The VEHICLe diagnostic process

Today, I would like to present a process for diagnosis I call VEHICLe. This is not intended to allow a layperson the ability to diagnose and repair his or her own vehicle, but to provide a beginning or novice professional with an outline or a framework for general diagnosis. The panel below outlines the process:

The VEHICLe process for mechanical diagnostics.

 V: A visual check often finds the problem in the shortest period of time, and should be done every time. For example: For drivability diagnostics, check for leaking fluids, connections to sensors, loose hoses and components. For electrical problems, look for loose or rubbing wires, loose connections, broken wires, loose grounds. For vibration or steering issues, check for low tires, rubbing suspension parts, bent parts. Look for recent repairs for possible clues to the present problem.

Next to your brain, your eyes are the most important diagnostic tool you have. Whenever you are looking at a vehicle keep your vision wide, looking AROUND what you are looking AT. Develop the skill of seeing everything in the area you are looking at. Don’t go under a truck to look at a low tire and miss the broken brake caliper that is right in front of you.

E: Make sure you check the easy things very early in the process. Don’t tear into a wiring harness until you know the fuses are good and the battery is hooked up. Don’t check a fuel pump until you know there is fuel in the tank. Don’t check a turbo or exhaust system for a power complaint until you have checked the air filter. These things may seem obvious, but the simple things are the answer in probably 3 out of 4 complaints. Checking them first will save a lot of time in the long run, and is a good habit to get into.

H: If the problem is with a system monitored by the vehicle computer, you should scan for codes, even if the check engine light is not on. Make sure you write them down right away, so you have them to look at without going back to the scantool. Even stored codes can sometimes give you a clue to the problem. Sometimes a code will lead to a solution, even if the problem seems to have nothing to do with that code.

If a code does address the problem at hand, this is a good time to hit the book or the online source for more information. You may find an update, a field change, or a service bulletin that takes care of the problem. You can almost certainly find a diagnostic tree or chart that will tell you how to proceed.

I: Isolate the problem to part of the whole system. This is one of the basic procedures in any diagnostic process. In fuel and electrical systems there is usually a way to eliminate part of the circuit as the problem. For instance, if the lights are blowing fuses because of a short, there is usually a harness connector in the middle of the vehicle somewhere to see if unhooking the rear lights will make the short go away. If A truck fuel system is sucking air and returning bubbles to the tank, you could take the fuel line off before the filter or pump and run the engine from a bucket of fuel to see if a tank connection or tank pickup is the problem. The harder a problem is to nail down, the more necessary it becomes to isolate it until the solution is found.

C: As you check things looking for the problem, don’t waste time checking something more than once. Check what you need to, check it properly and thoroughly, then move on. Don’t check it again unless you do something that may affect it. This again may seem obvious, but sometimes the tendency is to recheck things just to see if maybe you have moved something that made a difference. Avoid this temptation, as it just wastes time. All I am saying is don’t recheck unless you are pretty sure you found and fixed something that will make a difference.

L: The last step in the process is to verify that the vehicle is indeed fixed. Do not take this step lightly or skip it, even for a simple repair. Verification infers that you know exactly what the problem was in the first place, and that requires that you experienced the problem firsthand. This usually means you must have test driven the vehicle yourself, or seen the problem manifested, yourself. It is often impossible for another person to adequately describe a problem to you so that you can determine when it is gone. When the problem fails to appear under the same conditions as were present when you saw it before, you have done all you can do to verify the repair. 

As you can see this process is non specific, and won't necessarily help with any one particular problem. It is intended more as a general starting point and an outline. This set of rules is, however, universal to any diagnostic problem, and is sound practice to cultivate as a habit. 

Thank you for reading,

Bruce 

P.S. as this is original material, kindly cite me as the source if you use it.