Fig. 7.42 Result display of a voltage test on a ride height sensor

Although a test of this nature will show whether or not the sensor is operating electrically it should be noted that the mechanism that connects the sensor into the suspension system can also be a cause of trouble. Should the sensor's mechanical linkage be worn or damaged, or the sensor be wrongly positioned on the vehicle, the signal that the ECM receives will not accord with its programmed value and this may cause the height control system to malfunction. So, beware, always make a thorough visual check of the system at a very early stage in any diagnostic process.

7.10 Intermittent faults

The self-diagnosis capacity of modern computer controlled systems has provided technicians with an additional source of information that can be of great value in tracking down faults that occur occasionally. The processing power and memory capacity of the on-board computer can be of value in tracing the causes of this type of fault. In addition, many diagnostic scan tools have a data logger capability which is similar to, but of smaller capacity than, the flight recorder on aircraft. Because of this similarity, the data logger function of the scan tool is often known as the 'flight recorder' function.


The data logger aspect of test equipment capability permits the test equipment to store selected data that the test equipment 'reads' through the serial data diagnostic connector of the ECM. It is particularly useful for aiding the diagnosis of faults, such as an unexpected drop in power that occurs during the acceleration phase. When the test equipment is connected, and proper preparations have been made for a road test, the vehicle is driven by a person who should be accompanied by an assistant to operate the test equipment (for safety reasons). With the test equipment in 'record' mode, the vehicle is driven in an attempt to re-create the default condition and when the 'fault' occurs the test equipment control button is pressed. From this point, data from just before the incident and for a period after is recorded. The stored data can then be played back, on an oscilloscope screen, or printed out later for analysis in the workshop. Figure 7.43 shows an example of live data that was obtained from a test using the Bosch KTS 500 equipment on a Peugot vehicle.

Fig. 7.43 A display of live data as 'read' out from the diagnostic link

In this case the signals all appear to be in order. However, should a defect occur it will show up in displays such as these. Figure 7.44 gives an impression of the type of information that might be seen.


Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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