Ccc Theory/operation GMC Cab & Chassis R2500

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 computerized engine control system used on 1988 General Motors trucks monitors as many as 19 engine/vehicle functions. Refer to ECM CONDITIONS SENSED & SYSTEMS CONTROLLED in this article. This system controls engine operation and lowers exhaust emissions while maintaining fuel economy and driveability. The Electronic Control Module (ECM) is the "brain"of the CCC system.

The computerized engine control 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 3-way catalytic converter can control Oxides of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

Operating Conditions Sensed

1. A/C "ON" or "OFF"
2. Engine Coolant Temp
3. Engine Crank Signal
4. Exhaust Oxygen Sensor
5. Distributor Reference
6. Crankshaft Position
7. Engine Speed
8. Manifold Absolute Pressure (MAP)
9. Park/Neutral Switch Position (P/N)
10. System Voltage
11. Throttle Position (TPS)
12. Transmission Gear Position
13. Vehicle Speed (VSS)
14. Fuel Pump Voltage
15. Power Steering Pressure
16. EGR Vacuum
17. E-Cell (2.8L)
18. Engine Knock (ESC)
19. Manifold Air Temp (MAT)

Systems Controlled

1. Air Management
2. Exhaust Gas Recirculation (EGR)
3. Electronic Spark Timing (EST)
4. Fuel Control
5. Idle Air Control (IAC)
6. Electric Fuel Pump
7. Transmission Converter Clutch (TCC)
8. Downshift Control or Manual
9. Transmission Shift Light
10. Air Conditioning
11. Diagnostics
12. "Service Engine Soon" Light
13. Diagnostic Terminal (ALDL)
14. Data Output (ALDL)
15. Electronic Spark Control (ESC)

FUEL CONTROL OPERATION

An electrically pulsed injector is located in the intake manifold throttle body unit. The 2.5L engine uses one throttle body-mounted injector, while all other engines use two throttle body-mounted injectors. 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.

Model 700 Throttle Body Injection Unit Used on 2.5L Engine. Scheme 1

Model 220 Throttle Body Injection Unit Used on V6 & V8 Eng. Scheme 2

DATA SENSORS

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, a thermistor that changes value based upon temperature, is located in a coolant passage. The ECM sends a 5-volt signal to the CTS through a resistor in the ECM. 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 (1000,000 ohms at -40°F/-40°C)). When coolant temperatures are high, CTS resistance is low (70 ohms at 266°F/130°F). Engine coolant temperature is used by the ECM to control most CCC systems. A coolant sensor problem may set Code 14 or 15.

Manifold Absolute Pressure (MAP) Sensor

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 requiring more fuel; while low voltage indicates a low pressure condition requiring less fuel. The ECM uses this information to control fuel delivery and ignition timing. A failure in the MAP sensor problem may set Code 33 or Code 34.

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 voltage signal is always low, varying from a minimum of about 0.1 volt (lean mixture-high oxygen content) to a high of about 0.9 volt (rich mixture-low oxygen content).

Based on this input, the ECM produces injector pulse width command signals that provide a leaner or richer mixture. An oxygen sensor problem may set a Code 13, Code 44, or Code 45.

Throttle Position Sensor (TPS)

The TPS is a variable resistor connected to the throttle shaft of the TBI unit. The sensor is a potentiometer that has one end connected to the 5-volt ECM signal line and other end connected to ground. As throttle valve angle changes, a return voltage is sent back to the ECM through the third wire. Output voltage to the ECM is about .5 volts when throttle valve is closed, and about 5 volts when throttle valve is at wide open throttle.

A broken TPS can cause unstable idle and intermittent pulses of fuel from the fuel injectors because the ECM thinks the throttle valve is moving. 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 may think the throttle is at wide open position and should set Code 21. Once a trouble code is set, the ECM will use an artificial value for the TPS signal, and some vehicle performance will return.

Manifold Air Temperature (MAT) Sensor

The MAT sensor, a thermistor that changes value based upon temperature, is located in the intake manifold of 2.5L engines only. The MAT sensor measures changes in intake manifold temperature. These changes are converted to electrical signals to be used by the ECM. 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.

