DESCRIPTION
Note. Most Computer Command Control (CCC) problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the CCC system as a possible cause of problems, check ignition high tension wires, fuel supply, electrical connections and vacuum hoses. Failure to do so may result in lost diagnostic time.
The Computer Command Control (CCC) system controls engine operation and lowers exhaust emissions while maintaining good fuel economy and driveability. The Electronic Control Module (ECM) is the "brain" of the CCC system. The ECM controls as many as 13 different 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 operating conditions. When the ideal air/fuel ratio is maintained, the catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC), and carbon monoxide (CO) emissions.
SYSTEM OPERATION
The CCC system consists of the following sub-systems: Fuel Control, Data Sensors, Electronic Control Module (ECM), Spark Timing, AIR Management, Emission Control, Torque Converter Clutch (TCC), Diagnostic System and Catalytic Converter.
ECM Operating Conditions Sensed & Systems Controlled. Scheme 1
Throttle Body Injection
An electrically pulsed injector is located in the intake manifold throttle body unit. The ECM controls injector "on" time (pulse width) to provide the proper amount of fuel to the engine, resulting in a 14.7:1 air/fuel ratio under most conditions.
Carbureted Models
All carbureted models are equipped with "feedback" carburetors with an electric mixture control (M/C) solenoid. The M/C solenoid operates single or dual metering rods in the float bowl. The metering rod system supplements fuel supplied by the idle and main systems in the carburetor. It varies the air/fuel ratio within a pre-calibrated range. The M/C solenoid also controls air/fuel ratio through the use of an idle air bleed that operates in conjunction with the metering rod(s).
Sectional View of Mixture Control Solenoid M4ME 4-bbl. carb. shown, E2SE 2-bbl. models similar. Scheme 2
Throttle Body Injection Unit. Scheme 3
DATA SENSORS OPERATION
Each sensor furnishes electronic impulses to the ECM. Based on these inputs, the ECM computes spark timing and air/fuel mixture for proper engine operation.
Coolant Temperature Sensor (CTS)
The CTS is located in a coolant passage. The ECM sends a 5 volt signal to the CTS. This 5 volt signal is reduced by the resistance of the CTS and a return signal is sent to the ECM. When coolant temperatures are low, CTS resistance is high (high voltage signal to ECM). When coolant temperatures are high, CTS resistance is low (low voltage signal to ECM). A coolant sensor problem may set Code 14 or 15.
Manifold Absolute Pressure (MAP) Sensor
Used on fuel injected models only, the MAP sensor measures changes in intake manifold pressure resulting from engine load and speed changes. A 5-volt reference signal is sent to the sensor from the ECM. This signal is modified by the resistance of the sensor and sent back to the ECM.
Sensor resistance changes with manifold pressure. Therefore, sensor output voltage to the ECM is a direct indication of manifold pressure. High voltage indicates a high pressure condition while low voltage indicates a low pressure condition. The ECM uses this information to control fuel delivery and ignition timing.
The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes.
A failure in the MAP sensor circuit should set a Code 33 or Code 34.
Differential Pressure (Vacuum) Sensor
Used on carbureted models only, this sensor measures the difference between atmospheric pressure (outside air) and manifold pressure (vacuum), i.e. pressure differential. The sensor converts this difference into a voltage output signal to the ECM. A problem in this circuit may set a Code 34.
Gear Select Switch
A gear select switch is used on some 4.3L, 5.0L and 5.7L models with automatic transmissions. This switch sends a signal to the ECM telling it what gear the transmission is in. The ECM uses this information to vary the conditions under which the converter clutch applies or releases. The transmission does not have to be in high gear for the ECM to engage the clutch. Transmissions using gear select switches can be identified by 3 or 4 wires coming out of the TCC connector.
On some models, a third gear switch (normally open) is placed in series on the battery side of the TCC solenoid. This switch prevents TCC application until the transmission is in third gear.
