Computers and Control Systems: Description and Operation
Computerized Engine Control.:
Computerized Engine Control Schematic:
DESCRIPTION AND OPERATION:
POWERTRAIN CONTROL MODULE
The powertrain control module (PCM) uses signals from various sensors in order to optimize the control of the engine, transaxle, and electrical variable orifice power steering system. The subsystem also provides information to other subsystems and to service equipment.
The PCM is a microprocessor based computer that is the main component of the powertrain control system.
There are four different versions of the PCM:
1. Throttle body injection with manual transaxle.
2. Throttle body injection with automatic transaxle.
3. Multi-port fuel injection with manual transaxle.
4. Multi-port fuel injection with automatic transaxle.
Each powertrain control module (PCM) contains either one or two separate controllers located in one assembly. Manual transaxle versions contain only the engine controller (EC), the automatic transaxle versions contain an engine controller (EC) and a transaxle controller (TC). The physical size of the PCM will be the same, regardless of the number of controllers it contains.
TRANSMISSION CONTROL MODULE
The transmission control (TC) module is contained within the PCM on vehicles with automatic transmissions. For diagnosis and further information, refer to TRANSMISSION CONTROL SYSTEMS.
PCM EEPROM
The PCM EEPROM stores vehicle information such as engine and transaxle calibrations. Vehicle Identification Number, programmable vehicle option content and Malf history.
Vehicle information stored on the PCM EEPROM has a major effect on how the vehicle will operate.
Reprogramming of PCM EEPROM information is necessary when replacing a PCM, or when changing the engine and/or transaxle calibrations. It is essential that a replacement PCM be reprogrammed with the correct VIN. option content, tire size, and calibration information.
The term EEPROM Reprogramming means to electronically change the contents of the Electronically Erasable Programmable Read Only Memory (EEPROM) in the PCM.
The Saturn Service Stall System provides a means of reprogramming PCM EEPROM. Check with Saturn for this function.
CAUTION: The EEPROM is not serviceable and should not be removed for replacement. If the PCM is removed from the vehicle, reprogramming of the EEPROM will be necessary. Reprogramming of the EEPROM is only possible with the Service Stall System (SSS) hardware available at authorized dealer locations. Check with a Saturn dealer before performing PCM or EEPROM replacement.
PCM OPERATION
The PCM operates in two basic modes:
1. Normal operation
a. Computer and outputs operating properly and system voltage is between 6 and 16 volts.
b. Computer is operating properly, the outputs are non-functional and the system voltage is greater than 16 volts.
2. Backup operation
a. When system voltage is between 4.5 and 6 volts.
b. When system voltage is between 16 and 24 volts.
c. When the PCM is not operating properly.
NOTE: During this mode of operation the PCM will go into backup fuel operation, light the SES telltale. keep the fuel pump and electric engine cooling fan energized.
FUEL CONTROL
The PCM controls fuel by receiving input signals from various sensors. It then takes this information and calculates the amount of fuel required to achieve a specific air/fuel ratio. The PCM controls the fuel flow into the engine by controlling an electronic fuel injector on the throttle body injection (TBI) system. The PCM pulses the injectors at least once every crankshaft revolution.
Fuel Backup Control
If the PCM should become non-operational, the PCM will deliver a set amount of fuel, based on inputs received from the manifold absolute pressure (MAP) sensor and coolant temperature sensor (CTS).
Spark Timing Control
The PCM receives reference signals from an electronic ignition module so that it can calculate the spark output signal. The spark output signal controls the timing of the primary of the ignition coils. The PCM also provides spark retard when knock conditions are detected by a knock sensor mounted in the engine block.
Engine Cooling Fan Control
The PCM controls operation of the engine cooling fan based on coolant temperature, air conditioning state and vehicle speed. If the coolant rises above 105°C (22O°F) or if the air conditioning compressor is turned "ON". the PCM will supply a ground to the engine coolant fan relay, energizing the relay and engine cooling fan.
The PCM will shut off the cooling fan under the following conditions:
1. Vehicle exceeds 97 kph (70 mph) for three minutes.
2. When the PCM stops receiving an A/C request signal, it will run the cooling fan for three minutes, then de-energize the relay. turning "OFF" the cooling fan. The A/C request signal can be removed by the operator turning the A/C switch to "OFF", or by the A/C high or low pressure switches opening the A/C request circuit from the A/C control head to the PCM
NOTE: If the PCM is non-operational, the cooling fan will be turned "ON", as long as the engine is running.
DIAGNOSTICS
The PCM monitors the input and output signals to determine if the system is operating properly, or if a problem has occurred. Problem information will be stored for each individual failure for diagnostic and service procedures.
SERIAL DATA
Serial communications are used for communicating with diagnostic equipment like Scan tools.
DIAGNOSTIC INFORMATION
The PCM performs a continual self-diagnosis on certain control functions. The PCM's language for communicating the source of a malfunction is a system of diagnostic trouble codes. The codes are two digit numbers that can range from 11 to 99. When a malfunction is detected by the PCM, a code is set and the SERVICE ENGINE SOON telltale lamp is illuminated.
^ Code: Suspected failure detected in the engine or transaxle.
