Sable V6-183 3.0L DOHC VIN S MFI (1997)
Powertrain Control Module: Description and Operation
Powertrain Control Software
Multiplexing
The increased number of modules on the vehicle dictates a more efficient method of communication. Multiplexing is the process of communicating
several messages over the same signal path. This process allows multiple modules to communicate with each other through the signal path (BUS+/
BUS-). Modules communicate with the Powertrain Control Module (PCM) using Standard Corporate Protocol (SCP) which determines the priority in
which the signals are sent. Multiplexing reduces the weight of the car by reducing electrical wiring.
Standard Corporate Protocol
The Standard Corporate Protocol (SCP) is a communication language used by Ford Motor Company for exchanging bi-directional messages (signals)
between stand-alone modules and devices. Two or more signals can be sent over one circuit.
Included in these messages is diagnostic data that is output over the BUS+ and BUS- lines to the Data Link Connector (DLC). This information is
accessible with a scan tool.
Flash Electrically Erasable/Programmable Read Only Memory
The Flash Electrically Erasable Programmable Read Only Memory (FEEPROM) is an Integrated Circuit (IC) within the Powertrain Control Module (
PCM). This integrated circuit contains the software code required by the PCM to control the powertrain. One feature of the FEEPROM is that it can be
electrically erased and then reprogrammed without removing the PCM from the vehicle. If a software change is required to the PCM, the module no
longer needs to be replaced, but can be reprogrammed at the dealership through the Service Bay Diagnostic System(r)(SBDS). The reprogramming is
done through the Data Link Connector (DLC).
Idle Air Trim
PURPOSE
Idle Air Trim is designed to adjust the Idle Air Control (IAC) calibration to correct for wear and aging of components.
OPERATION
When engine conditions meet the learning requirement, the strategy monitors the engine and determines the values required for ideal idle calibration.
The Idle Air Trim values are stored in a table for reference. This table is used by the Powertrain Control Module (PCM) as a correction factor when
controlling idle speed. The table is stored in Keep Alive Random Access Memory (RAM) and retains the learned values even after the engine is shut
off. A Diagnostic Trouble Code (DTC) is output to indicate that the Idle Air Trim has reached its learning limits.
RELEARN PROCEDURE
Whenever an IAC component is replaced or cleaned or a service affecting idle is performed, it is recommended that Keep Alive RAM be cleared. This
is necessary so the idle strategy does not use the previously learned Idle Air Trim values.
To clear Keep Alive RAM, refer to Testing and Inspection / Procedures / Powertrain Control Module (PCM) Reset (Clearing DTCs). It is
important to note that erasing DTCs with a scan tool does not reset the Idle Air Trim table.
Idle Air Trim Learning Modes Chart
Once Keep Alive RAM has been reset, the engine must idle for 15 minutes (actual time varies between strategies) to learn new idle air trim values.
Idle quality will improve as the strategy adapts. Adaptation occurs in four separate modes shown in the table provided.
Fuel Trim
PURPOSE
The fuel control system uses the fuel trim table to compensate for normal variability of the fuel system components caused by wear or aging. During
closed loop vehicle operation, if the fuel system appears "biased" lean or rich, the fuel trim table will shift the fuel delivery calculations to remove the
bias. The fuel system monitor has two means of adapting - Short Term Fuel Trim (FT) and Long Term Fuel Trim. Short Term FT is referred to as
LAMBSE and Long Term FT reference the fuel trim table.
SHORT TERM FUEL TRIM
Short Term Fuel Trim (displayed as SHRTFT1 and SHRTFT2 on the New Generation STAR [NGS] tool) is a parameter that indicates short-term fuel
adjustments. Short Term FT is commonly referred to as LAMBSE. LAMBSE is calculated by the PCM from HO2S inputs and helps maintain a 14.7:1
