Vehicle - Basic Troubleshooting
GROUP 00Maintenance and General Hints
Bulletin Number
00 09 88 (1652)
Montvale, NJ
May 1988
Technical Dept.
SUBJECT:
"Basic" Troubleshooting
MODEL:
All
The development and introduction of the new 750iL demonstrates the application of new levels of Automotive Technology and System Self-Diagnostics. Further, many new troubleshooting possibilities are now available with the employment of the BMW Diagnostic Module. Clearly, the nature of system fault finding continues to expand to parallel product sophistication; and yet, one troubleshooting rule remains as true today as it ever was in far simpler times.
All troubleshooting must always begin by "checking the basics".
Certain basic faults can be undetectable to the Diagnostic Module and in some cases can actually interfere with Control Unit 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 and printed prior to any vehicle power interruption or troubleshooting.
The following list serves to remind the troubleshooter that today's (and tomorrow's) Advanced Automotive Systems are still dependent on basic requirements.
Prior to any teardown, repair or component replacement, the following steps/checks should always be considered.
Complaint Verification
Whenever possible the repairing technician should personally verify the complaint. Having experienced the malfunction himself, the technician is less likely to try to repair nonexistent 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 must compare the system operation to the nominal system operation as described in BMW Service Training Material or the BMW ETM. Prior to teardown, 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.
The following list of basic checks, while not complete, should always be considered prior to replacement of control units, transmissions, wiring harness', etc.
Battery State of Charge
Batteries in a state of partial discharge can have a dramatic effect on today's modern "smart box" electronic systems.
Positive Battery Cable Integrity
All B + connections must be in perfect condition for troublefree electronic system operation. The BMW ETM and related Service Information Bulletins alert the servicing technician to all B+ interconnects and service points. (See S.I. 61 04 88 (1619)).
Fusible Link Integrity
Fusible links where employed must be intact without dynamic resistance. The BMW ETM illustrates where fusible links are located in BMW automobiles.
NOTE:
Dynamic resistance can only be checked using the voltage drop method of testing. This test procedure is covered in the BMW Electrical I Service Training course and the BMW VIS Electrical Training Tapes.
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 "community" ground point will cause seemingly unrelated systems to influence one another. High current systems which-encounter a poor "community" ground can back feed thru 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 promote intermittent operation which will not set a fault in the DME fault memory.
Reference/Engine Speed Sensor
Monolithic sensors should be checked statically and dynamically per procedures found in Group 12 of the Repair Manual. Further, physical mounting and condition of the sensor(s) and pulse wheel must be assured.
High Tension Coil and Secondary Wiring
The secondary ignition system should be checked visibly as well as with the Service Tester. 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.
Air/Fuel Ratio
Today's 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 operation;
^ Excessive engine oil dilution;
^ Substandard fuel or unapproved additives.
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 conform to S.I. 12 03 85 (1085).
These basic checks are quick and in most cases do not require parts. Advanced electronic systems in most cases will react negatively to simple faults.
Therefore, all troubleshooting must begin with "checking the basics"!