Logical diagnostic processes

Aug. 1, 2016
Following the logic of the system is by far the quickest and most accurate path to the solution of the problem.

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.

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.  

Before any work is started on a project like this, the first step should always be to learn how the system works. Without this critical piece of information, time can easily be wasted and good parts can easily be replaced with no fix for the problem.

There are two ways to get this important information. One way is to use your service information and read the description of operation on the system, in this case, the charging system. The other way is to print out a wiring diagram and do a little study. Many times a picture (the wiring diagram) will be worth a thousand words and will be of more value than doing the reading.

Figure 1

Figure 1 shows the wiring of the charging system. From the wiring diagram it is easy to see this generator is not computer controlled. This only leaves three different places to test; the B+ terminal, terminal I and terminal F. Step one was to test the battery voltage with the engine running. The voltmeter read 12.2V, indicating that the generator is not charging the battery. If another new generator were installed the charging problem would most likely be fixed, but let’s spend a few minutes figuring out why this generator is not charging. I want to know if the problem is with the generator, or in the vehicle wiring.

To make the wiring diagram a little easier to understand, in Figure 2 I added some colors to the different wires in the system. This makes it easy to see the different voltages that should be present. If the system is tested with KOEO (key on engine off) the two wires colored red should be system voltage (B+) and the wire colored blue should have a voltage close to 0 volt. By identifying the voltage with colors, it can make the testing process quicker and more accurate. This circuit is quite simple, although if you are working on a more complicated circuit, the use of a few different colors becomes of great value in time savings.

Figure 2

Back to the charging system problem. When I went to start the vehicle to bring it into the shop and was waiting for the glow plugs to warm, I did not notice the battery light on. That leads me to wonder if the bulb burned out in the cluster, if the fuse that powers the light burned or if there is a problem with the generator. How can this problem be tested quickly without pulling the cluster? Do we want to hook up a scan tool and command a cluster bulb check, or can we use the wiring diagram to lead us to the problem?

Don’t Make It Harder Than It Has To Be

Because the battery light does not come on when the key is turned on, I wonder if the fuse that powers the light is burned. The wiring diagram shows the instrument cluster light is powered by fuse 45, a 10-amp fuse in the power distribution box located under the left end of the dashboard. You can get a test light, pull the left hush panel from the dash, find the fuse and test it, but there is a quicker and more accurate way to accomplish this task. Take a look in your service information power distribution wiring diagram for fuse 45 and find out what other things are powered by that fuse. In Figure 3, I used an OE wiring diagram of this circuit. Several things are shown powered by this fuse and of these, the 4-wheel drive control module is the easiest to find and operate. With the key on, just turn the 4-wheel drive knob one click to the right and see if the 4x4 light comes on. If it does, you are finished with your fuse testing.

Figure 3

Another way to find the fuse integrity and to test the whole circuit, fuse, bulb and the wiring is to turn the key on and then test for voltage at the light green/red wire (terminal F) at the generator. This wire is very easy to access since it is right on top of the generator. By testing this way, we will know if the fuse will flow current, the bulb in the instrument cluster is not burned out and the wiring is capable of flowing current all the way to the generator. All the system needs to light that bulb is a ground. There is no one correct and best way to do this sort of testing. Use a logical testing method and you will be good.

By applying some logic to the wiring diagram, we know the charge light needs a power and ground to work. In this case, the voltage regulator housed in the generator is supposed to supply the ground to the light. In this case, my voltmeter shows system voltage at the light green/red wire. Figure 4 is showing I have used insulation piercing probes to attach my meter leads to the wires. I do this instead of back-probing the plug because I do not want to disturb the circuit by pushing on the plug. I want my test equipment to find the problem for me, instead of me doing a temporary fix when backprobe pins are pushed into the back of the plug. Any time an electrical problem mysteriously goes away, or seemingly fixes itself, I always wonder what I have done, and how long it will be before the problem rears its ugly head again. I want the electrical problem to come to me, instead of me chasing the problem around.

