Electrical Problem Analysis

When vehicles with electrical concerns come to your bay, do you run and hide or jump up and down with joy with anticipation of another electrical challenge? If not, why not? Electrical problem analysis can be very rewarding if the technician is properly trained and has the proper equipment.

Ah, the proper equipment? This could be something as simple as a good quality digital multimeter or as advanced as a labscope with all the trimmings. While we are talking about equipment, let’s not forget about the need for a great source for wiring diagrams. Without the proper wiring diagram (the electrical road map), the technician is wandering around in the dark electrical world without a flashlight.

After working with electrical problems for many years, I have found a few things that can help simplify electrical circuits and make your troubleshooting challenges a little less challenging.

• Electricity goes in circles and without a completed circle, it does not go. In other words, the electrical circuit must be complete for the circuit to perform its required task.

• Any ungrounded circuit (open circuit) will have system voltage anywhere in the circuit between the power source and the place where the circuit is open.

• Any simple circuit will have the same current flow anywhere in the circuit.

These few pieces of information seem very simple, but they are very important things to keep in mind when working on electrical circuits. I always smile when I hear someone say, “I have a short to ground, since there is voltage everywhere in the circuit. A short to ground would have one of two things, a blown fuse or a fire!”

When working on late model vehicles, there are several different kinds of circuits. There are computer communication circuits, computer sensor circuits, very high current circuits (starter supply circuits) and high current circuits such as headlight and taillight circuits. Each of these circuits conducts electricity but perform different tasks. This is a very good reason to familiarize yourself with the circuit before you start the problem analysis. Knowing what the circuit is designed to do and the purpose for its function will give you a head start on a direction before you start the testing process.

One great way to get an understanding of the circuit function is to study a wiring diagram. This might give you enough information to start the diagnostic process. You might need to dig a little deeper and research how the component or system works. Armed with this information, the technician will have a good idea which tool will be needed to analyze the problem and what settings on the meter to use. Time spent in research is time well spent and in the end will be a great value.

In the photographs, I have listed a few pieces of diagnostic equipment: bulb test light, headlight bulb, graphing multimeter or lab scope, digital volt/ohm meter and a low amp current probe. Each piece of test equipment has its place, and each diagnostic tool can be used for multiple purposes. Have you ever considered using a test light or a headlight bulb as a power limiting source for a shorted circuit or a circuit with an intermittent short to ground?

Each circuit in an automobile has one paramount purpose: to conduct current. Sometimes this current will be very low (in the micro amp range) and some circuits are designed to carry current in the hundreds of amps. This leads to a different diagnosticapproach on each kind of circuit. Testing a

computer or sensor circuit for integrity will use the same testing principle as testing the integrity of a starter circuit, but the technician should use a different tool for the proper testing procedure. Failure to do so can result in component damage to some very expensive parts. For the very low current computer sensor circuit, an ohmmeter would be the tool of choice to test its integrity, but for the starter circuit, using an ohmmeter is not a very accurate way to test the circuit. When testing a starter circuit, I would recommend using the starter motor for the load, and using either a digital voltmeter, or a graphing voltmeter and test the voltage drop of the circuit. The starter motor will need to be turned on to get this valuable information. By using the voltage drop method of testing

the circuit the circuit is tested dynamically, under real world working conditions. Years ago, this test was referred to as a “dynamic resistance” test. In other words, the circuit is tested while it is working (dynamically) and the voltage drop method of testing will tell you the actual resistance of the working circuit.

Most electrical circuit problems can be narrowed down to four different kinds of problems: open circuits, short to ground, internal shorts between two wires in a harness or high resistance in the circuit. Each of these problems can be found easily, if you use the proper test equipment and proper test procedures. The object of diagnostics is to get the problem to come to you. Each type of electrical problem will require a different approach, although every electrical problem analysis should always start with learning how the circuit works, what the circuit does and how the circuit is laid out in the vehicle. Every piece of this information is found in the published service information for the vehicle.

