This section is very complex, so to make it easier to digest we have split it into 10 Parts.
Part1
Back in 1981, the majority of cars and trucks started to be equipped with engine management systems—”computer-controlled” engines. Since that time, there has been a great deal of mis-understanding of these systems, so over the next few months we will be dealing with all areas of this subject, especially trying demystify it!
Firstly, how do you know if your vehicle has an on-board management system? The easiest way is usually to look for a “Check Engine”, “Service Engine Soon” or other idiot light on the dashboard with a picture of an engine on it. When you first turn on the ignition, this light should illuminate (so that you can see the bulb has not blown) and should then go out once the engine has started. If, however, it does not extinguish or illuminates whilst driving, it is an indication that the computer has detected that there may be a fault with part of the management system. If an appearance of the light is intermittent, it is probably a transient problem that may not require any attention. If however it is seen on a regular basis, you should look into the causes.
This self-diagnostic system can be a very powerful aid in determining what is malfunctioning. However, there is a commonly held belief, that modern vehicles will tell a mechanic exactly what is wrong with them and virtually fix themselves—unfortunately, this is not the case. The latest OBD II systems (as introduced in the mid-90's), certainly go a lot further to help pinpoint problems, but even these are not infallible as we shall see later on.
Part2
The three major building blocks of any management systems are:-
The sensors. These are used to gather vital information about what is happening to an engine. The amount and types of sensors can vary from engine to engine; however, most Fuel Injected engines monitor a core set of functions. Typically, these might be Coolant Temperature, Camshaft and/or Crankshaft Position, Engine Knock, Exhaust Gas Oxygen, Manifold Absolute Pressure, Mass Air Flow, Throttle Position, Transmission gear, Vehicle Speed.
The signals that are generated by these sensors are then passed to the PCM ( P owertrain C ontrol M odule). This “computer” processes this information and then outputs signals to the control devices such as...
Actuators, motors and solenoids. These components are used to control various engine functions and might include: Air Management, Cooling Fan, Exhaust Gas Recirculation, Fuel Injectors, Idle Air Control, Spark Advance, and Torque Converter Clutch.
In this way, a modern engine can be made to run smoothly (for customer satisfaction), cleanly (in order to meet nationally mandated emission control requirements) and efficiently which is an overall benefit.
Part3
All input sensors monitor a change in temperature, pressure, chemical makeup or the change in position of a mechanical component. These changes are in some way converted to a variable electrical signal that is used to provide the PCM with information. Obviously, if a sensor sends the wrong information, the effect on engine operation could be detrimental.
Temperature sensors are usually simple thermistors. This means their resistance will change in accordance with a change in temperature. Typically they are used to monitor the coolant temperature, but can also be used to check the temperature of the air entering the engine.
Pressure sensors are commonly used to check intake manifold vacuum (negative pressure) and/or barometric pressure. The latter can vary according to weather or altitude (the latter is not too important to us in the UK , but the former often is!) Manifold pressure is determined by throttle opening, engine speed and load. These inputs are particularly important in determining correct ignition timing and air/fuel ratio.
Most pressure sensors incorporate a piezoelectric diaphragm that acts as a resistor and separates two chambers within the sensor. The diaphragm flexes in accordance with the varying pressure (or vacuum) applied to it which in turn modulates (varies) a reference voltage (usually 5 volts) supplied by the computer.
By checking the variation in this voltage, the PCM can send the appropriate signals to its output devices.
Part 4
The oxygen sensor is used to monitor the exhaust gasses. The comparison between the amount of oxygen in the exhaust and that in the surrounding atmosphere provides an indication of the air/fuel ratio. Once heated to around 600º F, the sensor outputs a voltage (around 0.1v to 0.9v) in response to the oxygen content. Below this temperature the computer does not use this information and the system is said to operate in “Open Loop” mode. During this time the computer uses a fixed strategy to control the air/fuel ratio.
Once normal operating temperature has been reached, the oxygen sensor signal provides feedback to the computer to adjust the ratio and the system is operating in “Closed Loop” mode. Some newer vehicles use a heated sensor, so that closed loop operation is reached more rapidly. This provides improvements in both economy and emissions.
Later engines built to OBD II emissions standards, use sensors both before & after the catalytic converter. Comparison between the upstream & downstream signals indicates converter efficiency and need for replacement.
A measure of airflow entering the engine is also an important parameter for the air/fuel control system. The so-called “speed-density” system monitors the air temperature and the MAP sensor to calculate the mass or density of the incoming air. Alternatively, the Mass Air Flow (MAF) sensor provides a direct measurement. This is usually achieved by monitoring the amount of power required to keep a wire heated to a controlled temperature. Since to provide an accurate measurement this wire must be kept extremely clean, a “burn-off” module is activated after closed loop operation to heat the wire and remove any dirt or residue that may have accumulated on it during operation.
