Test Instruments Connected To These Pins

Fig. 4.10 A breakout box

When the box is inserted it permits electrical access to most elements, such as sensors actuators and circuits, that make up the system that is being controlled by the ECM in question. A major advantage of the breakout box is that it permits resistance, continuity and dynamic tests of sensor and actuator circuits to be performed more easily because it overcomes the need to access the contacts inside the harness to ECM connector.

The breakout box's electrical connections are numbered to match those in the ECM connector, and instructions for performing circuit tests frequently refer to ECM pin numbers. For example, on a certain system the resistance reading between pins 12 and 22 should be 750 ohms. This would require the ohmmeter leads to be connected between connectors 12 and 22 on the breakout box - with the ignition switched off, of course, and the breakout box disconnected from the ECM. The reading of 750 ohms should then be obtained. The action to take if it is not, is considered in Chapter 7.

Some breakout boxes are designed for resistance checks only. In such cases the the main harness connector is removed from the ECM and is then connected to the breakout box with no additional connection to the ECM.

4.3 The digital multimeter

In section 3.3, the limitations of fault codes are described and an indication is given of the extra work that may be required to locate the cause of a defect. A digital multimeter is a valuable asset in this additional diagnostic work. Digital meters are preferred for work on electronic systems because they have a high impedance (internal resistance) and this prevents them from placing a load on electronic devices.

Fig. 4.11 Fluke digital multimeters

The 'Fluke 78' shown in Fig. 4.11 is an example of a digital multimeter that has been specially developed for work on automotive systems. Some uses of this type of meter are covered in later chapters.

These particular multimeters are supported by a range of test leads and adaptors that are shown in Fig. 4.12. It should be noted that test connections need to be securely made to ensure that good electrical contact is made. Test leads should also be supported so that they do not drop off during testing, or get tangled up in moving and/or hot parts.

Fig. 4.12 The Fluke test leads and connectors

4.4 Portable flat screen oscilloscopes

Oscilloscopes of the cathode ray type have long been part of the equipment for workshop-based diagnostic work. Recent developments in liquid crystal displays (LCDs) and thin film transistor (TFT) technology have made small portable oscilloscopes possible and they are available from several suppliers at reasonable cost. For electronic system diagnosis they have considerable value because they are versatile and can measure sensor and other components' performance very accurately.

The majority of the scope patterns that appear in Chapters 4 and 5 are derived from the Bosch PMS 100 oscilloscope, shown in Fig. 4.13(a). Figure 4.13(b) shows some screen menus for this scope and these give a good indication of the types of test that the instrument can be used for. Other oscilloscopes, such as the Lucas YWB 220 and the Crypton CPT 50, can be expected to perform a similar range of tests.

4.5 Diagnostic tool and oscilloscope combined

The Bosch KTS 500 is an example of a recent development that puts even greater diagnostic power at the disposal of the technician. This instrument is plugged into the diagnostic connector as shown in Fig. 4.14.

Fig. 4.13(a) The Bosch PMS 100 portable oscilloscope

Fig. 4.13(b) An overview of available test functions from the MENU key of the Bosch

PMS 100 portable oscilloscope

Fig. 4.13(b) An overview of available test functions from the MENU key of the Bosch

PMS 100 portable oscilloscope

This instrument is backed up by fault finding aids that can be viewed on screen, e.g. trouble shooting and repair guides, circuit diagrams etc. The instrument has a large memory capacity and it can display live data and also store live data 'captured' during a road test and display it on screen, or print it out for analysis in the workshop. The chart in Fig. 4.15 shows how vehicle identification followed

Fig. 4.14 The Bosch KTS 500 control unit diagnosis tester

by fault code read-out can be followed-up by taking readings of actual values. The use of data from this machine is discussed further in Chapter 7.

4.6 Pressure gauges

A good quality, accurate pressure gauge is an aid that is frequently used in diagnostic and routine maintenance work. An example of such use is the checking of fuel gallery pressure as shown in Fig. 4.16. The fuel pressure is a critical factor in determining the amount of fuel that passes through an injector nozzle, in a given time. Fuel gallery pressure is quite high, often several bar, and the procedure for connecting the gauge safely varies considerably from one make of vehicle to another. For this reason I do not propose to describe details of the test, except to emphasize that it is important to follow the manufacturers' prescribed procedure.

Do It Yourself Car Diagnosis

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