Fun with Fuse 15
As an example of an intermittent short circuit problem, I will use a case study from a 1997 Subaru Legacy/Outback. This Subaru has been ridden hard and put away wet many times. The vehicle lives on top of a mountain and is driven two miles each way on washboard road every time it comes to town. The odometer is showing 208,000 miles and still has the original automatic transmission. The customer concern is that intermittently the instrument cluster lights will go out, the high beam headlights will go out, numerous instrument cluster lights will come on and if driven long enough, the engine will finally stall.

Looking into the driver’s compartment, the floor is littered with several blue 15-amp fuses. There is also a good supply of fuses in the ashtray, too. This vehicle has been driven for 25 miles without the fuse blowing. Now since driving it until it blows the fuse will bring us no closer to knowing when and why this problem occurs, the best thing to do is to first learn how the circuit works and then what the circuit powers.

There are several diagnostic paths that can be taken on a problem like this. Some things I have seen used by techs to troubleshoot a shorted circuit include installing a higher amperage fuse to power the circuit or installing a circuit breaker in place of the fuse so the circuit breaker will still protect the circuit. This will close and supply power to the circuit and after all, this is an intermittent problem and we all know intermittent problems are hard, if not impossible to find. I do not like any of these diagnostic approaches so let’s think out of the box a little and use a solid diagnostic approach to the problem.

Any time I work on intermittent electrical problems, whether they be short to ground problems or intermittent open circuit problems, I always am afraid I am going to move a wire or wire harness and the problem will go away, never to resurface no matter what I do. This always leaves an unfixed car leaving the shop and wasted labor that is unbillable. Starting with a logical diagnostic process for this concern begins by examining a power distribution wiring diagram for fuse 15 (the location of the intermittent fuse failure), and it uncovers a lot of useful information. Some things I pick up on right away include learning that fuse 15 powers only small electrical loads like some relays and the instrument cluster, the fuse only has system voltage when the key is in the start or run

position and everything fuse 15 powers is located under the dash. This gives me a good direction where to look for the intermittent short to ground. Keep in mind, I want this problem to come to me. I do not want to take this whole dash apart and start opening up wiring harnesses to find the problem.

To accomplish this task, the system needs to be tested dynamically, which means to be able to make the short to ground happen in the shop where I can easily capture some data and pinpoint the location of the problem without having to take the car apart. The first thing is to install a fuse loop in place of fuse 15. This allows a low amp current probe and lab scope to record any current flow on the circuit.

Since fuse 15 will blow when the car is being driven on rough roads, the way to get this problem to happen will be to cause some vibration to the dash. With all loads on the circuit turned off and the ignition key turned on, I started using my fist to bump the top of the dash. Within a few seconds, the labscope started recording amperage spikes as seen in Figure 8. At this point, I know the problem lies inside of the dash, but I’m not sure exactly where.

The dash thumping was increased in intensity and the location was narrowed down to a place between the right end of the instrument cluster and the radio. Figure 9 shows the intensity of the amperage spikes increased as the vibration of the dash got closer to the place where the harness was shorting to ground. At this point, the 15 amp fuse burned and it is time to pull the instrument cluster to gain access to the wiring harness that lies behind the dash.With the instrument cluster removed, the wiring in the dash can be seen easily. There is only one large wiring harness that runs across the dash and the dash is all made from plastic. A headlight bulb was installed in place of fuse 15, the current probe was clipped around one of the headlight wires and the harness was moved very gently. A

few seconds of gently moving the harness made the headlight flash and the waveform in Figure 10 was recorded. At this point, I can make the electrical short happen and with the headlight limiting the current flow in the circuit, there is no possibility of doing harm to the harness, or setting the vehicle on fire.

Back in the dark dungeon in the interior of the dash is one small steel brace for the radio. This wiring harness had been rubbing on that one little black steel brace for 200,000 miles and finally had rubbed a very small hole in the insulation. When examining the harness, the hole in the insulation was about the size of a straight pin head. There was just one little tiny copper speck showing through the insulation, which was on the back side of the harness and I had to use a mirror to see the problem.