Flash Electrically Erasable Programmable Read Only Memory (EEPROM) - I

Flash Electrically Erasable Programmable Read Only Memory (EEPROM)

The flash 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. If a software change is required to the PCM, a now module is no longer necessary as the current one can be reprogrammed through the data link connector (DLC).

Fuel Trim

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 as 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.

High Speed Controller Area Network (CAN)
High speed CAN is a serial communication language protocol used to transfer messages (signals) between electronic modules or nodes. Two or more signals can be sent over one CAN communications network circuit allowing 2 or more electronic modules or nodes to communicate with each other. This communication or multiplexing network operates at 500kB/sec (kilobytes per second) and allows the electronic modules to share their information messages.

Included in these messages is diagnostic data that is outputted over the CAN + and CAN - lines to the DLC. PCM connection to the DLC is typically done with a 2-wire, twisted pair cable used for the network interconnection. The diagnostic data such as self-test or PIDs can be accessed with a scan tool.

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 the 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 the idle speed. The table is stored in the KAM and retains the learned values even after the engine is shut off. A DTC is set if the idle air trim has reached its learning limits.

Whenever an IAC component is replaced, or a repair affecting idle is carried out, it is recommended that the KAM be reset. This is necessary so the idle strategy does not use the previously learned idle air trim values.

To reset the KAM, see RESETTING THE KEEP ALIVE MEMORY (KAM). It is important to note that erasing DTCs with a scan tool does not reset the idle air trim table.

Once the KAM has been reset, the engine must idle for 15 minutes (actual time varies between strategies) to learn new idle air trim values. Idle quality improves as the strategy adapts. Adaptation occurs in 4 separate modes as shown in the following table.

Idle Air Trim Learning Modes:

Idle Speed Control Closed Throttle Determination
One of the fundamental criteria for entering RPM control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position (TP) voltage, seen since engine start. This lowest learned value is called ratch, since the software acts like a one-way ratch. The ratch value (voltage) is displayed as the TPREL PID. The ratch value is relearned after every engine start. Ratch learns the lowest, steady TP voltage seen after the engine starts. In some cases, ratch can learn higher values of TP. The time to learn the higher values is significantly longer than the time to learn the lower values. The brakes must also be applied to learn the higher values.

All PCM functions are done using this ratch voltage, including idle speed control. The PCM goes into closed throttle mode when the TP voltage is at the ratch (TPREL PID) value. An increase in TP voltage, normally less than 0.05 volts, puts the PCM in part throttle mode. Throttle mode can be viewed by looking at the TP MODE PID. With the throttle closed, the PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent the PCM from entering closed throttle mode. An incorrect part throttle indication at idle prevents entry into closed throttle RPM control, and could result in a high idle. Ratch can be corrupted by a throttle position sensor or a circuit that drops out or is noisy, or by loose/worn throttle plates that close tight during a deceleration and spring back at a normal engine vacuum.

Multiplexing
The increased number of modules on the vehicle necessitates a more efficient method of communication. Multiplexing is a method of sending 2 or more signals simultaneously over a single circuit. In an automotive application, multiplexing is used to allow 2 or more electronic modules to communicate simultaneously over a single media. Typically this media is a twisted pair of wires. The information or messages that can be communicated on these wires consists of commands, status or data. The advantage of using multiplexing is to reduce the weight of the vehicle by reducing the number of redundant components and electrical wiring.