Powertrain Control Software
POWERTRAIN CONTROL SOFTWARECommunications
The vehicle has three module communication networks one high speed and one medium speed CAN network, which are comprised of unshielded twisted pair cable, and the international standards organization (ISO) 9141 communications network which is a single wire network. All three networks are connected to the data link connector (DLC). For additional information, see COMMUNICATIONS NETWORK.
Flash Electrically Erasable Programmable Read Only Memory
The Flash Electrically Erasable Programmable Read Only Memory (FEEPROM) is an Integrated Circuit (1C) within the PCM. This 1C 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.
Idle Air Trim
Idle Air Trim is designed to adjust the Idle Air Control (IAC) calibration to correct for wear and aging of components. 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 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 DTC is output to indicate that the Idle Air Trim has reached its learning limits.
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.
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. The modes are shown in the table.
Fuel Trim
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 (FT). Short Term FT is referred to as LAMBSE and Long Term FT references the fuel trim table.
Short Term Fuel Trim (Short Term FT) (displayed as SHRTFT 1 and SHRTFT 2 on the WDS or equivalent tester) 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 H02S inputs and helps maintain a 147:1 air/fuel ratio during closed loop operation. This range is displayed in percentage (%). A negative percentage means that the H02S is indicating RICH and the PCM is attempting to lean the mixture Ideally, Short Term FT may remain near 0% but can adjust between -25% to +35%.
Long Term Fuel Trim (Long Term FT) (displayed as LONGFT1 and LONGFT2 on the WDS or equivalent tester) is the other parameter that indicates long-term fuel adjustments. Long Term FT is also referred to as Fuel Trim Long Term. FT is calculated by the PCM using information from the Short Term FT to maintain a 147:1 air/fuel ratio during closed loop operation. The Fuel Trim strategy is expressed in percentages. The range of authority for Long Term FT is from -35% to +35%. The ideal value is near 0% but variations of +20% are acceptable. Information gathered at different speed load points are stored in fuel trim cells in the fuel trim tables, which can be used in the fuel calculation.
Short Term FT and Long Term FT work together. If the H02S indicates the engine is running rich the PCM will correct the rich condition by moving Short Term FT in the negative range (less fuel to correct for a rich combustion). If after a certain amount of time Short Term FT is still compensating for a rich condition, the PCM "learns" this and moves Long Term FT into the negative range to compensate and allows Short Term FT to return to a value near 0%.
As the fuel control and air metering components age and vary from nominal values, the fuel trim learns corrections while in closed loop fuel control. The corrections are stored in a table that is a function of engine speed and load. The tables reside in Keep Alive Random Access Memory (RAM) and are used to correct fuel delivery during open and closed loop As changing conditions continue the individual cells are allowed to update for that speed load point. If, during the adaptive process, both Short Term FT and Long Term FT reach their high or low limit and can no longer compensate, the MIL is illuminated and a DTC is stored.
Whenever a fuel injector or fuel pressure regulator is replaced, Keep Alive RAM should be cleared. This is necessary so the PCM does not use the previously learned fuel trim values.
Failure Mode Effects Management
Failure Mode Effects Management (FMEM) is an alternate system strategy in the PCM designed to maintain engine operation if one or more sensor inputs fail.
When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value and continues to monitor the incorrect sensor input. If the suspect sensor operates within limits, the PCM returns to the normal engine operational strategy.
All FMEM sensors display a sequence error message on the WDS or equivalent tester. The message may or may not be followed by KOEO or Continuous Memory DTCs when attempting the KOER Self-Test.
Engine RPM/Vehicle Speed Limiter
The PCM will disable some or all of the fuel injectors whenever an engine rpm or vehicle overspeed condition is detected. The purpose of the engine rpm or vehicle speed limiter is to prevent damage to the powertrain. The vehicle will exhibit a rough running engine condition, and the PCM will store a Continuous Memory DTC P1270. Once the driver reduces the excessive speed, the engine will return to the normal operating mode No repair is required. However, the technician should clear the PCM and inform the customer of the reason for the DTC.
Excessive wheel slippage may be caused by sand, gravel, rain, mud, snow, ice, etc or excessive and sudden increase in rpm while in NEUTRAL or while driving.