Electronic Powertrain Control Diagnostics Overview of the 98+ ECM, PCM, JTEC, systems - Page 2

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3.2.3     Software Architecture

The 68HC16Z2 microcontroller is the main computing unit of PCM. The 68HCllD3 and K4 microcontrollers control fuel and spark respectively. They handle the critical timing requirements of their tasks, communicating with the Z2 using high level commands.

The Z2 operating system is the heart of the software and was written expressly for this PCM. Every 500 microseconds the Z2 interrupts what it is doing to perform periodic tasks such as updating sensor inputs values and checking for the occurrence of a crank position pulse. If this pulse is observed, a program known as the decision maker is executed and performs high priority tasks such as fuel and spark calculation, and RPM processing. Low priority tasks (i.e. OBDII processing, SCI and CCD communications) are executed during the time between crank pulses.

The Z2 software is divided into a main operating strategy and three separate calibration areas. The main strategy contains information specific to the various engines and transmissions supported by this PCM. Once installed, the information contained in this area is fixed for a given engine and transmission. Changes to this data, if required, can be performed only by computer programming personnel. The calibration areas (engine, transmission and OBDII) contain information relating to emissions, fuel economy and driveability and can be altered directly by calibration personnel.

3.2.4    On Board Diagnostics

The PCM and ECM have been programmed to monitor many different circuits of the Engine control system. This monitoring is called "on-board diagnosis."

Certain criteria, or "arming conditions," must be met for a trouble code to be entered into memory. The criteria may be a range of: engine rpm, engine temperature, and/or input voltage. If a problem is sensed with a monitored circuit, and all of the criteria or arming conditions are met, a trouble code will be stored. It is possible that a trouble code for a monitored circuit may not be entered into memory even though a malfunction has occurred. This may happen because one of the trouble code criteria (arming conditions) has not been met.
The PCM or ECM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria (arming conditions) are met, a trouble code will be stored in memory.

The On Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBDII. These OBDII Diagnostics control the functions necessary to meet the requirements of California OBDII and Federal OBD regulations. These requirements specify the inclusion of a Malfunction Indicator Light (MIL) located on the instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of the MIL is to inform the vehicle operator in the event of the malfunction of any emission systems and components which can affect emissions and which provide input to, or receive output from, the engine controller module.