Troubleshooting Hints For Electrical & Computer Problems
***UPDATED BY TSB #6106923582, JULY 1992GENERAL INFORMATION:
As electronically controlled systems continue to be developed and introduced in BMW vehicles, the level of complexity and sophistication of these systems has risen dramatically in the past ten years. This does not mean, however, that the diagnosis and repair of these systems is more difficult. On the contrary, these systems are possibly easier to diagnose, if a systematic troubleshooting method is used.
The purpose of all electrical troubleshooting is to identify and correct a malfunction which has caused a customer to complain. However, before repairs are attempted, the following steps should be taken:
VERIFY THE COMPLAINT
Whenever possible, the repairing technician should personally verify the complaint.
VERIFY THAT THE COMPLAINT IS A MALFUNCTION
If a customer does not fully understand a particular system, he or she may complain about an aspect of operation in that system that is normal (that is, the system is operating as designed, and therefore not malfunctioning). An example of this would be a customer complaining of "rapid vibrations in the brake pedal during a panic stop on wet pavement." While this is a customer complaint, if the vehicle is equipped with ABS this is not necessarily a malfunction. Determine that the system is operating correctly or incorrectly before proceeding with troubleshooting.
Refer to DESCRIPTION AND OPERATION for a description of system operation, and to any related Technical Service Bulletins for changed operation (for example, changes in General Module operation from MY 92 on).
Comparing the complaint to system operation in a known good vehicle is another acceptable means of verifying a malfunction.
CHECK FOR PREVIOUS REPAIRS
If a system had been working properly and now exhibits a malfunction, a review of the vehicle's repair history can provide significant insights into the problem. The improper installation of accessories, or the installation of non-approved accessories, can have detrimental effects on a variety of systems.
Once a malfunction has been verified, troubleshooting should be devoted to finding the cause of the problem, then correcting the fault; only if the cause is positively identified can the vehicle be confidently released to the customer.
Remember that the vast majority of causes for complaint concerning electronically-controlled systems are not exotic electronic malfunctions; rather, the cause is almost always a simple electrical fault, such as an open circuit, a short circuit (to B+ or B-), or excessive resistance (poor contact).
If the faulted system is equipped with self-diagnostics, the first step should be to call up any flash codes (for systems - without the diagnostic link) or to read out the control module's fault memory (for systems with the diagnostic link). Read out and print the contents of all systems' fault memories.
Refer to the appropriate testing procedures and examine the schematics of the system circuitry in CHASSIS ELECTRICAL DIAGRAMS. If several systems are faulted, common or shared circuits may provide a clue that will aid in more efficient diagnosis.
On systems which have no self diagnostic capabilities (or on any self-diagnostic systems which does not provide clear evidence of a fault location), an examination of all circuits of the system should be performed. Installation of the appropriate adapter ("break-out box") between the control module and harness will allow the technician to monitor the electrical activity at each pin of the control module.
Before any parts (such as a control module) are condemned as faulty, thorough investigation of all circuits connected to the control module should be performed. (If a system has self-diagnostic capabilities, this should be done if the troubleshooting for a specific fault is inconclusive.)
All electronically controlled systems should be approached with one rule in mind:
A control module will not produce the desired output if one or more of its inputs is impaired.
Consider a standard 4-pin relay:
When switch (S1) closes, the contacts should close and battery voltage should appear at terminal 87.
If any one of the inputs is impaired or missing, the desired output will also be impaired or missing. Since all inputs must be within nominal values at the control module (in this case, a relay), this is the point at which measurements are taken.
The same approach should be taken for all electronically controlled systems: verify that all inputs are present before replacing a control module. If a control module is producing the proper output (in the above example, battery voltage at pin 87) and the load is not operated, it is obvious that the load or the wiring between it and pin 87 is faulty. If all inputs are present, and no output is present at the control module, then and only then should the control module be replaced.
In all cases, verify proper voltage level at all power inputs, and check all grounds by measuring voltage drop. Depending on the type of input, other signals (e.g., A.C. voltage, frequency, etc.) may have to be verified. Refer to the appropriate diagnostic manual for this information.
ALWAYS "CHECK THE BASICS":
Look for such obvious problems as battery state of charge, loose battery cables, defective fuses, etc. Very few things are as discouraging as troubleshooting alternator noise in a sound system for four hours, only to discover that a partially discharged battery was causing increased alternator current and ripple.
When reading the fault memory of a control module with the diagnostic link, remember that the message "no faults found" simply means that all monitored circuits are intact and within limits. Some circuits cannot be monitored for faults; for example, it is "okay" for the stop light switch to be either open or closed as far as the control module is concerned - check the switch operation by monitoring its status.
INTERMITTENT PROBLEMS
Intermittent electrical problems can be difficult to diagnose. Gather as much information as possible from the customer concerning the conditions under which the problem occurs - this may provide valuable diagnostic clues.
When trying to locate an intermittent open or short in a circuit, attach the meter to the circuit (according to the test being performed) and flex, twist, pull and shake the harness or wire in question.
UNAPPROVED ACCESSORIES
If unapproved accessories are installed, they should be disconnected electrically from the vehicle's original harnesses - if they are not, voltage and resistance measurements may be erroneous. Such unapproved accessories may in fact be the cause of the malfunction being diagnosed.
DOCUMENTATION
Always write down all of your measurements. If ALLDATA Customer Service is contacted, the Technical Specialists must have all of this information.
Please follow these procedures whenever troubleshooting electrical problems. A systematic, thorough approach to troubleshooting is one of the easiest ways to ensure customer satisfaction, by finding and repairing electrical problems quickly and correctly.