Powertrain Control Module
GENERAL DESCRIPTION:The powertrain control module (PCM) is located in the upper dash area directly forward of the I/P cluster. It is mounted to the center dash cross car beam and is supported on the bottom side with a PCM/ABS lower bracket.
The PCM uses signals from various sensors in order to optimize the control of the engine, transaxle, and various accessories. The PCM is a microprocessor based computer that is the main component of the powertrain control system. The PCM contains either one or two separate controllers located in one assembly. Manual transaxle versions contain only the engine controller (EC) while auto transaxle models contain both the EC and transaxle controller (TC).
There are only two versions of the PCM dependent on transaxle usage:
1. SOHC (L24) and DOHC (LL0) with MANUAL transaxle (MP2/MP3).
2. SOHC (L24) and DOHC (LL0) with AUTOMATIC transaxle (MP6/MP7).
The PCM controls the following systems:
FUEL INJECTOR 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 each injector individually. This system is called sequential fuel injection (SFI).
SPARK TIMING CONTROL
The PCM receives reference signals from the crankshaft position sensor. The PCM also uses MAP and TP to 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.
FUEL PUMP CONTROL
The PCM controls the in-tank electric fuel pump by energizing or de-energizing the fuel pump relay. When the ignition key is first turned to the Run position, the PCM will energize the fuel pump relay, by supplying power, for approximately two seconds to pressurize the fuel system. If the vehicle is started and engine references pulses are received from the crankshaft position sensor, the fuel pump relay will be left energized. If the engine is not started within approximately two seconds, the relay will be de-energized. When the ignition key is turned to the Off position the fuel pump relay will be de-energized. The fuel pump relay will also be de-energized within approximately two seconds, if crankshaft pulses are not received. This is a safety feature, in case of an accident the fuel to the engine compartment is shut-off.
PCM FLASH PROGRAMMING
The PCM has both software and calibrations which are specific to model and option content. This information is stored in the PCM in an electronically erasable programmable read only memory (EEPROM). Reprogramming of the PCM/EEPROM is necessary when replacing a PCM, or when changing the EC or TC calibrations. Both the software and calibrations will be reprogrammed at the same time anytime a download is performed.
Refer to "Tools and Equipment" for further explanation of Saturn specific tools. Tools and Equipment
The service stall system (SSS) provides a means of reprogramming PCM EEPROM through the System Utilities Main Menu function. The "Service Stall System User's Guide" and/or system help (F1 = HELP) feature provides the procedure to follow for EEPROM flash reprogramming. When possible the vehicle information contained in the original controller being replaced should be removed and installed in the new PCM. This can only be accomplished by following the PCM Replacement functions through the SSS in the System Utilities Main Menu. if a PCM needs flash reprogramming and the PCM is incapable of communications, the vehicle identification number (VIN), vehicle option content, and tire size will have to be manually installed.
In the event the vehicle is not available (i.e. retailer over the counter parts sale), a bench reprogramming tool is available to flash the replacement PCM. The tool (SA9501Z) is used in conjunction with the SSS to allow connections for flash programming. if the original PCM is available, vehicle information should be pulled from the original and installed in the new. if the old PCM is not available or is incapable of communications, the VIN, vehicle option content, and tire size must be installed manually.
Diagram:
SERIAL COMMUNICATIONS
Scan tool operation is accomplished with bi-directional communication between the tool and the PCM. OBD II technology requires a much more sophisticated PCM than did OBD I technology. The PCM diagnostic management system not only monitors systems and components that impact emissions, but also runs active tests on these systems and components. To accommodate this expansion a Class 2 serial data bus is used which meets SAE J1850 specifications for serial data.
"Serial Data" refers to digital information which is transferred over a single line, one bit at a time. A "Data Bus" is an electronic pathway through which digital data travels.
96 Saturn vehicles use a 5 volt data bus called UART, which is an acronym for universal asynchronous receive and transmit. When neither the Scan tool nor the PCM are communicating, the voltage level of the bus at rest is 5 volts. The Scan tool and PCM communicate by toggling or switching the data bus from 5 volts to ground.
Class 2 data, which is used on OBD II vehicles, communicates at a much faster rate. The rest voltage on a Class II bus is 0 volts and is switched high (7 volts) when communications are taking place.
OBD II also standardizes the data link connectors (DLC). The DLC used is a 16 pin as compared to the 12 pin connector used in previous years. All manufacturers must conform to the 16 terminal standard.
CRANKSHAFT LEARN
The PCM uses crankshaft velocity calculations to determine misfire and run misfire self-diagnostics. The PCM must know precisely the variability in crankshaft notches for this function. The PCM has a "notch learn" process that learns the variability between notches which must be reset if the crankshaft in the engine is replaced. With use of the SSS, the crankshaft learn procedure can be set to relearn.
NOTE: Do not transfer PCM's from one vehicle to another. The learned crankshaft notches will be different from one vehicle to another.
NOTE: Anytime a replacement PCM is installed the PCM must relearn the crankshaft notches.
CRANKSHAFT RELEARN PROCEDURE
When a replacement PCM is installed, or the crankshaft or engine has been replaced, follow these procedures to relearn the crankshaft notches:
1. Allow the engine to idle until the SERVICE telltale flashes.
2. Hold rpm level to 3000 rpm until lamp goes Off (approx 10-20 seconds).
NOTE: This procedure will not be initiated if any misfire is detected. If misfire codes are present refer to Misfire Diagnosis section prior to attempting to relearn crankshaft notches. Specific/Random Cylinder Misfire Detection
NOTE: If the Scan tool or service stall system are used to "reset" the crank shaft learn, the ignition will have to be cycled off for 10 seconds to correctly initiate the process.