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 CCC system is designed to maintain a 14.7:1 air/fuel ratio under all engine 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.
The CCC system consists of the following sub-systems: Fuel Control, Data Sensors, Electronic Control Module (ECM), Electronic Spark Timing, Electronic Spark Control, AIR Management, Exhaust Gas Recirculation, Evaporative Emission Control, Torque Converter Clutch (TCC), Diagnostic System.
FUEL CONTROL - THROTTLE BODY INJECTION (TBI)
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.
Throttle Body Injection Unit. Scheme 1
DATA SENSORS OPERATION
Each sensor furnishes electronic impulses to the ECM. Based on these input signals, 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 (low voltage signal to ECM). When coolant temperatures are high, CTS resistance is low (higher voltage signal to ECM). A coolant sensor problem may set Code 14 or 15.
Manifold Absolute Pressure (MAP) Sensor
Used on the 2.5L, 2.8L, and 4.3L fuel injected engines 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. A failure in the MAP sensor circuit should set Code 33 or Code 34.
Differential Pressure (Vacuum) Sensor
Used on carbureted engines only, this sensor measures the difference between atmospheric pressure (outside air) and manifold pressure (vacuum). The sensor converts this difference into a voltage input signal to ECM. It has a high voltage output (about 5 volts) at high vacuum, and a low output at low vacuum. A problem in this circuit may set Code 34.
4TH Gear Switch
A 4th gear switch is used on all 4.3L engine equipped models with automatic transmissions. This switch is normally open, it closes in 4th gear. This switch sends a signal to the ECM telling it that the transmission is in 4th (high) gear. The ECM uses this information to vary the conditions under which the torque converter clutch is applied or released.
Knock Sensor
A knock sensor is used on the 2.8L and 4.3L fuel injected engines with Electronic Spark Control (ESC). 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 manifold 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 signal is always low, varying from a minimum of about 0.1 volt (lean mixture) to a high of about 0.9 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 Code 13. A shorted sensor circuit should set Code 44. A high voltage in the circuit should set Code 45.
| 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 (P/N) Switch
The P/N switch is used in conjunction with the 2.5L, 2.8L, and 4.3L fuel injected engines only. The switch is connected to gear selector on automatic transmission equipped models. The switch indicates when the transmission is in Park or Neutral. 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 Code 21.
On TBI models, an open circuit will cause the ECM to think the throttle is closed, and will normally set Code 22. If the circuit is shorted, the ECM will think the throttle is at wide open throttle and should set 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 fuel injected 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 Series Trucks and Astro Vans. Scheme 2
ELECTRONIC CONTROL MODULE (ECM) OPERATION
The Electronic Control Module (ECM) is the control center of the CCC system. The ECM is located in the passenger compartment behind the driver's seat on "G" series vans. The ECM is located behind right side of dash, near glove box, on all other models. (Scheme 2)
The ECM senses engine operating conditions, processes input signals, and controls the various systems that affect vehicle performance. The ECM also performs the diagnostic function of the system. It can recognize operational problems, alerts the driver through the "SERVICE ENGINE SOON" light, and stores trouble codes. The ECM senses and/or controls the following
Operating Conditions Sensed
- A/C on or off
- Engine coolant temperature
- Engine crank signal
- Exhaust oxygen sensor
- Distributor Reference (Crankshaft position & engine RPM)
- Manifold Absolute Pressure (MAP)
- Park/Neutral (P/N) switch position
- System voltage
- Throttle Position Sensor (TPS)
- Transmission gear position
- Vehicle Speed Sensor (VSS)
- Fuel pump voltage
- Power steering pressure
- EGR vacuum
- Engine knock (ESC)
Systems Controlled
- AIR management
- Canister purge
- Exhaust Gas Recirculation (EGR)
- Electronic Spark Timing (EST)
- Fuel control
- Idle Air Control (IAC)
- Transmission Converter Clutch (TCC) or shift light
- Electric fuel pump
- Air conditioning
- Diagnostics ("SERVICE ENGINE SOON", Diagnostic terminal (ALCL), Data output (ALCL))
- Electronic Spark Control (ESC)
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 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 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
The 2.8L and 4.3L fuel injected engines use the Electronic Spark Control (ESC) system. This system consists of two main components: the ESC module and the knock sensor. 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, Exhaust Gas Recirculation (EGR) and Evaporative Emission Control (EEC)
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 the 4.3L Carb. engine. 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.
Note. The AIR management system is not used on all engines and may or may not be controlled by ECM.
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.
Note. The EGR system is used on all engines and may or may not be controlled by ECM.
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.
Note. The EEC system is used on all engines and may or may not be controlled by ECM.
A/C CLUTCH CONTROL
The ECM controls the A/C compressor clutch to improve idle quality on A/C equipped models. The A/C compressor will be engaged or disengaged as deemed necessary by ECM.
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, 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.
SHIFT LIGHT CONTROL
The shift light system is used on 2.5L engine, manual transmission equipped models only. The ECM controls the shift light that is used to indicate to driver the best shift point for maximum fuel economy.
DIAGNOSTIC SYSTEM
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 "SERVICE ENGINE SOON" light 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 "SERVICE ENGINE SOON" light to glow and remain on until the malfunction is repaired. If the "SERVICE ENGINE SOON" light comes on and remains on during vehicle operation, the cause of the malfunction must be determined.
- "Intermittent failures" cause the "SERVICE ENGINE SOON" light 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 "SERVICE ENGINE SOON" light 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. A "SERVICE ENGINE SOON" light driver is used on C, K, and G models. This unit is installed in light driver circuit, between "SERVICE ENGINE SOON" light and ECM.