Knock Sensor
A knock sensor is used on all 4.3L engines with the Electronic Spark Control (ESC) system. Mounted in the engine block near the cylinders, this sensor detects abnormal engine vibration due to "detonation" and/or "pre-ignition". This information is sent to the ECM via the ESC module. The ECM then alters ignition timing as needed to reduce engine knock.
Oxygen (O2) Sensor
The O2 sensor is mounted in the exhaust system where it can monitor oxygen content of exhaust gases. The oxygen content reacts with the sensor to produce a voltage output signal which is sent to the ECM. This voltage signal is always low, varying from a minimum of about .1 volt (lean mixture) to a high of about .9 volt (rich mixture).
Based on this input, the ECM signals the injector (TBI) or M/C solenoid (carb.) to produce a leaner or richer mixture. An open O2 sensor circuit should set a Code 13. An extended lean or rich mixture signal should set a Code 44 or 45, respectively.
| CAUTION | Do not attempt to measure O2 sensor output voltage. Current drain of voltmeter could damage the sensor. Do not connect any wiring or test equipment to the sensor. |
Park/Neutral Switch (P/N)
The P/N switch is connected to the transmission gear selector on 2.5L TBI models, only. The switch indicates when the transmission is in Neutral or Park. Information from the P/N switch is used for TCC and IAC valve operation.
Throttle Position Sensor (TPS)
The TPS is a variable resistor connected to the throttle shaft on TBI units, or mounted in the carburetor. The ECM provides the TPS with a 5-volt reference signal which is modified according to throttle position and returned to the ECM. This return signal varies, being lowest with the throttle closed and highest during wide open throttle conditions.
On carbureted models, an open TPS circuit will cause the ECM to think the vehicle is at wide open throttle, causing the ECM command to go full rich. This should set a Code 21.
On TBI models, an open circuit will cause the ECM to think the throttle is closed, and will normally set a Code 22. If the circuit is shorted, the ECM will think the throttle is at wide open throttle and should set a Code 21.
On all models, once a trouble code is set, the ECM will use an artificial value for the TPS signal, and some vehicle performance will return.
Vehicle Speed Sensor (VSS)
The VSS is used on TBI models, only. It sends a pulsing voltage signal to the ECM which uses it to determine vehicle speed. TCC control is based largely on this information.
Component Locations for S/T Series Trucks and Astro Vans. Scheme 4
Component Locations for C/K Series Trucks and G Series Vans. Scheme 5
ELECTRONIC CONTROL MODULE (ECM) OPERATION
The ECM is located in the passenger compartment behind the driver's seat on "G" series vans and behind right side of dash (near glove box) on all other models. The ECM consists of input/output devices, a Central Processing Unit (CPU), power supply and memories.
Input/Output Devices
These devices are an integral part of the ECM. They convert electrical signals, received by the ECM from the various engine sensors, into digital signals for use by the CPU.
Central Processing Unit (CPU)
Digital signals received by the 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 the diagnostic system.
Power Supply
The main source of power for the ECM is from the battery, through the 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).
- Read Only Memory (ROM) - ROM is programmed information that can only be read by the 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. It works like a calculator. Data sensor input, diagnostic codes and results of calculations are temporarily stored in RAM. If battery voltage is removed from the ECM, all information stored in this memory is lost.
- Programmable Read Only Memory (PROM) - This memory is factory programmed information, including engine calibration data, transmission, vehicle weight and rear axle ratio application. The PROM can be removed from the ECM. If battery voltage is removed, PROM information will be retained.
HEI-EST
All models are equipped with a High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains a 7-terminal HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to the external electronic control module (ECM).
When engine speed reaches about 400 RPM, the ECM transmits a constant 5-volt signal to the distributor HEI-EST module. This activates the HEI-EST module by-pass circuit, switching spark timing control from the HEI module to the ECM.
The PROM unit in the ECM has a basic spark advance curve built into it. Engine sensor values are used by the ECM to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions.
Spark timing is calculated by the ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking).
ESC
On vehicles with the 4.3L V6, additional spark timing control is provided by the Electronic Spark Control (ESC) system. This system consists of two main components: the ESC module and the ESC knock sensor. See Data Sensors.
