Powertrain Control Software
POWERTRAIN CONTROL SOFTWAREAdaptive Airflow
Some vehicles equipped with electronic throttle control (ETC) have an adaptive airflow strategy that allows the powertrain control module (PCM) to correct for changes in the airflow. During idle, the PCM monitors the throttle angle and airflow. If the airflow is determined to be less than expected, the PCM adjusts the throttle angle to compensate.
The PCM only learns the adaptive airflow when the vehicle is at idle and normal operating temperature and the airflow is less than a calibrated limit. Whenever the battery is disconnected or the keep alive memory (KAM) is reset, it is necessary for the PCM to learn the new value and not use the default value. For additional information on a KAM reset, refer to Diagnostic methods, Resetting The Keep Alive Memory (KAM). Reading and Clearing Diagnostic Trouble Codes / With Manufacturer's Scan Tool
Brake Over Accelerator
Note: On some vehicles, for off road use the brake over accelerator feature can be disabled along with the electronic stability control system by pressing and holding the traction control OFF button for 5 seconds (Mustang and Raptor).
The brake over accelerator feature may not be active during low speed operating conditions. This enables unique drive maneuvers, such as trailer tow, boat launch and retrieval, or operation in hilly environments, where the operator may require the application of both the accelerator pedal and the brake pedal during low speed maneuvering. The brake over accelerator feature will be active at speeds greater than 16 km/h (10 mph).
In the event the accelerator pedal becomes entrapped, such as by an object lodging the pedal, the brake over accelerator feature will reduce engine power when the brake pedal is applied.
Operators that rest a foot on the brake pedal when also applying the accelerator pedal may activate the brake over accelerator feature. The brake activation is detected by the PCM from the electrical brake switch. In addition to brake over accelerator comments, the customer may bring the vehicle in for repair to address concerns such as a hesitation/stumble or a lack/loss of power. In the event of a hesitation/stumble or a lack/loss of power concern, carry out normal vehicle diagnostics for the appropriate symptom code. If the brake over accelerator feature is suspect, the BRKOVR_ACTION, BRKOVRD_POSS and DIST_BRKOVRD PIDs will display a brake over accelerator event occurred.
In the event the brake over accelerator feature is suspected as the cause of the customer concern, explain to the customer the details of the override system as described above. Additionally, make sure the customer is aware that resting a foot on the brake pedal while driving may cause the activation of this feature. This also results in activation of the brake lights on the vehicle while driving. For additional information, refer to the Owners Literature.
Check Fuel Cap Indicator
The check fuel cap indicator is a communications network message sent by the PCM. The PCM sends the message to illuminate the lamp when the strategy determines there is a concern in the EVAP system due to the fuel filler cap or capless fuel tank filler pipe not being sealed correctly. This is detected by the inability to pull vacuum in the fuel tank after a fueling event.
Computer Controlled Shutdown
The PCM controls the PCM power relay when the ignition is turned to the ON or START position, by grounding the PCM relay control (PCMRC) circuit. After the ignition is turned to the OFF, ACC or LOCK position, the PCM stays powered up until the correct engine shutdown occurs.
The ignition switch position run (ISP-R) and the injector power monitor (INJPWRM) circuits provide the ignition state input to the PCM. Based on the ISP-R and INJPWRM signals the PCM determines when to power down the PCM power relay.
Deceleration Fuel Shut Off (DFSO)
During a DFSO event the PCM disables the fuel injectors. A DFSO event occurs during closed-throttle, deceleration; similar to exiting a freeway. This strategy improves fuel economy, allows for increased rear heated oxygen sensor (HO2S) concern detection, and allows for misfire profile correction learning.
Engine Fluid Temperature Management
The engine fluid temperature management can be activated when high temperature or high load conditions take place. When the engine fluid temperature management is activated, the PCM sends a controller area network (CAN) message to the instrument cluster (IC) or instrument panel cluster (IPC). The IC (IPC) then displays a power reduced to lower temp message. The engine coolant temperature gauge needle moves toward the H (hot) zone. In order to manage the engine's fluid temperatures, the PCM starts to reduce engine power and vehicle speed. The air conditioning may cycle ON and OFF to protect overheating of the engine.
Engine RPM Limiter
The PCM disables some or all of the fuel injectors whenever an engine RPM over speed condition is detected. The purpose of the engine RPM limiter is to prevent damage to the powertrain. Once the driver reduces the excessive engine speed, the engine returns to the normal operating mode. No repair is required. However, the technician should clear the diagnostic trouble codes (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 with a cylinder head temperature (CHT) sensor have the fail-safe cooling strategy.
The fail-safe cooling strategy is only activated by the PCM when 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. For additional information about the CHT sensor, refer to Engine Control Components.
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 an overheat condition exists.
