Car Automotive

Archive for November, 2009

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.

Without a good quality spark, in the right place at the right time, the engine performance will be affected, as will the operation of the emissions control system. A misfire can lead to unburnt fuel reaching the exhaust and this will quickly harm the catalyst, often irreparably. For this reason, modern systems monitor the performance of each cylinder, in relation to combustion.

One method of doing this is to ‘sense’ the angular acceleration of the engine flywheel; a firing cylinder will produce more acceleration than a misfiring one. In order to identify the cylinder that is misfiring the ECM requires a reference signal and this is often provided by the camshaft position sensor.

On modern systems, the ECM has the ability to detect misfires because the unburnt fuel that results can cause serious damage to the exhaust catalyst. The ECM achieves this diagnosis by reading the time interval between pulses from the crankshaft speed sensor. Persistent misfires will activate the MIL and a fault code (DTC) will be recorded. Urgent remedial work will then be required if serious catalyst damage is to be avoided.

In the case of poor compression on one cylinder, given above as an example, the analysis would take the form of tests to determine the cause of low compression,e.g. burnt valve, blown head gasket etc. The analysis of evidence that is performed will vary according to the system under investigation. But these steps are obviously important otherwise one may embark on a drawn out electronics test procedure which will prove unproductive.

The procedure for doing this on an electronics system varies according to the type of test equipment available. It may be the case that the system has  some self-diagnostics which will lead you to the area of the system which is defective. Let us assume that this is the case and the self-diagnostics report that an engine coolant temperature sensor is defective. How do you know whether it is the sensor, or the wiring between it and the remainder of the system? Again this is where a good basic knowledge of the make-up of the system is invaluable.