Part 5
The throttle position sensor or throttle potentiometer monitors the throttle opening to see if the engine is accelerating, decelerating or cruising at a fixed speed. Based on this information, the computer can accurately adjust the air/fuel mixture and ignition timing. The information from a TPS is based on the difference between a reference voltage and the output or return voltage. As the throttle moves, resistance changes causing a change in the voltage drop across the sensor. Typically, with a reference voltage of 5 volts, at closed throttle a voltage of 0.5 volts would be output and at wide open throttle 4.5volts.
Determination of the vehicle speed is also an important parameter used by the computer. Most vehicle speed sensors count the revolutions of a speedometer cable or are gear driven from the output shaft of the transmission. The signal takes the form of a pulse and by counting the pulses over a given time the vehicle speed can be computed. A typical VSS might use a magnetic pulse generator, a Hall –Effect switch or an LED/photo-transistor combination. Some manufacturers use a VSS with a resolution of more than 10,000 pulses per mile and so any change in speed is almost immediately noticed.
A triggering device is also required to activate the ignition system. Initially this was contained within the distributor and was either a magnetic pulse generator or Hall-effect switch. Later engines that are distributor less usually use a crankshaft position sensor, a camshaft position sensor or both in combination. These may take a signal from an external component such as the crankshaft damper or directly from the component within the engine.
Finally we shall cover the detonation or knock sensor. This device is a piezoelectric sensor that produces an electrical signal in response to pressure or vibration. It “listens” and generates a signal whenever a vibration is present. However, only when this frequency is typical to detonation, the computer will respond and retard the ignition accordingly. This allows the engine to run at the maximum possible advance without causing abnormal combustion.
Part 6
Many of these output controls make adjustments to lower the exhaust emissions of the engine. This is often achieved by controlling a so-called “stepper” motor. Unlike a conventional motor that can move to any position, a stepper motor operates with discrete movements or steps. This is an ideal type of control system for the on-board management computer, which by its nature is a digital processor.
Whilst early systems made use of an electronically controlled carburettor that contained a mixture control solenoid, the use of computerized systems really came into their own with the adoption of fuel injection. However, the idle air control valve or air bypass valve was first used in carburettors. By controlling the amount of air that bypasses the throttle plate, the engine idle speed can be regulated.
To reduce CO emissions, air is often pumped into the exhaust manifolds and/or the catalytic converter. The injection of this air is controlled by electrical diverter or switching valves (solenoids). The amount of exhaust gas that is re-circulated back through the engine can be controlled by either the management of a vacuum solenoid that is attached to a conventional EGR valve or on later engines using a digital solenoid—another adaptation of the stepper motor concept.
Ignition timing is also under direct computer control. Initially using a conventional distributor later superseded by distributorless - “wasted spark” systems and now by coil on (or near) plug designs. The fuel injectors are also directly controlled. By precisely varying the “pulse width” - the time during which fuel is sprayed into the engine - an accurately determined air/fuel mixture can be obtained.
Typically, the lock-up of the transmission torque converter is also controlled by the Powertrain Control Module. Transmissions that use solenoids to control shifting are now commonplace, although they often use a dedicated transmission control module (TCM).
Part 7
In order for these controls to be effected, a central processing unit, “brain” or computer is employed.
This is commonly referred to as an Electronic Control Module ( ECM ), although you may also find it referred to as a PCM (Powertrain Control Module) or VCM (Vehicle Control Module). Some vehicles also use additional computers to control Body (HVAC, Cruise control, Air Bag etc.) functions ( BCM) or the Transmission ( TCM ).
In fact, automotive computers do not “think” they merely follow a set of pre-programmed instructions. Several times each second, the computer reads the information that is provided by the sensors, processes this information and then sends out signals to activate or modify the operation of key components.
Communication to and from the computer is accomplished by a variation in voltage. This can either take the form of an analogue signal (the voltage is proportional to a variation in conditions—for example engine temperature) or a digital signal (mostly outputs). The latter is usually frequency modulated—faster pulses (on/off cycles) equate to a higher frequency.
To allow the digital computer to process analogue signals, an interface is used. The interface converts the input signals from analogue to digital and output signals from digital to analogue.
The microprocessor makes the calculations and decisions for the system. It bases these decisions on the instructions that are programmed and stored within the computer's memory. In most cases, there are three types of memory Read Only Memory ( ROM ), Programmable Read Only Memory ( PROM ) and Random Access Memory ( RAM ). ROM & PROM cannot be changed and were originally plugged into the computer as specific chips. Later designs do not allow these chips to be removed, but can be updated or “flashed” by connecting to specific equipment. In this way, software updates can easily be accomplished.
The instructions, called “maps” or “look-up tables”, tell the computer what to do when certain conditions exist. Again, as designs and capabilities have improved, adaptive controls have been introduced. This allows the computer to take other factors e.g. engine wear, into consideration. This improves their capabilities through a learning process to maintain drivability and efficiency as the vehicle ages.
Part 8
Unlike non-computerised systems that change ignition timing in response only to engine speed & load, computerised systems will also look at many other parameters before determining the correct amount of ignition advance and other operating parameters. The information received from the inputs is processed in accordance with the “map” used to determine the proper calibration for the engine.
