Diagnosis

Diagnostic Theory
Diagnosis is more than just following a series of interrelated steps in order to find the solution to the 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
- know how the parts go together.
- know how the system operates as well as its limits and what happens when the system goes wrong.
- sometimes this means checking the system against one that is known to be working correctly.

Know the History of the System
A clue in any one of these areas may save time:
- how old or new is the system?
- what kind of treatment has it had?
- has it been repaired in the past in such a manner that might relate to the present condition?
- what is the repair history?

Know the History of the Condition
- did it start suddenly or appear gradually?
- was it related to some other occurrence such as a collision or previous part renewal?
- know how the condition made itself known; it may be an important clue to the cause.

Know the Probability of Certain Conditions Developing
- Look for the simple rather than the complex.
- For example:
- Know the difference between impossible and improbable. Certain failures in a system can be improbable but still happen.
- New parts are just that, new. It does not mean they are always good functioning parts.

Do Not Cure the Symptom and Leave the Cause
Lowering the pressure in a front tire may correct the condition of a vehicle leaning to one side, but it does not correct the original condition.

Be Positive the Cause is Found
- double check the findings.
- what caused a worn component?
- a loose transmission or engine mount could indicate that other mounts are also loose.

Diagnostic Charts
Charts are a simple way of expressing the relationship between basic logic and a physical system of components. They help discover the cause of a condition in the least time. Diagnostic charts combine many areas of diagnosis into one visual display:
- probability of certain things occurring in a system
- speed of checking certain components or functions before others
- simplicity of carrying out certain tests before others
- elimination of checking huge portions of a system by carrying out simple tests
- certainty of narrowing down the search to a small portion before carrying out in-depth testing

The fastest way to find a condition is to work with the tools that are available. This means working with proven diagnostic charts and the correct special equipment for the system.

Inspection and Verification
1. Verify the customer concern.
2. Visually inspect for obvious signs of mechanical damage.
3. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.
4. If the concern(s) remains after the inspection, determine the symptom(s) and go to the Symptom Chart.

How To Use This Diagnostic Procedure Section
- Noise, vibration and harshness (NVH) concerns have become more important as vehicles have become more sensitive to these vibrations. This section is designed as an aid to identifying these situations.
- The section provides diagnostic procedures based on symptoms. If the condition occurs at high speed, for instance, the most likely place to start is under High Speed Shake.
- The road test procedure will tell how to sort the conditions into categories and how to tell a vibration from a shake.
- A series of Road Test Quick Checks is provided to make sure that a cause is either pinpointed or eliminated.
- Name the condition, proceed to the appropriate section and locate the correct diagnosis. When the condition is identified, the job is partly done.
- Follow the diagnostic procedure as outlined.
- Quick Checks are described within the step, while more involved tests and adjustments are outlined in General Procedures.
- Always follow each step exactly and make notes to recall important findings later.

Customer Interview
The road test and customer interview (if available) provide information that will help identify the concerns and will provide direction to the correct starting point for diagnosis.

Identify the Condition
NVH usually occurs in a number of areas:
- tires
- engine accessories
- suspension
- driveline
- air leakage (wind noise)
- squeaks and rattles
- Heating ventilation and air conditioning (HVAC)
- electrical (e.g. motor noise)

It is important, therefore, that an NVH concern be isolated into its specific area(s) as soon as possible. The easiest and quickest way to do this is to carry out the Road Test as outlined.