^ Flag: A diagnostic aid used when a code or intermittent problem occurs. It does not necessarily indicate a failure.
SERVICE ENGINE SOON TELLTALE LAMP
This telltale lamp is in the instrument cluster and has the following functions:
1. It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
2. It displays trouble codes stored by the PCM which help the technician diagnose system problems.
3. It indicates Open Loop or Closed Loop engine operation.
4. As a bulb and system check, the lamp will come "ON" with the key "ON" and the engine not running. When the engine is started, the lamp will turn "OFF". If the lamp remains "ON", the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases after 10 seconds, but a trouble code will remain stored in the PCM.
5. When the lamp remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability problem a Diagnostic Circuit Check must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely. Refer to TESTING AND INSPECTION PROCEDURES / DIAGNOSTIC CIRCUIT CHECK.
INTERMITTENT SERVICE ENGINE SOON TELLTALE LAMP
In the case of an intermittent problem, the SERVICE ENGINE SOON telltale lamp will light for 10 seconds and then will go out. However, the corresponding trouble code or flag will be stored in the memory of the PCM until removed.
An intermittent problem may or may not set a code or flag. If it is an intermittent failure, a Diagnostic Trouble Code Chart is not used. Consult the diagnostic chart corresponding to the intermittent trouble code or flag. A physical inspection of the applicable sub-system most often will resolve the problem.
OPEN AND CLOSED LOOP OPERATION
NOTE: 1992 vehicles will not go into closed loop until a change in throttle position is seen.
The PCM operates in two different fuel control modes:
Open Loop and Closed Loop. Whenever the vehicle is first started the PCM operates in Open Loop fuel control. When the PCM determines that the coolant temperature sensor (CTS) has reached at least 20°C (68°F), and the oxygen sensor has reached operating temperature of 318°C (600°F), it will go into Closed Loop fuel control operation. Close Loop fuel control operation will be maintained as long as certain parameters are met.
In Close Loop fuel control, the PCM varies the fuel to the engine according to signals received from the oxygen sensor, located in the exhaust manifold. The oxygen sensor varies a voltage signal to the PCM indicating the oxygen content of exhaust gases. If the oxygen sensor signals that the air/fuel mixture is lean, the PCM will increase the amount of fuel to the engine. If the oxygen sensor signals that the air/fuel mixture is rich, the PCM will decrease the amount of fuel to the engine. During Closed Loop fuel control operation, the PCM is constantly adjusting the amount of fuel to the engine, according to signals received from the oxygen sensor, to try and obtain a 14.7:1 air fuel ratio.
If there are any oxygen sensor related PCM/EC codes or if the oxygen sensor does not switch between rich and lean, or if the CTS does not obtain 20°C (68°F), the PCM will not go into Closed Loop fuel control operation.
PCM LEARNING ABILITY
The PCM has a learning ability that allows it to make corrections for minor variations in the fuel system to improve driveability and engine idle. If the battery is disconnected, or the PCM is replaced, the PCM must go through a leaning process. Until the PCM has gone through this learning process, a change may be noted in the driveability, or idle of the vehicle.
READING CODES
The provision for communicating with the PCM is the assembly line diagnostic link (ALDL) connector. It is located under the instrument panel. It is used in the assembly plant to receive information in checking that the engine is operating properly before it leaves the plant.
The code(s) stored in the PCM's memory can be read either through a hand-held diagnostic scanner plugged in the ALDL connector or by counting the number of flashes of the SERVICE ENGINE SOON telltale lamp when the diagnostic terminal of the ALDL connector is grounded.
SATURN also uses information flags. These flags do not indicate a failure and will not turn "ON" the SERVICE ENGINE SOON telltale lamp. Information flags are used as diagnostic aids to the technician when hard codes or intermittent problems occur.
Engine information flags will not be flashed on the SERVICE ENGINE SOON telltale lamp and they can only be read with a Scan tool, but automatic transaxle flags will flash on the SHIFT TO D2 telltale lamp.
DIAGNOSTIC MODES
When the diagnostic terminal is grounded with ignition "ON" and the engine "OFF", the system will enter what is called the Diagnostic Mode. In this mode the PCM will:
^ Display a Code 12 by flashing the SERVICE ENGINE SOON telltale lamp (indicating the system is operating correctly).
^ Display any stored trouble codes by flashing the SERVICE ENGINE SOON telltale lamp. Each code will be flashed three times, then code 12 will be flashed again, if no transaxle codes are present
^ Energize all PCM controlled relays and solenoids except fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
^ The IAC valve is pulsed in and out with a pause before moving each direction.
FIELD SERVICE MODE
If the diagnostic terminal is grounded with the engine running, the system will enter the Field Service Mode. In this mode, the SERVICE ENGINE SOON telltale will indicate whether the system is in Open Loop or Closed Loop.
In Open Loop the SERVICE ENGINE SOON telltale lamp flashes two and one-half times per second.
In Closed Loop, the lamp flashes once per second. Also, in Closed Loop, the telltale lamp will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.
While the system is in the Field Service Mode and the Closed Loop timer is bypassed, new trouble codes cannot be stored, but information flags can be stored in the PCM.