412 Obd Ii Type Diagnostic Equipment

Much of the diagnostic capability that is known as OBD II has been in use for some time. The main effect of the OBD II legislation seems to have been to focus attention on the desirability of standardization, particularly in the areas of means of access to diagnostic data on vehicle systems and the availability of supporting documentation. This means that some of the techniques described under this heading will, in some cases, be available on non-OBD II systems.

Firstly the principal features that relate to OBD II are outlined. This is followed by a description of the equipment and its diagnostic functions. The principal features are as follows.

1. A standard 16-pin SAE J 1962 diagnostics plug-in point, described in Chapter 3.

2. The diagnostic plug should be accessible from the driving seat and the preferred location is as shown in Fig. 4.3.

In addition to the pin assignment, the Standard also defines the installation location for the diagnostics plug.


It must be accessible from the driving seat. The preferred installation location is on the dashboard between the steering column and the median line of the vehicle.

Fig. 4.3 The preferred location of the SAE J 1962 diagnostic interface

3. The communication between the scan tool and the ECM takes place according to one of a small number of protocols.

4. Initializing (starting) the diagnostic communication takes place via the diagnostics equipment as follows: hexadecimal 33 is signalled to the ECM, by the scan tool at a transmission speed of 5 binary bits per second, i.e., 5 baud.

5. The ECM then sends a 'header label' to the scan tool in response to the intialization prompt. This header label consists of information about the baud rate and two keywords.

6. To check that the communication is correctly set up, the scan tool inverts the second keyword and sends it back to the ECM. (Inversion in binary language means turning 0s into 1s and vice versa.)

7. The ECM sends the inverted memory address (hexadecimal 33) back to the scan tool.

As far as actual practice is concerned, the user merely prepares for the test. The above procedure takes place automatically via the control buttons on the scan tool. Some of the main points about the OBD II test tool are that it must:

• automatically recognize the type of data transfer used by the engine control system that is being checked;

• display any fault codes relating to emissions;

• display current (live) values relevant to exhaust emissions;

• be able to erase the fault codes;

• contain an 'on-line' help facility that can be called up from the instrument panel.

Readers who are familiar with scan tools will recognize that many of these features have been available on scan tools for some years. As modern vehicles are frequently equipped with several computer (ECM) controlled systems a technician will expect a scan tool to be able to perform diagnostic functions on a range of systems. The additional pins on the 16-pin diagnostic interface permit access to other systems on the vehicle.

Fig. 4.4 The Bosch KTS 300 pocket system tester

Figure 4.4 shows the Bosch KTS 300. For OBD II purposes, the KTS 300 pocket system tester is supplied with two leads, as shown in Fig. 4.5. One of these leads (Bosch reference 1 684 463 361) provides connection to the control units related to exhaust emissions. The other lead, which includes the adapter box shown in Fig. 4.6, provides the diagnostic link to other control units such as ABS and transmission control etc.

Fig. 4.5 The leads for the Bosch KTS 300 pocket system tester
Fig. 4.6 The CARB adaptor box

When the test instrument is connected and diagnostic communication is established, the instrument guides the user through the test procedure. In addition to the OBD II functions, the KTS 300 tester can also be used on vehicle systems that are provided with an ISO 9141 diagnostic connector. The extent of the diagnostics that can be performed on these systems is dependent on the vehicle that is being tested and the availability of diagnostic data. Vehicle specific connector leads are also required.

Fig. 4.7 The flow chart that describes the fault memory processing under degree of extension 1

Fig. 4.7 The flow chart that describes the fault memory processing under degree of extension 1

The software is described as having two degrees of extension. With degree of extension 1, the KTS 300 provides for the selection of the fault code memory and the 'help' menu, processing the communications between the tester and the ECM. The flow chart shown in Fig. 4.7 shows what is available under degree of extension 1. (Where 'key' is stated it means that this is the key to press to move to the next step.) From the flow chart it may be seen that fault codes (DTCs) are 'read out' and cleared, as required. Pressing the '>' key takes the user forward to the next screen menu and pressing the 'N' key takes the user back to the beginning of the previous step.

With degree of extension 2, the user is presented with three options as shown in Fig. 4.8. Option 1 gives the fault memory processing capacity of degree of extension 1, plus extra items. Options 2 and 3 give access to screen menus that provide considerable diagnostic capacity.

If you examine the flow chart shown in Fig. 4.9, you will gain an impression of the diagnostic work that can be performed under option 1. For example, if the

Menu Degree 2

Key N

Key 1 Key 2 Key 3

Key 1 Key 2 Key 3

Fig. 4.8 The options in degree 2
Fig. 4.9 Flow chart for option 1

fault is sporadic (intermittent) it is possible to 'wiggle' the connectors. If loose connections are detected, the '>' key will direct the user down the dotted line route to A and then to A at the top right-hand side and so on, through the steps on the flow chart.

Option 2 permits the KTS tester to perform a range of actuator tests. Option 3 permits actual component values to be read from the ECM, e.g. duty cycle, injection time etc., and compare them with values that are stored in the KTS 300 software.

4.2 Breakout boxes

The breakout box is a diagnostic aid that is normally connected to the main ECM harness connector as shown in Fig. 4.10.

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