Diagnosing engine management system faults


As with all systems, the six stages of fault finding should be followed.

  1. Verify the fault.
  2. Collect further information.
  3. Evaluate the evidence.
  4. Carry out further tests in a logical sequence.
  5. Rectify the problem. 6. Check all systems.

The procedure outlined in the next section is related primarily to stage 4 of the process. Table 10.1 is based on information available from ‘Autodata’ in its excellent range of books. It relates in particular to the Bosch LH-Jetronic fuel system but it is also a good guide to many other systems. The numbers relate to the order in which the systems should be checked.

ECU auto-diagnostic function

Most ECUs are equipped to advise the driver of a fault in the system and to aid the repairer in the detection of the problem. The detected fault is first notified to the driver by a dashboard warning light.

Each fault detected is memorized as a numerical code and can only be erased by voluntary action. Often, if the fault is not detected again for 50 starts of the engine, the ECU erases the code automatically. Only serious faults will light the lamp but minor faults are still recorded in memory.

The faults are memorized in the order of occurrence. Certain major faults will cause the ECU to switch over to an emergency mode. In this mode, the ECU substitutes alternative values in place of the faulty signal. This is called a ‘limp home facility’.

Faults can be read as two-digit numbers from the flashing warning light by shorting the diagnostic wire to earth for more than 2.5 s but less than 10 s. Earthing this wire for more than 10 s will erase the fault memory, as does removing the ECU constant battery supply. Earthing a wire to read fault codes should only be carried out in accordance with the manufacturer’s recommendations. The same coded signals can be more easily read on many after-sales service testers. On some systems, it is not possible to read the fault codes without a code reader.

Testing procedure

Caution/Achtung/Attention – Burning fuel can seriously damage your health! Caution/Achtung/Attention – High voltages can seriously damage your health!

The following procedure is very generic but with a little adaptation can be applied to any fuel injection system. Refer to the manufacturer’s recommendations if in any doubt.

Injection duration signals

shows typical injector signals as would be shown on an oscilloscope during a test procedure. These will vary depending on the particular system but, in principle, are the same. The most important parts of the traces are marked.

These are the open time or dwell, current limiting phase and the back EMF produced when the injector is switched off. The traces showing variations in the dwell represent how the quantity of fuel injected is varied. The difference in how the dwell is varied is due to the method of injector switching.

Last word

If a simple on/off technique is used then the trace will be as shown in the first two sketches; if current limiting is used then the trace will be slightly different, as shown by the lower two sketches.

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