Information is temporarily held in the computer's RAM and remains there until new information is sent. Since RAM can both read from and written to, it can be used to store information that may be need for future decisions and is usually only held for a short time. However, if the computer determines that there is a faulty component or other problem, it is also used to store this information. It will remain there until either the fault is corrected or the memory is erased (usually by disconnecting the battery). It is this functionality that provides computerised systems with their self-diagnostic capabilities. When the computer determines the existence of a fault, it will base future decisions around that fault to provide the best drivability considering the faulty condition.
On some vehicles, there is an additional type of computer memory, which can be updated but not erased. Non-volatile RAM is primarily used in conjunction with a digital odometer. This allows for the accurate and permanent storage of mileage information.
Whilst the information stored in the computer's memory is used to control the engine under all operating conditions, when cold only limited information is processed and the engine is said to be operating in “open-loop” mode. Once warmed up (determined by inputs from the coolant temperature and O² sensors) the system enters “closed-loop” mode. All of the sensor inputs will now be taken into account.
As well as determining operating parameters from the pre-programmed data, the signal from the O² sensor(s) is of particular importance. Since this input is a good indicator of a rich/lean mixture, the strategy provided by the map will be modified to try to maintain an optimum air/fuel ratio and ignition timing. As well as lowering overall engine emissions, this also improves drivability and fuel economy.
Having briefly mentioned the self-diagnostic abilities of computerized engine management systems, we shall next examine this in more depth and see how it can be used as an aid to repairing malfunctioning components & systems.
Part 9
One of the most widely touted benefits of computerised engine managements systems are their built in diagnostic capabilities. With just the press of a few buttons, the insertion of jumper wires or hook up to simple machinery will fix the vehicle instantly—alas this is a pipe-dream, but nevertheless there is an element of truth here.
With any managed vehicle, there seems to be an instant reaction that all problems associated with the powertrain are the direct result of faults within the management system. In reality, there are many other factors that must be taken into consideration. Just because a management system is fitted, this does not mean that all other common engine problems are suddenly irrelevant. For example, a simple visual inspection of wires or vacuum hoses that are damaged or broken may reveal the source of problems. Other checks such as contaminated fuel or for correct cylinder compression are still important indicators. Never forget the basics when trying to find the root of the trouble.
As always with electronic devices, when first introduced they are of limited capacity and are relatively expensive. The economies of scale and continued research & development lead to cheaper and more powerful systems. Thus as time has proceeded the diagnostic information has become more accurate and comprehensive.
The information about what may be a problem is usually communicated back to the user via Diagnostic Trouble Codes (DTC's). The extraction of this information varies from vehicle to vehicle and in many cases this can be obtained without specialist equipment, however in the case of later model (OBDII) systems that have far more codes available this is invariably not the case.
Typically on earlier vehicles, the DTC's are displayed via the “Check Engine” or “Service Engine Soon” lights by jumpering connections within the onboard diagnostic plug, by cycling of the ignition key or through a driver information panel. The lights flash in a particular sequence, cycling through any codes that are stored—hence they are often known as “flash codes”.
Unfortunately, early Ford EEC-IV systems are less friendly, although arguably more powerful than their contemporary counterparts. It is possible to extract DTC's using an analogue voltmeter connected to the diagnostic plug under the hood, but for these cars cheap (less than £50) scanners are available that simplify the job and also activate the functional tests built-in to this system's diagnostic capability.
Part 10
Once the DTC's have been identified and recorded, by referring to a shop manual or other diagnostic book, these codes will then point to a particular type of failure and are used as an entry point to specific procedures used to diagnose a problem. When checking for the “meaning” of these codes, make sure you refer to a specific listing for your vehicle. Before the standardization of OBDII, there were a limited number of codes that could be stored and displayed (usually two digits). As a result manufacturer's often “re-used” codes on different vehicles to indicate different problems, so failure to use the correct chart could lead you astray! OBDII allows for 4digit codes with prefixes for Engine, Body etc. allowing far more precise (and consistent) diagnostic procedures.
Inevitably, repair of most computerized engine control systems often involves the replacement of faulty components; although since diagnostic procedures are primarily designed to check the operation of circuits, rather than individual components. The cause of a problem may therefore lie in a bad connection or just a poor earth.
The procedures are usually in the form of a flow chart with tests for voltage etc. When using diagnostic flow charts, make sure you follow the steps carefully. In most cases a cheap digital multimeter (measuring voltage and circuit continuity) are often all that is required. Although with later systems a scan tool is often needed to perform certain tests. In some cases, tests can be performed and monitored using on-board displays e.g. certain Cadillacs & Corvette models. These will use the digital readout to display diagnostic trouble codes and perform functional tests. As originally stated, there are no miracle cures for ailing powertrains, but at least you can get a head start. Use of a scan tool can also help to find problems when no DTC is found. Sometimes a component will provide an incorrect reading but not one unexpected by the computer. Some familiarity of the systems will often pinpoint this.