Engine temperature is controlled by alternating the number of disabled fuel injectors, allowing all cylinders to cool. When the fuel injectors are disabled, the 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 cylinder head 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 IC or IPC temperature gauge to the H (hot) zone and setting DTC P1285. 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, 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 (FMEM)
The 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.
Flash Electrically Erasable Programmable Read Only Memory (EEPROM)
The flash EEPROM is an integrated circuit within the PCM. This integrated circuit 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 through the data link connector (DLC) without removing the PCM from the vehicle.
Fuel Level Input (FLI)
The FLI is a communications network message. Most vehicle applications use a potentiometer type FLI sensor connected to a float in the fuel pump (FP) assembly to determine fuel level.
Fuel Trim
Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines the engine can operate near stoichiometric air to 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 to 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 to 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 between -25% and 25%. Some calibrations have time between switches and short term fuel trim excursions that are not equal. These unequal excursions 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 nitrogen oxides (NOx).
Values for SHRTFT1 and 2 may change significantly on a scan tool as the engine is operated at different RPM and load points. This is because SHRTFT1 and 2 react 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 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 and 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 currently being used at that RPM and load point.
High Speed Controller Area Network (CAN)
The 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 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 output over the CAN (+) and CAN (-) lines to the DLC. The 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. For additional information on scan tool equipment, refer to Diagnostic Methods. Scan Tool Testing and Procedures
Idle Air Trim
Idle air trim is designed to adjust the idle air control 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 idle air control component is replaced, or a repair affecting idle is carried out, it is recommended 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) Reading and Clearing Diagnostic Trouble Codes / With Manufacturer's Scan Tool. 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.
International Standards Organization (ISO) 14229 Diagnostic Trouble Code (DTC) Descriptions
The ISO 14229 is a global, diagnostic communication standard. The ISO 14229 is a set of standard diagnostic messages that can be used to diagnose any vehicle module in use and at the assembly plant. The ISO 14229 is similar to the Society of Automotive Engineers (SAE) J2190 diagnostic communication standard that was used by all Original Equipment Manufacturers (OEMs) for previous communication protocols, like J1850 standard corporate protocol (SCP).
The ISO 14229 changes the way PIDs, DTCs, and output state control (OSC) is processed internally in the PCM and in the scan tool software. Most of the changes are to make data transfer between electronic modules more efficient, and the amount and type of information that is available for each DTC. This information may be helpful in diagnosing driveability concerns.
DTC Structure
Like all digital signals, DTCs are sent to the scan tool as a series of 1s and 0s. Each DTC is made up of 2 data bytes, each consisting of 8 bits that can be set to 1 or 0. In order to display the DTCs in the conventional format, the data is decoded by the scan tool to display each set of 4 bits as a hexadecimal number (0 to F). For example, P0420 Catalyst System Efficiency Below Threshold (Bank 1).
The table below shows how to decode the bits into hex digits.
The first 4 bits of a DTC do not convert directly into hex digits. The conversion into different types of DTCs (P, B, C and U) is defined by SAE J2012. This standard contains DTC definitions and formats.
ISO 14229 sends 2 additional bytes of information with each DTC, a failure type byte and a status byte.
All ISO 14229 DTCs are 4 bytes long instead of 3 or 2 bytes long. Additionally, the status byte for ISO 14229 DTCs is defined differently than the status byte for previous applications with 3 byte DTCs.
Failure Type Byte
The failure type byte is designed to describe the specific failure associated with the basic DTC. For example, a failure type byte of 1C means circuit voltage out of range, 73 means actuator stuck closed. When combined with a basic component DTC, it allows one basic DTC to describe many types of failures.
For example, P0110:1C-AF means intake air temperature (IAT) sensor circuit voltage out of range. The base DTC, P0110, means IAT sensor circuit, while the failure type byte 1C means circuit voltage out of range. This DTC structure was designed to allow manufacturers to more precisely identify different kinds of faults without always having to define new DTC numbers.
The PCM does not use failure type bytes and always sends a failure type byte of 00 (no sub type information). This is because OBD II regulations require manufacturers to use 2 byte DTCs for generic scan tool communications. Additionally, the OBD II regulations require the 2 byte DTCs to be very specific, so there is no additional information that the failure type byte could provide.
A list of failure type bytes is defined by SAE J2012 but is not described here because the PCM does not use the failure type byte.
Status Byte
The status byte is designed to provide additional information about the DTC, such as when the DTC failed, when the DTC was last evaluated, and if any warning indication has been requested. Each of the 8 bits in the status byte has a precise meaning that is defined in ISO 14229.
The protocol is that bit 7 is the most significant and left most bit, while bit 0 is the least significant and right most bit.