When manifold temperatures are low, MAT resistance is high (100,000 ohms at -40°F/-40°C). When manifold temperatures are high, MAT resistance is low (70 ohms at 266°F/130°F). Manifold temperature is used by the ECM to allow EGR flow after manifold temperature reaches about 40°F (5°C), and to retard ignition timing slightly when ambient air temperatures are high. A MAT sensor problem may set a Code 23 or Code 25.

High Gear Switch

A high gear switch is installed inside automatic transmission equipped vehicles. This switch is normally open, it closes in high gear. This switch sends a signal to the ECM telling it that the transmission is in high gear. The ECM uses this information to delay disengagement of the torque converter clutch.

Knock Sensor

A knock sensor is used on 2.8L, 4.3L, 5.0L and 5.7L engines. Mounted in the engine block near the cylinders, this sensor detects abnormal engine vibration due to "detonation" and/or "pre-ignition". The knock sensor supplies detonation information to the ECM and the Electronic Spark Control (ESC) module. The ECM then alters ignition timing as needed to maintain maximum timing under most operating conditions.

Park/Neutral (P/N) Switch

The P/N switch is mounted on the steering column of automatic transmission equipped vehicles. The switch indicates when the trans- mission is in Park or Neutral. Information from the P/N switch is used by the ECM to control operation of the torque converter and idle air control.

Vehicle Speed Sensor (VSS)

The VSS is located behind the speedometer or on transmission on "C/K" or "P" Series. It sends a pulsing voltage signal to the ECM which uses it to determine vehicle speed. The ECM uses speed information to control operation of the torque converter (automatic transmission), shift indicator light (manual transmission), and cruise control.

Power Steering Pressure Switch

This switch is used on 2.5L only and is located near the power steering gear. The switch is normally open. When steering gear is at full left or full right position, the switch closes to indicate to the ECM that idle speed should be increased and that spark timing should be retarded.

A/C "On" Switch

This switch signals the ECM that the A/C system is on and that the pressure switch is closed. This signal is used by the ECM to increase idle speed, and on some systems, engages the A/C compressor clutch.

"E" Cell Timer Module

This device is used only on 2.8L engines. It is located next to the ECM. The "E" cell timer module is a cold start modifier programmer. It provides an added enrichment signal to the ECM for about 4 seconds during cold starts. After a specific number of operating hours, this device is no longer required and becomes inoperative.

Crank Signal

Although no switch or sensor is used, the ECM requires this information for fuel enrichment during starting. This information is relayed to the ECM through the ignition switch and starter solenoid.

Distributor Reference Signal

Although no switch or sensor is used, the ECM requires crankshaft position and engine speed (RPM) information for ignition timing and fuel supply. This information is relayed to the ECM through terminal "C" of the distributor 4-terminal connector (terminal "B" on "S" Series 2.5L engine).

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 on all vehicles. It is located behind right kick panel on Astro and Safari vans, beneath driver's seat on "G" series vans, behind left side of dash on "P" series vans, or behind right side of dash on "P" series vans, or behind right side of dash, near glove box, on all other models.

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 engine operating conditions and controls the systems shown.

Two different types of ECM's are used in these vehicles. All 2.5L engine equipped vehicles use a "GMP4" computer with 2 parts: controller and "MEM-CAL" unit. All other engines use a "GMCM" computer with 3 parts: controller, "PROM", AND "CALPAK".

CALPAK (Calibration Package)

The CALPAK contains back-up fuel control circuitry required to keep the vehicle operable enough to get it to a repair facility in the event the ECM becomes damaged or defective. If battery voltage is removed, CALPAK information will be retained. The CALPAK can be removed from the ECM. If the CALPAK is missing from the ECM, a no start and run condition will be exhibited. A missing CALPAK can also set a Code 52.

Controller

The controller is the microprocessor unit (ECM without CALPAK, PROM and/or MEM-CAL). This unit performs all the calculations and translations necessary for controlling the various systems.

MEM-CAL (Memory And Calibration Unit)

The MEM-CAL unit incorporates the functions of the PROM and CALPAK into one unit. It contains factory programmed information on engine calibration, vehicle weight, transmission usage, and rear axle ratio. It also contains the back-up fuel control circuitry required to keep the vehicle operable enough to get it to a repair facility in the event the ECM becomes damaged or defective. If battery voltage is removed, MEM-CAL information will be retained. The MEM-CAL unit can be removed from the ECM.

