Maintenance and Fault Diagnosis of Engine Management Systems
Modern engine-control systems incorporate many sensors to communicate operational data to the ECU to control the engine effectively. Cables from these sensors and associated circuitry use many multi-pin connectors.18.9.1.
Connector Problems
Under ideal conditions the connectors used on automobiles normally perform satisfactorily but when they are exposed to an under-bonnet high temperature environment, to water, salt, oil and dirt, these are likely to develop problems. Although to minimize these problems, some form of flexible cover is provided to prevent the ingress of contaminates, but the possibility of even partial failure can not be eliminated, especially after the connector has aged.During routine maintenance, and when a fault is being diagnosed, attention should be given to the condition of the connectors, specifically the plugs located in exposed conditions. Security and assessment of the condition of the connectors plug requires two important routine checks. Where an intermittent fault is experienced, it is often impossible to locate a cable connector
fault by normal meter tests. In these cases the contact pins of all connectors in the circuit that have the problem, should be cleaned.
Some manufacturers recommend the actual method for cleaning the contact surfaces. These methods range from the use of an ink eraser for spraying the surfaces with a special cleaning fluid. Emery cloth should not be used, because it removes the contact surface and is likely to create a short-circuit due to the electrical conductivity of the emery dust.
18.9.2.
Self-diagnosing Systems
It is expected that in due course of time most management systems will incorporate their own fault-diagnosis circuit. Already these systems in use have a monitoring circuit, which either signals the driver when a fault occurs, or controls the system in such a way that the fault does not seriously damage the engine. In such a case the ECU resets the control system to enable the vehicle to 'limp home' and be driven to the garage for repair.Some engine management computers incorporate a built-in self-diagnosing feature, which displays, when instructed by a diagnostician, the faulty area. Systems of more advanced design use a facility, which permits the transmission of information relating to a fault to a larger computer installed in the workshop.
Self-diagnosis by Light Signal.
The Toyota computer controlled system (TCCS) uses a coded light signal to indicate the cause of a malfunction in the system. When a fault develops, the ECU registers the sub-system in which the fault is present, into its memory. This information is stored in the memory even after the engine is switched-off.The possible faults are monitored by the system. Some of these faults are capable of producing an engine stall, so in such a case a warning light on the instrument panel indicates the driver to check engine. When the malfunction is corrected the warning light goes out, but the ECU still holds the information in its memory. This is specifically helpfull to the diagnostician when an intermittent fault recurs.
Access to the memory data is obtained by short-circuiting a test terminal. This causes the panel lamp to flash at a rate, which indicates a particular fault that can be identified by referring to the code shown in the repair manual. Figure 18.28 illustrates the lamp behavior for three examples and the fault associated with each one of them.
Fig. 18.28. Fault diagnosis by light signal.
In addition to this memory feature, this computer also incorporates a fail-safe function that avoids an engine stall due to faulty operation of MAP, coolant temperature and intake air temperature sensors. Malfunctions in one or more of these areas cause the computer to make the following adjustments.
MAP : Sets the ignition timing to 10 degrees before TDC
and maintains a constant injection duration.
Coolant temperature : Sets injection duration by assuming a temperature of
353 K.
Air temperature : Sets injection duration by assuming a temperature of
293 K.
Breakout Box Tests.
A breakout box provides a series of contact plug sockets that allows test meters to be connected into the various circuits to carryout fault diagnosis. A breakout box used on Ford vehicles is shown in Fig. 18.29. This box has 60 sockets and a provision for the connection of these sockets to the multi-plug, which normally fits into the ECU.
Fig. 18.29. Break-out box (Ford).
Tests conducted with the aid of this box cover many sub-sections. The use of the box minimizes the problems of connecting test equipment to the wrong pin and making ineffective connections to the test meters.
Each socket is numbered, which simplifies the test procedure when used in conjunction with a fault-diagnosis chart. As with many other tests of electronic equipment, proper care must be taken when a multi-meter is used to measure resistance. This is because the current supplied by the meter can damage many of the components in an electronic control unit.