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Applying principle to reality

To be successful in electrical diagnostics, you have to be solid in your fundamentals
Wednesday, February 3, 2016 - 09:00
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Experts in every field have something in common: They possess the ability to perform at a high level repeatedly. In our field, diagnostic technicians like John Thornton, Scott Manna and Bernie Thompson come to mind as top performers. While automotive is their craft, they share traits with the greats in other areas such as sports and music. The secret to their success started early in each one of their careers with mastering the fundamentals. If you master electrical fundamentals, you will fix cars right the first time on a consistent basis. Here we will take a different look at some fundamental concepts that apply to your daily diagnostic routine, whether you are the newbie in the shop or the seasoned foreman. While principles of electricity work well in the classroom, the application to reality can sometimes be hard to see. Basic principles of electricity are used in a multitude of diagnostic applications whether you realize it or not. Let’s look at some examples.

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Voltage essentials

Whether you are fixing a simple circuit problem or an advanced computer input or output fault, knowledge of voltage and voltage testing is essential. Voltage is a unit of electrical pressure that is necessary for current to flow. Without voltage there is no flow, therefore testing available voltage is typically a good starting point in any electrical diagnostic routine. When using voltage testing, keep in mind that voltage is only the command as the work in the circuit is done by electron flow, or current/amperage. Voltage can be measured digitally with your DVOM or graphically with a graphing meter or lab scope. Knowledge of your equipment is key.

When diagnosing ECU inputs and outputs, always remember that ECUs communicate only in voltage. Sensor inputs can be both analog and digital, but an ECU can only process a signal that is digital or on/off. For that reason, analog to digital (AD) converters are used. A common scenario when diagnosing input faults is that data will not always make sense. For example, if a two-wire analog wheel speed sensor reading jumps between speed values yet the scope waveform at the ECM looks normal, you may have a bad A to D converter.

Knowing how and when to use your electrical diagnostic tools, such as a digital multimeter, is a necessity for getting to the correct diagnosis quickly.
Using voltage drop to test for parasitic draw is built upon foundational knowledge of Kirchhoff's law.

Scoping inputs such as crank, cam, wheel speed, etc. is a great way of diagnosing input problems. While these sensors are outputting a voltage of some sort, it is important to remember that a repeatable pattern’s frequency and signature is often used by the ECU to determine input speed and position. Also keep in mind that triggering and slope come in to play when an ECU is interpreting a signal. I remember a case study a few years back of a wiring repair on a two-wire crank sensor. The tech mixed up the polarity of the wires and wound up with an inverted slope, which led to all sorts of problems for the poor truck owner.

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