Powertrain Control Software
CommunicationsThe vehicle has three module communication networks:
one high speed and one medium speed controller are network (CAN), 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 (IC) within the PCM. This IC 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 through the data link connector (DLC).
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.
Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7:1 for gasoline), the PCM enters closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy continuously adjusts the desired air/fuel ratio between rich and lean causing the oxygen sensor to switch around the stoichiometric point. If the time between rich and lean switches are the same, then the system is actually operating at stoichiometric. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometric is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/-25%. Some calibrations have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometric. This practice is referred to as using bias. For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce oxides of nitrogen (NOx).
Values for SHRTFT1 and 2 may change significantly on a diagnostic tool a& the engine is operated at different RPM and load points. This is because SHRTFT1 and 2 reacts to fuel delivery variability that changes as a function of engine RPM and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel control, the short term fuel trim corrections are learned by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in the keep alive memory (KAM) fuel trim tables. Fuel trim tables are based on engine speed and load and by bank for engines with 2 heated oxygen sensor (HO2S) forward of the catalyst. Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:
- short term fuel trim does not have to generate new corrections each time the engine goes into closed loop
- long term fuel trim corrections can be used both while in open loop and closed loop modes.
Long term fuel trim is represented as a percentage, similar to the short term fuel trim, however it is not a single parameter. A separate long term fuel trim value is used for each RPM/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (RPM and load), ambient air temperature, and fuel quality (% alcohol, oxygenates). When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1/2 PID(s) display the long term fuel trim correction that is currently being used at that RPM/load point.
Failure Mode Effects Management (FMEM)
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.