomplished by injecting air into exhaust port of each cylinder.
The ECM energizes an air control solenoid which allows air to flow to air switching valve, directing air to exhaust ports. During warm engine (closed loop) operation, the ECM de-energizes air switching valve, directing air to dual-bed converter, which lowers HC and CO emissions.
If air control valve detects rapid increase in manifold vacuum (deceleration condition), or ECM detects any failure in CCC system, air is diverted to air cleaner or dumped to atmosphere.
Note. Vehicles may use separate air switching and air control valves, integral control valves or an air diverter valve.
Early Fuel Evaporation (EFE)
The ECM controls EFE system by either of the following methods: Vacuum operated valve and actuator, or ceramic heater grid located underneath carburetor primary bore. The vacuum operated valve and actuator is opened by a control solenoid mounted on valve cover. This solenoid controls vacuum to EFE valve by an electrical signal from the ECM.
The ceramic heater grid system is part of carburetor insulator. When ignition is turned on and engine coolant temperature is low, voltage is applied to EFE relay through ECM, energizing EFE heater. When coolant temperature increases, ECM de-energizes EFE relay, which shuts off voltage to EFE heater.
Note. EFE may not be used on all vehicles. Some vehicles may incorporate EFE control through EGR or AIR system.
Exhaust Gas Recirculation (EGR)
The ECM controls ported vacuum to EGR valve with an electrically operated solenoid valve. When engine is cold, solenoid is energized, blocking vacuum to EGR valve. When engine is warm, solenoid is de-energized and EGR operation is allowed.
Note. Vehicles may use an integral EFE/EGR valve, TCC/EGR valve or an EFE/EGR/Canister Purge Valve.
Evaporative Emission Control System (EECS)
This system controls vapor canister purging. The ECM controls vacuum to purge valve in charcoal canister through a solenoid valve. When engine is operating in open loop mode, solenoid valve is energized and blocks vacuum to purge valve. When engine is operating in closed loop mode above prescribed RPM, the solenoid valve is de-energized and vacuum is applied to purge valve. This draws stored vapors into manifold.
Note. Some vehicles control canister purge operation through an integral EFE/EGR/Canister Purge Valve.
Catalytic Converter
Proper emission control is accomplished with a special 3-way catalytic converter which converts all 3 major pollutants. The converter is a dual-bed type. The "upstream" section of the converter contains a reducing/oxidizing bed to reduce NOx while at the same time oxidizing HC and CO.
An air supply pipe from the AIR system introduces air between the dual beds (during closed loop mode), so the second bed can oxidize any remaining HC and CO with a high conversion efficiency to minimize overall emissions.
TORQUE CONVERTER CLUTCH (TCC)
The ECM controls a solenoid (mounted on automatic transmission) to allow torque converter to directly connect engine to transmission, providing direct drive. When vehicle speed is high enough, ECM energizes TCC solenoid and engine is mechanically coupled to transmission.
When operating conditions indicate that transmission should operate as normal (during rapid acceleration or deceleration), solenoid is de-energized. The transmission also returns to normal automatic operation when brake pedal is depressed.
DIAGNOSTIC SYSTEM OPERATION
Note. A "CHECK ENGINE" lamp driver is installed in the wiring harness from ECM to the "CHECK ENGINE" lamp. This driver amplifies the power to the "CHECK ENGINE" lamp to reduce amperage draw on the battery.
The ECM of the CCC system is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, ECM will light the amber "CHECK ENGINE" lamp located on instrument panel. When malfunction is detected and lamp is turned on, a corresponding trouble code is stored in ECM memory. Malfunctions are recorded as "hard failures" or "intermittent failures".
- "Hard failures" cause "CHECK ENGINE" lamp to glow and remain on until malfunction is repaired. If the "CHECK ENGINE" lamp comes on and remains on during vehicle operation, cause of malfunction MUST be determined.
- "Intermittent failures" cause lamp to flicker or go out after 10 seconds when fault goes away. However, the associated trouble code will be retained in ECM memory. "Intermittent failures" may be sensor related. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, service is not mandatory; but loss of driveability may be encountered. If the related fault does not recur within 50 ignition cycles, related trouble code will be erased from ECM memory.
As a bulb and system check, the "CHECK ENGINE" lamp will glow when ignition switch is turned on and engine is not running. When engine is started, the lamp should go out after 1-4 seconds. If not, a malfunction has been detected in CCC system.
Note. Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others require operation for 5 minutes or longer.
DIAGNOSIS & TESTING
For Diagnosis of the CCC system, refer to the appropriate CODE TESTS or COMPONENT TESTS article(s) in this section.
For basic trouble shooting of the CCC system, refer to the TROUBLE SHOOTING information in the CCC TESTS W/O CODES article in this section.