Powertrain Control Software
Powertrain Control SoftwareMultiplexing
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 PCM using controller area network (CAN) which determines the priority in which the signals are sent. (See Controller Area Network for more information.) Multiplexing reduces the weight of the vehicle by reducing electrical wiring.
Controller Area Network
The HS-CAN network uses an unshielded twisted pair cable of data (+) and data (-) circuits. The data (+) and the data (-) circuits are each regulated to approximately 2.5 volts during neutral or rested network traffic. As bus messages are sent on the data (+) circuit, voltage is increased by approximately 1.0 volt. Inversely, the data (-) circuit is reduced by approximately 1.0 volt when a bus HS-CAN (+) message is sent. Multiple bus messages can be sent over the network circuits allowing multiple modules to communicate with each other. The HS-CAN is a high speed communication network used for the instrument cluster, the powertrain control module (PCM), the four-wheel drive (4WD) control module, the restraints control module (RCM), and the occupant classification sensor module communications, and designed for real time information transfer and control. The HS-CAN network will not communicate while certain faults are present, but will operate with diminished performance with other faults present. The HS-CAN bus may remain operational when 1 of the 2 termination resistors are not present.
Flash Electrically Erasable Programmable Read Only Memory
The EEPROM is an integrated circuit (IC) within the PCM. This IC contains the software code required by the PCM to control the powertrain. One feature of the EEPROM is that it can be electrically erased and then reprogrammed without removing the PCM from the vehicle.
Manual data entry must be performed if the old module is damaged and/or incapable of communicating. Remove and replace the old PCM. Using the IDS or equivalent tester, select and execute module/parameter re-programming referring to the manufacturers users manual. Important, make certain that all parameters are included. Failure to properly program tire size in revolutions per mile, (rev/mile = 63,360 divided by the tire circumference in inches) axle ratio, 4WD/2WD, and/or manual/electronic shift on the fly (MSOF/ESOF) may result in codes: P1635, and P1639.
Making Changes to the VID Block
A PCM which is programmed may require changes to be made to certain VID information to accommodate vehicle hardware. See PCM/Module Reprogramming on the IDS or equivalent tester.
Making Changes to the PCM Calibration
At certain times, the entire EEPROM will need to be completely reprogrammed. This is due to changes made to the strategy or calibration after production or the need to reset. See PCM/Module Reprogramming on the IDS or equivalent tester.
Idle Air Trim
Idle air trim is designed to adjust the 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 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, 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.
NOTE: 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 fuel trim (short term FT) (displayed as SHRTFT 1 and SHRTFT 2 on the IDS or equivalent tester) is a parameter that indicates short-term fuel adjustments. Short term FT is calculated by the PCM from HO2S inputs and helps maintain a 14.7:1 air/fuel ratio during closed loop operation. This range is displayed in percentage (%). A negative percentage means that the HO2S 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 IDS 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 14.7:1 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 HO2S 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 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 IDS or equivalent tester. The message may or may not be followed by key on, engine off (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.