The Electronic Control Module (ECM) monitors and controls engine performance to ensure maximum performance and adherence to emissions standards. The ECM is also able to monitor and control vehicle features such as cruise control, transmission control, starter engagement, etc. See Figure
“Electronic Control Module”
.
To understand how the ECM functions and how it can monitor input signals and exert control over the actuators, it is necessary to view the four primary functions of the ECM.
Figure 1. Electronic Control Module
Section 5.1.1
Reference Voltage
The ECM supplies a 5 volt reference signal to many of the input sensors in the control system. The ECM is able to determine pressures, speeds, positions and many other variables that are important to engine and vehicle functions by comparing the regulated 5 volts sent to the sensors with their respective returned signals.
The VREF (voltage reference) signal is supplied by the ECM by three separate and independent circuits. See Figure
“Electronic Control Module 5 Volt Reference”
.
- VBRef for the cab sensors
- VCRef for the body builder sensors
- VDRef for the engine sensors
Figure 2. Electronic Control Module 5 Volt Reference
Different VREF signal supplies make the system more robust against VREF circuit failures. A current limiting resistor protects the microprocessor of the ECM in the event of an external short to ground of the VREF signal.
Section 5.1.2
Signal Conditioner
The signal conditioner conditions the input signals for the internal microprocessor by converting analog signals to digital signals, squaring up sine wave signals or amplifying low intensity signals to a level that the ECM microprocessor can process. See Figure
“Electronic Control Module Signal Conditioning”
.
Figure 3. Electronic Control Module Signal Conditioning
Section 5.1.3
Microprocessor
The ECM contains an internal microprocessor. The processor stores operating instructions (control strategies) and tables of values (calibration parameters). It compares these stored instructions and values to conditioned input values to determine the correct operating strategy for any given engine operating condition. Calculations in the ECM occur at two different levels or speeds referred to as the foreground and the background calculations. These calculations are performed on a continuous loop basis.
Diagnostic strategies (instructions) are also programmed into the ECM. Some instructions monitor inputs on a continuous basis and command the necessary outputs to achieve the correct performance of the engine.
The microprocessor of the ECM is equipped with two types of memory. They are Random Access Memory (RAM) and Read Only Memory (ROM). They allow the processor to store the necessary instructions, calibration tables and input values to control the engine. See Figure
“Electronic Control Module Microprocessor Memory”
.
Figure 4. Electronic Control Module Microprocessor Memory
These strategies will instruct the ECM to continuously perform certain diagnostic procedures and other diagnostic tests upon the demand of the technician.
Diagnostic codes can be generated by the microprocessor in the event of any inputs or conditions not complying with pre-programmed expected values. Calculations in the ECM occur at two different levels or speeds referred to as the foreground and the background calculations. These calculations are performed on a continuous loop basis.
Foreground calculations occur at a much faster rate than the background calculations and are normally the more critical functions to engine operation. Engine speed control is an example of foreground calculations.
Background calculations are normally variables that change at a slower rate such as engine temperature.
The microprocessor of the ECM is equipped with two types of memory, Random Access (RAM) and Read Only (ROM) memory. These memories allow the processor to store the necessary instructions, calibration tables and input values necessary to control the engine.
Section 5.1.3.1
ROM
Read Only Memory is the memory where calibration tables and operating strategies are stored. Information in the ROM is permanent. It cannot be changed or lost by turning the engine off or disconnecting the batteries.
All the information regarding the engine rating are stored in this memory. Vehicle and engine personality information are also stored in this memory and can be modified with the EST to comply with the needs of each individual customer. This includes truck configuration, available features, warning and protection modes and many other parameters that will be described in more detail in this manual.
Section 5.1.3.2
RAM
Random Access Memory is a temporary storage memory for current events such as current engine temperature, engine r/min, pedal position, etc. It is the memory to which information is temporarily stored so that it can be compared to the information in the ROM. Unlike the ROM, information stored in the RAM is lost every time the key is turned off or when power is interrupted to the ECM.
| Series 40E Troubleshooting Manual – 6SE241 |
| Generated on 10-13-2008 |