Diagnostic Notes
^ Before using this section you should have performed the SELF-DIAGNOSTIC PROCEDURES which verify that the
DME control unit and "CHECK ENGINE" light are operational, as well as no trouble codes were stored.
^ Always use the FLOW OF DIAGNOSIS for proper testing sequence.
^ Several of the following symptom procedures call for a careful visual/
physical check. This step is very important - it can lead to
correcting a problem without further checks and can save valuable time.
^ Certain basic faults can be undetectable by the self-diagnostic system of the DME control unit and in some cases can actually interfere with the self-checking and fault memory operation.
Low battery voltage, for example, can cause erroneous faults to set in control unit fault memories or can cause a system to go "Fail Safe" without setting a fault in memory. On the other hand, system fault memories are cleared whenever the control unit or the battery is disconnected. Therefore, all fault memories should be read prior to any vehicle power interruption or troubleshooting.
CHECK THE FOLLOWING:
NOTE: It is essential that the vehicle has been properly warmed up and is at operating temperature, except when attempting to diagnose a cold driveability problem. The catalytic converter and the oxygen sensor will not function as designed if this is not achieved. Failure to do so will result in inaccurate test results.
COMPLAINT VERIFICATION
Whenever possible the repairing technician should personally verify the complaint. Having experienced the malfunction, the technician is less likely to try to repair non-existent faults.
MALFUNCTION VERIFICATION
Today's sophisticated automotive systems are easily misunderstood, which can lead to repairs that attempt to force a particular system to perform in a way that it was never intended to operate. Therefore, the troubleshooting technician should compare the system operation to the nominal system operation as described in the section DESCRIPTION AND OPERATION. Furthermore, the technician is also encouraged to compare the problem vehicle system operation with a known good vehicle.
PREVIOUS REPAIRS
The vehicle repair history can provide explanations to unusual complaints which seem to elude normal troubleshooting attempts. Incorrect components or unapproved repairs can have subtle influences on seemingly unrelated systems.
BATTERY STATE OF CHARGE
Batteries in a state of partial discharge can have a dramatic effect of DME control units and related components.
POSITIVE BATTERY CABLE INTEGRITY
All B+ connections must be in perfect condition for trouble-free electronic system operation. Refer to ELECTRICAL AND ELECTRONIC DIAGRAMS for B+ interconnects.
FUSIBLE LINK INTEGRITY
Fusible links are employed to prevent possible damage to electrical components and wiring harnesses. These links and their connections must be without dynamic resistance. Dynamic resistance can only be checked using the voltage drop method of testing.
NEGATIVE GROUND CONNECTIONS
As all electrical circuits are a circle, all B- connections must also be checked and verified to be in perfect condition. A poor "common" ground point will cause seemingly unrelated systems to influence one another. High current systems which encounter a poor "common" ground can back feed through other electrical systems causing unusual operation and perhaps inexplicable component failure. As with the B+ side of the electrical system, the ground side should be checked dynamically using the voltage drop technique.
POWER TRAIN CABLE ATTACHMENTS
Metal cables attached to the engine or transmission which appear overheated and/or discolored indicate the need to thoroughly test all ground connections.
POWER SUPPLY RELAYS
Fuel pump and main relays as well as their plug connections can be a source of intermittent operation which will not set a fault in the DME fault memory.
ENGINE REFERENCE/SPEED SENSOR
Monolithic sensors should be checked statically and dynamically. Physical mounting and condition of the sensor and pulse wheel must be assured.
HIGH TENSION COIL AND SECONDARY COMPONENTS WIRING
The secondary ignition system should be checked visibly as well as with a suitable engine analyzer and scope. All components should be examined for tight connections and freedom from carbon tracking, moisture and corrosion.
FUEL DELIVERY AND FUEL RAIL PRESSURE
Fuel delivery must begin at once when cranking and the pressure in the fuel rails must be within specifications. Fuel pressure retention must be maintained between the fuel pump outlet and the pressure regulator after engine shutdown.
VALVE CLEARANCE AND SPARK PLUGS
These two service items, if out of specification, can significantly impair engine performance. Valve clearance should be adjusted "cold" with each piston at top dead center when adjusting the respective valves. Spark plug type, condition and gap must verified according to specification.
AIR/FUEL RATIO
Adaptive engine management systems efficiently compensate for conditions which may affect the combustion process. However, when troubleshooting idle quality or driveability complaints, it is still necessary to consider the following:
^ Injector spray pattern quality
^ Presence of unmetered air leaks
^ Evaporative purge system
^ Excessive engine oil dilution
^ Substandard fuel or unapproved additives
^ Carbon build-up
^ The following symptom charts contain groups of possible causes for each
symptom and cover several engines. These procedures are not
meant to be done in consecutive order. Proceed in a logical order, easiest to check or most
likely cause first. To determine if a particular system or component is
used on a specific vehicle, refer to DESCRIPTION AND OPERATION and applicable wiring diagrams in
ELECTRICAL AND ELECTRONIC DIAGRAMS.