PROM (Programmable Read Only Memory)

This 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. The PROM is listed as a calibrator in the parts catalogs.

The ECM incorporates a "learning" process that can make corrections to its preprogrammed information on air/fuel ratio to improve driveability. The learning process must be renewed whenever the battery is disconnected from the vehicle. To begin the process, the vehicle must be at normal operating temperature. Then drive the vehicle under part throttle, moderate acceleration, and idle conditions until normal performance characteristics return.

CONTROLLED SYSTEMS OPERATION

The following systems are controlled by the Electronic Control Module (ECM) in response to input from the sensed systems

1. AIR Management
2. Exhaust Gas Recirculation (EGR)
3. Electronic Spark Timing (EST)
4. Fuel Control
5. Idle Air Control (IAC)
6. Electric Fuel Pump
7. Torque Converter Clutch (TCC)
8. Downshift Control
9. Manual Transmission Shift Light
10. Air Conditioning
11. Diagnostics
12. "SERVICE ENGINE SOON" Light
13. Diagnostic Terminal (ALDL)
14. Data Output (ALDL)
15. Electronic Spark Control

HEI-EST

All models are equipped with a High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains an 8-terminal (7-terminal on 2.5L engine) HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to the ECM.

On 2.5L engines the HEI-EST system consists of a Hall effect switch, distributor module, ECM, and necessary electrical connections. The function of each terminal in the 4-terminal connector is as follows

1. Terminal "A" (Electronic Spark Timing) is the circuit that triggers the module. The ECM does not know what the actual timing is, but it does know when it receives the reference signal. When the reference signal is received, the ECM advances or retards timing from the point. If base timing is set incorrectly, the engine spark curve will also be incorrect.
2. Terminal "B" (Distributor Reference High) provides RPM and crankshaft position information to the ECM.
3. Terminal "C" (By-Pass) is the circuit to which the ECM applies 5 volts to switch spark timing from the ignition module to the ECM when engine speed reaches about 400 RPM. An open or grounded bypass circuit will set a Code 42, and the engine will run at base timing plus small amount of advance built into the ignition module.
4. Terminal "D" (Reference Ground Low) is grounded in the distributor. This ensures the ground circuit has no voltage drop that could affect performance. If this circuit is open, it may cause poor vehicle performance.

On all other engines the HEI-EST system consists of a distributor module, ECM, and necessary electrical connections. The function of each terminal in the 4-terminal connector is as follows

1. Terminal "A" (Reference Ground Low) is grounded in the distributor. This ensures the ground circuit has no voltage drop that could affect performance. If this circuit is open, it may cause poor vehicle performance.
2. Terminal "B" (By-Pass) is the circuit to which the ECM applies 5 volts to switch spark timing from the ignition module to the ECM when engine speed reaches about 400 RPM. An open or grounded by-pass circuit will set a Code 42, and the engine will run at base timing plus small amount of advance built into the ignition module.
3. Terminal "C" (Distributor Reference High) provides RPM and crankshaft position information to the ECM.
4. Terminal "D" (Electronic Spark Timing) is the circuit that triggers the module. The ECM does not know what the actual timing is, but it does know when it receives the reference signal. When the reference signal is received, the ECM advances or retards timing from that point. If base timing is set incorrectly, the engine spark curve will also be incorrect.

The PROM (MEM-CAL on 2.5L engine) in the ECM has a basic spark advance curve built into it. Engine sensor values are used by the ECM to modify PROM (MEM-CAL) 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, 4.3L, 5.0L, and 5.7L engines use the Electronic Spark Control (ESC) system. This system consists of three main components: ESC module, knock sensor, and ECM. 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 result in sluggish performance and set Code 43.

SYSTEM OVER-VOLTAGE OPERATION

This system is used on 2.5L engines. If there is voltage greater than 17.1 volts for 2 seconds at ECM terminal "B1", a Code 53 will be set. Code 53 indicates a basic alternator problem.

EMISSION CONTROL OPERATION

The ECM electrically controls the following emission control systems: Air Injection Reaction (AIR) Management and Exhaust Gas Recirculation (EGR).