DTC Status Bit Definitions
Refer to the following status bit descriptions:
Bit 7
- 0 - The ECU is not requesting warning indicator to be active
- 1 - The ECU is requesting warning indicator to be active
Bit 6
- 0 - The DTC test completed this monitoring cycle
- 1 - The DTC test has not completed this monitoring cycle
Bit 5
- 0 - The DTC test has not failed since last code clear
- 1 - The DTC test failed at least once since last code clear
Bit 4
- 0 - The DTC test completed since the last code clear
- 1 - The DTC test has not completed since the last code clear
Bit 3
- 0 - The DTC is not confirmed at the time of the request
- 1 - The DTC is confirmed at the time of the request
Bit 2
- 0 - The DTC test completed and was not failed on the current or previous monitoring cycle
- 1 - The DTC test failed on the current or previous monitoring cycle
Bit 1
- 0 - The DTC test has not failed on the current monitoring cycle
- 1 - The DTC test failed on the current monitoring cycle
Bit 0
- 0 - The DTC is not failed at the time of request
- 1 - The DTC is failed at the time of request
For DTCs that illuminate the MIL, a confirmed DTC means the PCM has stored a DTC and has illuminated the MIL. If the fault has corrected itself, the MIL may no longer be illuminated but the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased.
For DTCs that do not illuminate the MIL, a confirmed DTC means the PCM has stored a DTC. If the fault has corrected itself, the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased.
To determine if a test has completed and passed, such as after a repair, information can be combined from 2 bits as follows:
If bit 6 is 0 (the DTC test completed this monitoring cycle), and bit 1 is 0 (the DTC test has not failed on the current monitoring cycle), then the DTC has been evaluated at least once this drive cycle and was a pass.
If bit 6 is 0 (the DTC test completed this monitoring cycle) and bit 0 is 0 (the DTC test is not failed at the time of request), then the most recent test result for that DTC was a pass.
The status byte bits can be decoded as a 2 digit hexadecimal number, and displayed as the last 2 digits of the DTC, for example for DTC P0110:1C-AF, AF represents the status byte info.
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 CAN communication language protocol to communicate with the PCM.
For additional information about the module communications network, refer to the Information Bus, Module Communications Network for Description and Operation.
Permanent Diagnostic Trouble Code (DTC)
The software stores a permanent DTC in non-volatile random access memory (NVRAM) whenever a DTC is set and the MIL has been illuminated. Permanent DTCs can only be cleared by the module strategy itself. After a permanent DTC is stored, 3 consecutive test passed monitoring cycles must complete before the permanent DTC can be erased. At that time, both the permanent DTC is erased and the MIL is extinguished. The PCM clears permanent DTCs after one monitoring cycle if a request to clear DTCs is sent by the scan tool, and the test subsequently runs and passes (test must continue to pass for the entire driving cycle for continuous monitors) and a Permanent DTC Driving Cycle has been completed. A Permanent DTC Driving Cycle requires a total of 10 minutes of engine run time, consisting of 5 minutes of vehicle operation above 40 km/h (25 mph) and 30 continuous seconds of vehicle operation at idle. After clearing DTCs, running the OBD Drive Cycle ensures that all monitors complete, the Permanent DTC Driving Cycle completes, inspection/maintenance (I/M) readiness codes are set to a ready status and any permanent DTCs are erased. A permanent DTC cannot be erased by clearing the KAM. The intended use of the permanent DTC is to prevent vehicles from passing an in-use inspection simply by disconnecting the battery or clearing the DTCs with a scan tool prior to the inspection. The presence of permanent DTCs at an inspection without the MIL illuminated is an indication that a correct repair was not verified by the on board monitoring system.
Vehicle Speed Limiter
The PCM disables some or all of the fuel injectors whenever a vehicle over speed condition is detected. The purpose of the vehicle speed limiter is to prevent damage to the powertrain. Once the driver reduces the excessive vehicle 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.
Either excessive wheel slippage caused by sand, gravel, rain, mud, snow, ice or excessive RPM increase in neutral may cause the vehicle speed limiter to activate even though the vehicle has not exceeded the maximum speed limit.
Powertrain Control Module - Vehicle Speed Output (PCM-VSO)
The PCM-VSO speed signal subsystem generates vehicle speed information for distribution to modules and subsystems that require vehicle speed data. This subsystem senses the transmission output shaft speed with a sensor. The data is processed by the PCM and distributed as a message on the vehicle communication network.
The key features of the PCM-VSO system are to:
- infer vehicle movement from a speed sensor.
- convert transmission output shaft rotational information to vehicle speed information.
- compensate for tire size and axle ratio with a programmed calibration variable.
- distribute vehicle speed information as a multiplexed message.
The signal from a non-contact shaft sensor, such as an OSS or VSS, mounted on the transmission is sensed directly by the PCM. The PCM converts the OSS or VSS information to 8,000 pulses per mile, based on a tire and axle ratio conversion factor. This conversion factor is programmed into the PCM at the time the vehicle is assembled and can be reprogrammed in the field for changes in the tire size and axle ratio. The PCM transmits the computed vehicle speed and distance traveled information to all vehicle speed signal users on the vehicle.