Under normal conditions (no spark knock), the ESC module sends a voltage signal of 8-10 volts to the ECM, and the ECM provides normal spark advance. If the knock sensor detects spark knock, it signals the ESC module which then turns off the voltage signal to the ECM. The ECM retards ignition timing as needed to reduce knock. A fault in the ESC system should set Code 43.
EMISSION CONTROL
The ECM electrically controls the following emission control systems: Air Injection Reaction (AIR) Management (except 2.5L TBI), Exhaust Gas Recirculation (EGR) on 4.3L, 5.0L and 5.7L models, and Evaporative Emission Control (EEC) on 4.3L, 5.0L and 5.7L models.
Air Management System
This system helps reduce hydrocarbon (HC) and carbon monoxide (CO) exhaust emissions. Air is injected into the exhaust ports, allowing for completion of the combustion process after exhaust gases leave the combustion chamber.
When the ECM energizes the air control valve, air flow from the air pump to the valve is directed to the exhaust ports. During warm engine operation (closed loop), the ECM de-energizes the air control valve, diverting air flow to the air cleaner or atmosphere.
A deceleration valve is used to prevent backfiring on 2.8L and 4.3L V6 engines. During high vacuum conditions (deceleration), this valve allows air flow from the air cleaner to the intake manifold, leaning out the rich air/fuel mixture created by high vacuum when the throttle valve closes.
EGR System
An ECM controlled solenoid valve is used to control EGR valve function. This valve is located in the vacuum line to the EGR valve and is operated by the ECM in response to coolant temperature, throttle position and manifold pressure.
Under conditions of low coolant temperature, engine cranking, wide open throttle, or engine idle, the solenoid valve is energized, blocking vacuum to the EGR valve. At normal operating temperatures, the solenoid valve is de-energized, allowing normal EGR valve function.
EEC System
This system controls purging of the vapor canister. The ECM controls vacuum to the purge valve with a solenoid. When the engine is in open loop, the solenoid valve is energized. This blocks vacuum to the purge valve.
The solenoid is de-energized, allowing the vacuum signal to reach the purge valve, when the engine is at normal operating temperature, above idle speed and the control system is in closed loop (system not in ALCL mode). Fuel vapors are then drawn into the intake manifold and burned.
TORQUE CONVERTER CLUTCH (TCC)
The ECM controls a solenoid valve mounted in the transmission to allow the torque converter to directly connect the engine to the transmission. This reduces slippage and improves fuel economy. The ECM uses information concerning vehicle speed (2.5L TBI), coolant temperature, throttle position and gear position (some models) to determine when to apply the TCC.
When operating conditions indicate that the transmission should function normally, or when the brake pedal is applied, the TCC solenoid is de-energized. This allows the transmission to return to normal automatic operation.
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 power to the "CHECK ENGINE" lamp to reduce amperage draw on the battery.
The ECM of the CCC system is equipped with self-diagnostic capabilities which detect system failures or abnormalities. When a malfunction occurs, the ECM will light the "CHECK ENGINE" lamp in the instrument panel. At the same time, a corresponding trouble code is stored in ECM memory. Malfunctions may be recorded as "hard failures" or "intermittent failures".
- "Hard failures" cause the "CHECK ENGINE" lamp to glow and remain on until the malfunction is repaired. If the "CHECK ENGINE" lamp comes on and remains on during vehicle operation, the cause of the malfunction must be determined.
- "Intermittent failures" cause the "CHECK ENGINE" lamp to come on, then flicker or go out after about 10 seconds when the fault goes away. However, the corresponding trouble code will be retained in the ECM memory.
- "Intermittent failures" may be sensor related. If a sensor fails, the ECM will use a substitute value in its calculations to continue engine operation. In this condition, service is not mandatory; but loss of good driveability is likely. If the related fault does not happen again within 50 engine restarts, the related trouble code will be erased from ECM memory.
As a bulb and system check, the "CHECK ENGINE" lamp will glow when the ignition switch is turned on and the engine is not running. When the engine is started, the lamp should go out. If not, a malfunction has been detected in the 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.