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, airflow from the air pump to the valve is directed to the exhaust ports. During warm engine operation (closed loop) on all except Federal 2.8L engine, the ECM de-energizes the air control valve, diverting airflow to the air cleaner. On Federal 2.8L engine, air is diverted to air cleaner only during deceleration of the engine.

Note. The AIR management system is controlled by the ECM. It is not used on 2.5L engines.

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 de-energized, blocking vacuum to the EGR valve. At normal operating temperature with engine speed above idle, the solenoid valve is energized, allowing normal EGR valve function.

Two EGR valves and systems are used depending upon vehicle weight and engine application. Vehicles under 8500 GVW with 2.5L, 4.3L, 5.0L or 5.7L engines use a negative backpressure EGR valve with ECM-controlled solenoid valve. Vehicles over 8500 GVW with 2.8L, 5.7L or 7.4L engines use a ported EGR valve with ECM-controlled solenoid valve. In addition, 2.8L engine vehicles use an EGR vacuum switch to monitor vacuum to the EGR valve.

Note. The EGR system is used on all engines and is controlled by the ECM.

A/C CLUTCH CONTROL OPERATION

The ECM controls the A/C compressor clutch to improve idle quality on A/C-equipped 2.5L and 2.8L engines. The A/C compressor will be engaged or disengaged as deemed necessary by ECM.

A/C "ON" SIGNAL OPERATION

On 4.3L, 5.0L, 5.7L and 7.4L engines, the ECM monitors the A/C control switch and pressure cycling switch. When A/C is activated, the ECM increases the idle air rate to maintain idle speed. The ECM does not control A/C compressor clutch on these vehicles.

POWER STEERING PRESSURE OPERATION

On 2.5L engines, idle speed is increased and ignition timing is retarded when engine is at idle, and the steering wheel is turned full right or full left. This prevents engine from stalling when power steering system is working at full capacity under idle conditions.

TORQUE CONVERTER CLUTCH (TCC) OPERATION

The ECM controls a solenoid valve mounted in the THM 700-R4 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.

DOWNSHIFT CONTROL OPERATION

Vehicles equipped with THM 400 transmissions are equipped with an ECM-controlled downshift system. When vehicle is at speeds below 70 MPH, a forced (detent) downshift is possible by fully depressing the accelerator. When the ECM recognizes the drop in manifold vacuum (high MAP sensor voltage reading), the downshift control relay is energized by the ECM. The relay directs battery voltage to the detent solenoid in the transmission to open an orifice to force the transmission to downshift.

SHIFT LIGHT CONTROL OPERATION

The shift light system is used on all manual transmission equipped vehicles. The ECM monitors coolant temperature, throttle position, vehicle speed, and engine speed to control the shift light.

The ECM calculates what gear position the vehicle should be in and uses this information to turn on the light. The light indicates the best shift point to the driver for maximum fuel economy.

FUEL PUMP MODULE OPERATION

All 7.4L and some 5.7L engines use a fuel module to correct vapor lock (hot restart) during high ambient temperature conditions. The module overrides the ECM 2-second pump operation and will run the fuel pump for 20 seconds when ignition is first turned on.

An inoperative fuel module may cause a hot stall/no start condition. Check for power and ground circuit to fuel module and check complete circuit to pump from terminal "A". If okay, and pump does not run for the specified 20 seconds when ignition is first turned on, replace the fuel module.

DIAGNOSTIC SYSTEM OPERATION

The ECM of computerized engine control system is equipped with a self-diagnostic system which detects system failures or abnormalities. As a bulb and system check, "SERVICE ENGINE SOON" light will glow when ignition switch is turned to "ON" position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in the computerized engine control system or "SERVICE ENGINE SOON" light circuit is faulty.

When a malfunction occurs, ECM will illuminate the "SERVICE ENGINE SOON" light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. Malfunctions are recorded as "hard failures" or as "intermittent Failures".

"Hard Failures"

Hard failures cause "SERVICE ENGINE SOON" light to glow and remain on until the malfunction is repaired. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is driveable, but loss of good driveability will most likely be Encountered.

"Intermittent Failures"

Intermittent failures cause "SERVICE ENGINE SOON" light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in appropriate TROUBLE SHOOTING or TESTING article in this section.

Note. Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds. Others may require operation for 5 minutes or longer under engine load.

See also:
• ECM CONDITIONS SENSED & SYSTEMS CONTROLLED