Powertrain Control Software
POWERTRAIN CONTROL SOFTWAREFail-Safe Cooling Strategy
The fail-safe cooling strategy is activated by the powertrain control module (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 cylinder head temperature (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 may 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 period of time when an overheat condition exists.
The 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.
On the hybrid vehicle, the PCM provides a fail-safe cooling status information to the instrument cluster through the controller area network (CAN). The PCM sends a CAN message signal to the cluster indicating what fail-safe cooling mode the vehicle is in. There are 3 levels of this message, which are: normal operating mode, fail-safe mode 1, and fail-safe mode 2. The cluster turns the red temperature indicator OFF if normal operating mode is received, turns the red temperature indicator ON if it receives a fail-safe mode 1 message, and it flashes the red temperature indicator if it receives a fail-safe mode 2 message. During fail-safe mode 1 the PCM sets a diagnostic trouble code (DTC) P1285 and during fail-safe mode 2 the PCM sets a DTC P1299.
NOTE: The instrument cluster red temperature indicator is also used by the motor electronic cooling loop and may be illuminated by an over temperature condition in that subsystem. The motor electronic cooling loop includes the generator, DC/DC converter, and the traction motor. The motor electronic cooling loop also contains a motor electronic coolant temperature (MECT) sensor and motor electronic coolant pump (MECP) to circulate the coolant.
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 scan tool. The message mayor 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 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 using a scan tool.
Fuel Trim
Short Term Fuel Trim
If the heated oxygen sensors (HO2S) are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7:1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air/fuel ratio rich and lean to get the oxygen sensor to switch around the stoichiometric point. If the times between switches are the same, then the system is actually operating at stoichiometry. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1) where stoichiometry 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%. Sometimes the calibration can run the system slightly lean or rich of stoichiometry. 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 NOx.
Values for SHRTFT1 may change a great deal on a scan tool when the engine is operated at different RPM and load points. This is because SHRTFT1 reacts to fuel delivery variability that can change 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, the short term fuel trim corrections can be learned by the PCM as long term fuel trim (LONGFT1) corrections. These corrections are stored in keep alive memory (KAM) in tables that are referenced by engine speed and load. 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, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that 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 or oxygenates). When viewing the LONGFT1 PID, the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1 PID displays the long term fuel trim correction that is currently being used at that RPM/load point.
High-Speed Controller Area Network (CAN)
The high-speed CAN is based on SAE J2284, ISO-11898 and 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 network operates 500 kilobytes per second (kb/sec) and allows the electronic modules to share their information messages.
Included in these messages is diagnostic data that is output over the CAN high (+) and CAN low (-) lines to the data link connector (DLC). The diagnostic data such as self-test DTCs or PIDs can be accessed with the scan tool. Information on scan tool equipment is described in Diagnostic Methods.
Multiplexing
The increased number of modules on the vehicle necessitates a more efficient method of communication. Multiplexing is a method of designating a system for 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, mode 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
The multiplexing can be implemented by using a communication language protocol such as controller area network (CAN). Vehicle network protocols such as CAN allow module-to-module communication to become possible. This communication allows several modules to share information within the vehicle network. The hybrid vehicle uses a high-speed CAN protocol for its powertrain communication. For more information about the entire communication network, refer to Information Bus (Module Communications Network), Module Communications.
Vehicle Speed Functional Overview
The hybrid vehicle uses 3 methods to calculate vehicle speed.
Vehicle Speed From The Anti-Lock Brake System (ABS) Module
The ABS module calculates wheel speed from the front 2 wheel speed sensors and sends this information to the PCM through the communication network.
Vehicle Speed From The Transaxle Control Module (TCM)
The TCM calculates traction motor speed from the traction motor shaft speed sensor and combines it with (PCM stored) tire size and axle ratio data to determine vehicle speed. This calculation is then sent to the PCM over the communication network.
Vehicle Speed From Engine And Generator Speed
The TCM calculates generator speed from the generator shaft speed sensor and sends this information to the PCM. The PCM combines input information from the generator speed and engine speed along with tire size and gear ratio to calculate a vehicle speed.
The PCM strategy then cross checks all the inputs to determine if they agree with one another. If there is a discrepancy between inputs, a vehicle speed fault flag is set in the PCM and a DTC is stored.