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Logical diagnostic processes

Make the problem come to you by using a common sense troubleshooting plan
Monday, August 1, 2016 - 07:00
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Day in and day out, vehicles come to service bays with problems. These problems could be anything from an illuminated Check Engine light, an electrical problem or even a vibration or noise in the vehicle. How do you get these problems to come to you, to tell you where that elusive problem is and what the real cause is?

In all my years of working on machinery, I don’t think I have found any one single test that was best for all electrical problems or one single way that is best to find an NVH (noise, vibration or harshness) problem or even to find a mechanical problem. There are times when you just need to play it by ear, but one thing for sure: every problem I have ever found had a logical explanation. Following the logic of the system is by far the quickest and most accurate path to the solution of the problem.


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Stop and think for a bit, if we are working on an electrical problem, the electrical circuit will have some sort of logic involved in its design. If the problem is a hydraulic problem, the hydraulic system will be designed with some sort of logic in its design, and the same can be said about a mechanical problem. Many times the quickest way to find the solution to the problem will be to gain an understanding of the logic of the system you are working with.

In this article, I will use two different types of problems to demonstrate what I mean. The first problem is a charging system concern on a 2005 F250 Ford pickup. The vehicle is powered with a 6.0 diesel engine, is using an automatic transmission and generates its electrical power with a single generator charging system. There are 225,000 miles on the odometer and the vehicle is no cream puff. The other vehicle is a 2002 Mercedes Benz ML 320. The ML320 is a nice, clean vehicle with 137,000 on the odometer. The Mercedes has a noise complaint that sounds like a wheel bearing noise.

First, The Ford

 The Ford vehicle has had a problem with the batteries going dead, so a new generator was installed as a first attempt to fix the truck. This didn’t take care of the issue, so the owner installed two new batteries. This second attempt also failed to fix the concern, and the vehicle was brought to my shop.

 The vehicle owner reported the battery light would come on at times and sometimes the light would be dim, other times the light would be bright. Many times when I hear stories like this, I will make note, but most times, I want to deal with the logic of the system, how it works and be able to use sound diagnostic principles to come to the resolution of the problem.  

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