The above descriptions give a reasonable overview of the methods of ‘reading’
diagnostic trouble codes. I now propose to look at more recent developments that have arisen from legislation and advances in technology.
From the foregoing review of methods for accessing fault codes it will be evident that there are many different methods in use. In many cases, vehicle manufacturers have developed a version of diagnostic equipment that is unique to their range of vehicles. When an equipment manufacturer makes a piece of diagnostic equipment which has capabilities to perform diagnostic work on a range of vehicles, it is normally accompanied by an extensive range of leads and adapters so that it can be configured to suit a particular vehicle.
For several years now there have been arguments for and against bringing a degree of standardization into automotive computer applications, particularly in the area of access to diagnostic information. Generally speaking, there are two driving forces that cause changes to occur; one is legislation and the other is changes in technology. In the automotive field, as in other areas where computers are used, the technology has changed rapidly.
In the area of legislation, the world-wide concern with the effects of atmospheric pollution has had a major effect on vehicle design and, quite naturally, enforcement authorities are anxious to ensure that vehicles comply with the current emissions regulation. The result is that there have been major developments that cause vehi-cles in the USA to be equipped with a standard means of access to emissions related
Developments in self-diagnosis 79 data and it is anticipated that Europe will follow a similar path in the near future. As the USA has a major influence on events in technology, it is to be expected that their developments will have an effect on vehicle technology in Europe and elsewhere.
There is plenty of evidence to show that this is the case and readers should benefit from a review of some recent developments, such as on board diagnostics (OBD).
The term on board diagnostics refers to the self-diagnosing capabilities that are carried in the computers on the vehicle, and the aids that are provided to make the diagnostic data available to authorized users. Off-board diagnostics is equipment such as scan tools, oscilloscopes and other test equipment. In most cases both types of equipment are required for vehicle repair work. To date (January 2000) there have been two versions of OBD, i.e., OBD I and OBD II. Both apply to the USA, but introduction of similar legislation for Europe is imminent.
3.2.1 OBD I
This required vehicles produced from 1988 onwards to be equipped with electron-ically (computer) controlled systems that were capable of monitoring themselves.
Any malfunction (defect) that affected exhaust emissions must be displayed on a warning lamp, known as the malfunction indicator lamp (MIL), on the dashboard.
The malfunction must be stored in the ECM’s memory and it must be readable with the aid of ‘on board’ facilities, e.g. a flash code on a lamp.
3.2.2 OBD II
OBD II strengthens the requirements of OBD I on vehicles of model year 1994 and afterwards. OBD II applies to spark-ignition cars and light vans, and from 1996 onwards to diesel-engined vehicles. The main features are that the following emissions related systems must be continuously monitored:
ž combustion
ž catalytic convertor
ž oxygen (lambda) sensors
ž secondary air system
ž fuel evaporative control system
ž exhaust gas recirculation system.
The requirements for diesel-engined vehicles vary and glow plug equipment may be monitored instead of the catalytic converter.
Features of OBD II are as follows.
ž The malfunction indicator lamp (MIL) is provided with an additional ‘flashing’
function.
ž The DTCs can be read out by a standard form of scan tool, via a standardized interface that uses a 16-pin diagnostic connector of the type shown in Fig. 3.20.
ž The emissions-related components must be monitored for adherence to emis-sions limits in addition to monitoring them for defects.
ž Operating conditions (performance data) can be logged and stored in a ‘freeze frame’.
80 Self-diagnosis and fault codes
CARB plug:
Pin 7 and 15: Data transfer in accordance with DIN ISO 9141-2
Pin 2 and 10: Data transfer in accordance with SAE J 1962
Pin 1, 3, 6, 8, 9, 11-14 are not assigned to CARB.
(OBD II data administration guideline "OBD II-DV") Pin 4: Vehicle ground (body)
Pin 5: Signal ground Pin 16: Battery positive
Fig. 3.20 The SAE J 1962 standardized diagnostic link connector (DLC)
Fig. 3.21 The structure of standard fault codes for OBD II
Diagnostic equipment and limitations of DTCs 81 The malfunction indicator lamp (MIL) should light up when the ignition is first switched on and then go out after about 3 seconds, during which time the ECM is performing a series of ‘self checks’. After this, when the engine is running, the MIL should only light up when a malfunction occurs. If the MIL does not light up when the ignition is first switched on, it is an indication that there is a fault in the MIL, or in the ECM itself, assuming that the battery is not flat.
From the repair shop point of view, OBD II provides some features which should produce benefits. Examples of such benefits are: (1) a standardized diagnostic interface and connector (see Fig. 3.20); and (2) standardized fault codes. The fault codes, as presented at the scan tool, comprise five digits, e.g. P0125. Digit 1, at the left-hand end, identifies the vehicle system. Digit 2 identifies the subgroup.
Digit 3 identifies the subassembly. Digits 4 and 5 identify the localized system components.
Figure 3.21 shows how a range of fault codes can be constructed by using the recommended standard approach.
The example quoted, i.e., P0125, has the following meaning under this coding system: ‘insufficient coolant temperature for closed loop fuel control’. There are many hundreds of codes and full details are given in the SAE J 2012 publication.