Powertrain Control Software
POWERTRAIN CONTROL SOFTWAREComputer Controlled Shutdown
The powertrain control module (PCM) controls the electronic engine control (EEC) power relay when the ignition key is turned to the ON or START position, by grounding the PCM relay control (PCMRC) circuit. After the ignition key is turned to the OFF, ACC or LOCK position, the PCM will stay powered up until the proper engine shutdown occurs.
The ignition switch position run (ISP-R) and the injector power monitor (INJPWRM) circuits provide the ignition key state input to the PCM. Based on the ISP-R and INJPWRM signals the PCM determines when to power down the EEC power relay.
Engine RPM/Vehicle Speed Limiter
The powertrain control module (PCM) disables some or all of the fuel injectors whenever an engine RPM or vehicle over speed condition is detected. The purpose of the engine RPM or vehicle speed limiter is to prevent damage to the powertrain. The vehicle exhibits a rough running engine condition, and the PCM stores one of the following continuous memory diagnostic trouble codes (DTCs): P0219, P0297, or P1270. Once the driver reduces the excessive speed, the engine returns to the normal operating mode. No repair is required. However, the technician should clear the DTCs and inform the customer of the reason for the DTC.
Excessive wheel slippage may be caused by sand, gravel, rain, mud, snow, ice, or excessive and sudden increase in RPM while in NEUTRAL or while driving.
Fail-Safe Cooling Strategy
NOTE: Not all vehicles equipped with a cylinder head temperature (CHT) sensor has the fail-safe cooling strategy.
The fail-safe cooling strategy is activated by the PCM only in the event that an overheating condition has been identified. This strategy provides engine temperature control when the cylinder head temperature exceeds certain limits. The cylinder head temperature is measured by the CHT sensor.
A cooling system failure such as low coolant or coolant loss could cause an overheating condition. As a result, damage to major engine components could occur. Along with a CHT sensor, the fail-safe cooling strategy is used to prevent damage by allowing air cooling of the engine. This strategy allows the vehicle to be driven safely for a short time with some loss of performance when a overheat condition exist.
Engine temperature is controlled by varying and alternating the number of disabled fuel injectors. This allows all cylinders to cool. When the fuel injectors are disabled, their respective cylinders work as air pumps, and this air is used to cool the cylinders. The more fuel injectors that are disabled, the cooler the engine runs, but the engine has less power.
A wide open throttle (WOT) delay is incorporated if the CHT temperature is exceeded during WOT operation. At WOT, the injectors function for a limited amount of time allowing the customer to complete a passing maneuver.
Before injectors are disabled, the fail-safe cooling strategy alerts the customer to a cooling system problem by moving the instrument cluster temperature gauge to the hot zone and a PCM DTC P1285 is set. Depending on the vehicle, other indicators, such as an audible chime or warning lamp, can be used to alert the customer of fail-safe cooling. If overheating continues, the strategy begins to disable the fuel injectors, a DTC P1299 is stored in the PCM memory, and a malfunction indicator lamp (MIL) illuminates. If the overheating condition continues and a critical temperature is reached, all fuel injectors are turned off and the engine is disabled.
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 determined to be out-of-limits by the PCM, an alternative strategy is initiated. The PCM substitutes a fixed value for the incorrect input and continues to monitor the suspect sensor input. If the suspect sensor begins to operate within limits, the PCM returns to the normal engine operational strategy.
All FMEM sensors display a sequence error message on the diagnostic tool. The message may or may not be followed by key on engine off (KOEO) or continuous memory DTCs when attempting key on engine running (KOER) self-test mode.
Flash Electrically Erasable Programmable Read Only Memory (EEPROM)
The flash EEPROM is an integrated circuit (IC) within the powertrain control module (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, the module no longer needs to be replaced, but 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 500 kB/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 High (+) and CAN Low (-) lines to the data link connector (DLC). Powertrain control module (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 diagnostic tool.
Idle Air Trim
Idle Air Trim Learning Modes:
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 keep alive memory (KAM) and retains the learned values even after the engine is shut off. A diagnostic trouble code (DTC) is output if the idle air trim has reached its learning limits.
Whenever an IAC component is repaired or cleaned, 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, refer to Diagnostic Methods, Resetting The keep Alive Memory (KAM). It is important to note that erasing DTCs with a diagnostic tool does not reset the idle air trim table. Resetting The Keep Alive Memory (KAM)
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 will improve as the strategy adapts. Adaptation occurs in 4 separate modes. The modes are shown in the table.
Idle Speed Control Closed Throttle Determination - Applications Without Electronic Throttle Control
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 will learn 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, will put 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 will prevent 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.
Multiplexing Implementation
Currently Ford Motor Company uses 2 different types of communication language protocols to communicate with the powertrain control module (PCM). These protocols are controller area network (CAN) and standard corporate protocol (SCP). The Ford GT and Ranger use the SCP protocol for PCM communication with the diagnostic tool. All other vehicles use the CAN protocol for PCM communication with the diagnostic tool.
Standard Corporate Protocol (SCP)
SCP is a communication language protocol based on SAE J1850 and is used by Ford Motor Company for exchanging bi-directional message (signals) between electronic modules. Two or more signals can be sent over one SCP network circuit. Ford's SCP communications network operates at 41.6 kB/sec (kilobytes per second).
Included in these messages is diagnostic data that is output over the BUS + and BUS - lines to the data link connector (DLC). Powertrain control module (PCM) connection to the DLC is typically done with a 2-wire, twisted pair cable used for network interconnection. The diagnostic data such as self-test or PIDs can be accessed with a diagnostic tool.