General System Description
Control Module System:
DESCRIPTION
The control module system has a computer, Powertrain Control Module (PCM) to control fuel delivery, timing, and some emission control systems.
The control module system, monitors a number of engine and vehicle functions and controls the following operations:
^ Fuel control.
^ Fuel injection timing.
^ Exhaust gas recirculation.
^ Transmission shift and shift quality functions. Specific transmission control diagnostics are in Transmission Control Systems/Transmission and Drivetrain.
The PCM is designed to process the various input information and then sends the necessary electrical responses to control fuel delivery, timing and other emission control systems. The input information has an interrelation to more than one output, therefore, if the one input failed it could effect more than one systems operation.
OPERATION
The PCM supplies a buffered 5 or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are so high in value that a test light will not light when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, the use of a 10 megohm input impedance digital voltmeter is necessary to assure accurate voltage readings.
The input/output devices in the PCM include analog to digital converter, signal buffers, counters, and special drivers. The PCM controls output circuits such as the inject solenoids, etc. by controlling the ground circuit through transistors or a device called a driver in the PCM.
MEMORY
There are three types of memory storage within the PCM: ROM, RAM and PROM.
Read Only Memory (ROM)
ROM is a permanent memory that is physically soldered to the circuit hoards within the PCM. The ROM contains the overall control algorithms. Once the ROM is programmed, it cannot be changed. The ROM memory is non-volatile, and does not need power to be retained.
Random Access Memory (RAM)
RAM is the microprocessor "scratch pad." The processor can write into, or read from this memory as needed. This memory is volatile and needs a constant supply of voltage to be retained. If the voltage is lost, the memory is lost.
Programmable Read Only Memory (PROM)
PROM is the portion of the PCM that contains the different engine calibration information that is specific to year, model and emissions.
The PROM also contains specific calibration information used to allow fuel delivery if other parts of the PCM are damaged. The PROM is a non-volatile memory that is read only by the PCM.
While one PCM part number can be used by many vehicle lines, a PROM is very specific and must be used for the right vehicle. For this reason, it is very important to check the latest parts book and service bulletin information for the correct part number when replacing a PROM.
A PCM used for service comes without a PROM. The PROM should be retained with the vehicle following PCM replacement. The PROM from an old PCM must be carefully removed and installed in the new PCM refer to Prom or Mem-Cal Replacement.
PCM SENSORS AND INPUT SIGNALS
In addition to the PCM, the control module system has some or all of the following information sensors:
^ Engine Coolant Temperature (ECT) sensor.
^ Accelerator Pedal Position (APP) module.
^ Optical sensor.
^ Fuel temperature sensor.
^ Crankshaft position sensor.
^ Intake Air Temperature (IAT) sensor.
^ EGR control pressure sensor.
^ Barometric Pressure (BARO) sensor.
^ Boost sensor.
^ Vehicle Speed Sensor (VSS).
And some or all of the following input signals:
^ Fuel inject signal.
^ Brake switch signals.
^ Glow plug signal.
^ Cruise control signals.
^ A/C signal.
^ Automatic transmission sensors and signals.
DIAGNOSTIC INFORMATION
The diagnostic "tree" charts and functional checks are designed to locate a faulty circuit or component through logic based on the process of elimination.
The charts are prepared with the requirements that the vehicle functioned correctly at the time of assembly and that there are no multiple failures.
The PCM performs a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The PCM's language for communicating the source of a malfunction is a system of Diagnostic Trouble Codes (DTCs). The DTCs are two digit numbers that can range from 12 to 99. When a malfunction is detected by the PCM, a DTC is set and the Malfunction Indicator Lamp (MIL) is illuminated.
MALFUNCTION INDICATOR LAMP (MIL) SERVICE ENGINE SOON
This light is on the instrument panel and has the following functions:
^ It informs the driver that a problem has occurred and that the vehicle should be taken in for service as soon as reasonably possible.
^ It displays DTCs stored by the PCM which help the technician diagnose system problems.
As a bulb and system check, the light will come "ON" with the key "ON" and the engine not running. When the engine is started, the light will turn "OFF." If the light remains "ON", the self- diagnostic system has detected a problem. If the problem goes away, the light will go out in most cases after 10 seconds, but a DTC will remain stored in the PCM.
When the light remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic (OBD) System.
Check" must be performed. This check will expose malfunctions which may not be detected if other diagnostics are performed prematurely.
SERVICE THROTTLE SOON LAMP
This light is on the instrument panel and has the following functions:
^ It informs the driver that a problem has occurred in the Accelerator Pedal Position (APP) circuit and the vehicle should be taken in for service as soon a reasonably possible.
^ If APP DTCs are stored by the PCM, the MIL will display these. The "Service Throttle Soon" Lamp will not display DTCs.
As a bulb and system check, the light will come "ON" with the key "ON" for 2 seconds. When the engine is started, the light will turn "OFF" If the light remains "ON," the self-diagnostic system has detected a problem. If the problem goes away, the light will go out in most cases after 10 seconds, but a DTC will remain stored in the PCM.
When the light remains "ON" while the engine is running, or when a malfunction is suspected, an On-Board Diagnostic (OBD) System Check must be performed. This check will expose malfunctions which may not be detected if other diagnostics are performed prematurely.