Section 00-04: Noise, Vibration and Harshness
1996 F-150, F-250, F-350, Bronco and F-Super Duty Workshop Manual

Diagnostic Theory


Diagnosis is more than just following a series of interrelated steps in order to find the solution to a specific condition. It is a way of looking at systems that are not functioning the way they should and finding out why. Also, it is knowing how the system should work and whether it is working correctly.

There are basic rules for diagnosis. If these rules are followed, the cause of the condition is usually found the first time through the system.


Know the System

Sometimes this means checking the system against one that is known to be working properly.


Know the History of the System

A clue in any one of these areas might save time.


Know the History of the Condition

Know how the condition made itself known; it may be an important clue to the cause.


Know the Probability of Certain Conditions Developing

It is true that most conditions are caused by simple things rather than by complex ones and they occur in a fairly predictable pattern. Electrical conditions, for instance, usually occur at connections rather than in components. An engine no-start is more likely to be caused by a loose wire or some component out of adjustment than a sheared-off camshaft. Know the difference between impossible and improbable. Many good technicians have spent hours diagnosing a system thinking certain failures were impossible, only to find out that the failures were just improbable and actually happened. Remember to verify that new parts are functioning properly.


Do Not Cure the Symptom and Leave the Cause

Lowering the air pressure in a front tire may correct the condition of a vehicle leaning to one side, but it does not correct the cause of the condition.


Be Positive the Cause Is Found

Double-check findings. If a worn component is found or something is out of adjustment, determine what else could be wrong. If one of the engine mounts was loose, could the other one be loose too? What about the transmission mounts?


Diagnosis Charts

Charts are simply a way of expressing the relationship between basic logic and a physical system of components. They help corner the cause of a condition in the shortest possible amount of time. Diagnosis charts combine many areas of diagnosis into one visual display:

The fastest way to find a condition is to work with the tools that are available, which means working with proven diagnosis charts and the proper special equipment for the system.