C 1500 Truck 2WD V8-393 6.5L DSL Turbo VIN S (1996)
Engine Control Module: Description and Operation
Description
The PCM processes the various input information. The PCM sends the necessary electrical responses to the control fuel delivery, the spark timing. and
the other emission control systems. The input information has an interrelation to more than one output. One failed input can affect more than one
operation of a system.
On-Board Diagnostic System Check
The On-Board Diagnostic (OBD) System Check is an organized approach that is used in order to identify a problem which is caused by an electronic
engine control system malfunction. The OBD must be the starting point for any driveability complaint diagnosis. The OBD directs the technician to the
next logical step in diagnosing the complaint.
Use the Engine Scan Tool Data Definitions list for a comparison after fulfilling the following items:
^
The OBD system check is completed.
^
The on-board diagnostics is functioning properly.
^
No diagnostic trouble codes are displayed.
NOTE: Do not use a scan tool that displays malfunctioning data. Report the problem to the manufacturer. Using a malfunctioning scan tool can result in
misdiagnosis and unnecessary parts replacement.
Only the parameters listed in the Engine Scan Tool Definitions are used for diagnosis. When a scan tool reads other parameters, tie values are not
recommended by General Motors for use in diagnosis.
For more description on the values and the use of the scan tool to diagnose the PCM inputs, refer to the applicable diagnostic file. When all of the values
are within the range which is illustrated in the Engine Scan Tool Data Definitions, refer to Driveability Symptoms. See: Computers and Control
Systems/Testing and Inspection
The Engine Scan Tool Definitions are an average of values which are recorded during normal vehicle operation. The Definitions are intended to
represent what a normal functioning system should display.
Malfunction Indicator Lamp (MIL)
The MIL (Malfunction Indicator Lamp) is on the instrument panel. The MIL has the following functions:
^
The MIL informs the driver that a fault that affects the vehicle's emission levels has occurred. The owner should take the vehicle for service as
soon as possible.
^
As a bulb and system check, the MIL (Malfunction Indicator Lamp) comes on with the key on and the engine not running. When the engine is
started, the MIL turns off if no DTCs are set.
When the MIL remains on while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an
On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the
technician may not detect if other diagnostics are performed first.
Reprogramming (Flashing) The Control Module
Some vehicles allow the reprogramming of the control module without any removal from the vehicle. This provides a flexible and a cost-effective
method of making changes in the software and the calibrations. Refer to the latest Techline information on the reprogramming or the flashing procedures.
High Resolution Signal (256X)
The high resolution signal is an optical device which generates 64 equally spaced pulses per cylinder combustion stroke from the slotted disc mounted on
the injection pump timing cam ring. The pulses are counted by the PCM and are used in order to measure the angular pump displacement. The fuel is
metered by using the angular indication. The timing is measured by counting the angular pulses between the pump cam signal and the crankshaft position
sensor signal.
