The Computer ECM

Whilst vehicle computers (ECMs) are not made to be repaired in garage workshops, there are certain factors that require technicians to have an appreciation of computer technology. For example, diagnostic trouble codes (DTCs) are an important part of fault finding and DTCs are stored in the computer memory. The means by which these codes are read out varies from vehicle to vehicle and it is helpful for technicians to understand why a procedure for reading DTCs on one vehicle may not work on another vehicle. It is also the case that technicians in some main dealer workshops are required to use special equipment to amend the computer operating program. Increasingly, use is being made of 'freeze frame' data. This is 'live' data that is captured whilst the system is in operation and it is useful in helping to determine the causes of a system fault. Whilst these operations are normally performed through the use of 'user friendly' diagnostic equipment, it is still the case that an understanding of what can and what cannot be done via the ECM is useful.

2.1 The fundamental parts of a computer

Figure 2.1 shows the general form of a computer that consists of the following parts:

• a central processing unit (CPU)

• a clock for timing purposes.

Data processing is one of the main functions that computers perform. Data, in computer terms, is the representation of facts or ideas in a special way that allows it to be used by the computer. In the case of digital computers this usually means binary data where numbers and letters are represented by codes made up from 0s and 1s. The input and output interfaces enable the computer to read inputs and to make the required outputs. Processing is the manipulation and movement of data


Fig. 2.1 The basic components of a computer system


Fig. 2.1 The basic components of a computer system and this is controlled by the clock. Memory is required to hold the main operating program and to hold data temporarily while it is being worked on.


Read only memory (ROM) is where the operating program for the computer is placed. It consists of an electronic circuit which gives certain outputs for predetermined input values. ROMs have large storage capacity.

Read and write, or random access memory (RAM), is where data is held temporarily while it is being worked on by the processing unit. Placing data in memory is referred to as 'writing' and the process of using this data is called 'reading'.


The clock is an electronic circuit that utilizes the piezoelectric effect of a quartz crystal to produce accurately timed electrical pulses that are used to control the actions of the computer. Clock speeds are measured in the number of electrical pulses generated in one second. One pulse per second is 1 Hertz and most computer clocks operate in millions of pulses per second. One million pulses per second is 1 megahertz (1 MHz).

2.2 A practical automotive computer system

Figure 2.2 shows a computer controlled transmission system. At the heart of the system is an electronic module. This particular module is a self-contained

Computer System Transmission
Fig. 2.2 A computer controlled transmission system

computer which is also known as a microcontroller. Microcontrollers are available in many sizes, e.g. 4, 8, 16 and 32 bit, which refers to the length of the binary code words that they work on. In this system it is an 8-bit microcontroller.

Figure 2.3 shows some of the internal details of the computer and the following description gives an insight into the way that it operates.

The microcomputer (1)

This is an 8-bit microcontroller. In computer language a bit is a 0 or a 1. The 0 normally represents zero, or low voltage, and the 1 normally represents a higher voltage, probably 1.8 V.

The microcontroller integrated circuit (chip) has a ROM capacity of 2048 bytes (there are 8 bits to one byte) and a RAM that holds 64 bytes. The microcontroller also has an on-chip capacity to convert four analogue inputs into 8-bit digital codes.

The power supply (2)

The power supply is a circuit that takes its supply from the vehicle battery then provides a regulated d.c. supply of 5 V to the microcontroller, and this is its

Fig. 2.3 Internal details of the computer

working voltage. The power supply also includes protection against over voltage and low voltage. The low voltage protection is required if battery voltage is low and it often takes the form of a capacitor.

The clock circuit (3)

In this particular application the clock operates at 4 MHz. The clock controls the actions of the computer, such as counting sensor pulses to determine speed and timing the output pulses to the electrovalves so that gear changes take place smoothly and at the required time.

The input interface (4)

The input interface contains the electronic circuits that provide the electrical power for the sensors and switches that are connected to it. Some of these inputs are in an electrical form (analogue) that cannot be read directly into the computer and these inputs must be converted into computer (digital) form at the interface.

The output (power) interface (5)

The power driver consists of power transistors that are switched electronically to operate electrovalves that operate the gear change hydraulics.

Feedback (6)

At (6) on the diagram the inscription reads ' Reading electrical state'. This means that the computer is being made aware of the positions (on or off) of the electrovalves.

The watchdog (7)

The watchdog circuit is a timer circuit that prevents the computer from going into an endless loop that can sometimes happen if false readings occur.

The diagnostic interface (8)

The diagnostic interface is a circuit that causes a warning lamp to be illuminated in case of a system malfunction. It can also be used to connect to the diagnostic kit.

2.3 Principles of operation

As with all automotive computer controlled systems, this one relies on inputs from sensors. The computer compares these input values with values that are held in the program memory (ROM) and then determines what signals are to be delivered to the actuators (electrovalves) in order to cause gear changes.

In this particular application the computer program makes use of a concept known as the vehicle operating point. The operating point is dependent on two sensor inputs, vehicle speed and load. Speed is determined by an electromagnetic sensor and the load is determined from the throttle position sensor. (Details of both types of sensor are given in Chapter 5.)

A set of vehicle operating points is stored in the ROM and these are used, whilst the vehicle is operating, as references for making gear changes. When the vehicle speed, as measured by the speed sensor, is greater than the speed held in the ROM for a given operating point the computer will call for a change to a higher gear. A lower gear (change down) will be called for when the vehicle speed falls below that held in the ROM.

Should the microcomputer detect a sensor reading that is out of limits for a sequence of readings (probably three readings) then the fault detection system comes into operation. In the case of this transmission control the types of failure that may be detected are:

• electrohydraulic valve failure

• throttle position (load) sensor failure

• speed sensor failure

• power source (battery) voltage failure.

In the event of a failure being detected, the warning lamp(s) are illuminated and the 'limp home' facility is activated and, in all probability, a code will be entered in a section of RAM for later use in diagnosing cause and effect. In this case the 'limp home' mode consists of a computer subroutine that causes the system to operate in high gear. This is intended to permit the driver to drive to a service garage where the required rectification work can be performed.

2.4 Computer data

At the base level inside the computer, the values that the computer processor works on are all expressed in digital form. That is to say that sensor readings, such as engine speed, coolant temperature etc., will be presented in the computer as binary codes made up of 0s and 1s. The 0s and 1s are electrical signals and values often used are binary 0 = 0.0-0.8 V and binary 1 = 2-5 V. The computer thus operates on electrical signals.

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