Figure 4

With a voltmeter hooked to the light green/red wire and the key turned on, my voltmeter showed system voltage and the charge light was not lit. At this point, what I know about this circuit is more important that what I don’t know. Since I have B+ voltage at the I terminal wire (light green/red), I know the circuit is intact and working as designed. The only thing missing is the ground through the voltage regulator. I’m wondering if the wire is broken at the generator plug or if the pin fit on the plug has been compromised. With one stiff push on the generator plug, I heard the nice sharp “click” of the plug snapping into its place. With the KOEO, the volt meter reads 0 volt and the battery light is now illuminated When the engine was started, the battery light went out and the battery voltage showed above 14 volts. The problem is fixed.

On To The Mercedes

The next vehicle problem is a Mercedes Bens ML 320, which has a complaint of a noise in the rear of the vehicle. The vehicle owner thinks the noise is caused by the rear wheel bearings.

When test-driving the vehicle, the vehicle has that distinct humming of a noisy wheel bearing, even going around corners causes the noise to change. With the vehicle on the hoist and all wheels turning, I used a stethoscope to listen to the wheel bearings, and the right rear bearing is the one with the most noise. Am I sure this is a wheel bearing, or could it be something else masquerading as a rough noisy wheel bearing? There are tools that make finding noises and vibrations like this very accurate, but there are times when a sharp eye and sharp ear will accomplish the same job.

Because every vehicle going through my shop gets a vehicle inspection, I found a few interesting things. Many times when doing an undercar inspection, I will have the engine idling so I can listen for vibrations and exhaust noises. This time while doing the inspecting, the engine was idling and I moved the exhaust pipe a little while checking for proper exhaust mounting and things, and I heard a slight rattle, sort of a buzzing sound. A closer look found the exhaust from the passenger side engine bank was rubbing on the front suspension torsion bar, as seen in Figure 5. Is there a possibility the noise that seemed to be rear wheel bearings is caused by this exhaust pipe and the sound is being transmitted through the frame and suspension to the rear wheel bearings?

Figure 5

Search For A Logical Answer

As I have mentioned before, every problem has a logical answer, so looking for the logic in this problem will lead to the fix. While inspecting the exhaust system I did not see where any repair had been done to the exhaust pipe. There were also no rusted exhaust pipes, as well as no damage from the vehicle being backed into something, which can ram the exhaust pipes forward and bend them. Since there is no physical damage to the exhaust, why has it been lowered to the point of rattling on the torsion bar?

In front of the exhaust pipe is an engine mount. The right side engine mount is the one that gets compressed when the engine makes power; the left side engine mount is the one that gets torn apart from the engine torque. Inspecting the right side engine mount, I found the mount was compressed to the point where the steel pieces of the engine mount were hitting each other. Figure 6 shows the engine mount with less than 1/8 inch between the steel top and bottom parts of the mount. Keep in mind that this engine mount is out of the vehicle with no weight load on it. Looking at the bottom of the engine mount, I saw a shiny spot where part of the steel had been bumping on the vehicle frame. The collapsed engine mount was the cause of the exhaust pipe hitting the front torsion bar. By using a logical inspection process, the process led me to the collapsed engine mount.

Figure 6

A new engine mount has about 1/2 inch between the two parts of the engine mount. With a new engine mount installed, the noise was gone and the exhaust pipe had about  1/2 inch clearance between the pipe and the torsion bar.

In talking with the vehicle owner, both rear wheel bearings had been replaced a few months ago trying to get rid of this noise. I was glad to find the real cause of the seemingly rear wheel bearing noise. This made the cost of the repair a lot cheaper than replacing the rear wheel bearings, and it actually fixed the real problem. There are times when taking your time on both your vehicle inspections, and your analysis of electrical problems will be a great investment in both our time and money.

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