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Master the basics of voltage drop to improve your diagnostic ability

Tuesday, December 13, 2016 - 09:00
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The modern vehicle contains over a mile of wire, up to 100 modules and thousands of electrical connectors. Looking at a wiring diagram of one of these vehicles can be a bit overwhelming. Without a doubt, the complexity of these vehicles has reached an all-time high. With onboard radar and laser detection systems, it may seem like you are working on some sort of aircraft or spaceship, and not that of a road vehicle. So how does one navigate these complicated systems? When diagnosing this amount of wire, modules and connections, one can no longer guess at where the problem may lie.  

Establishing a solid foundation

Whether you are working on a spaceship or a modern vehicle, it will be necessary to understand basic electrical principles. Perhaps one of the most important principles in diagnosing electrical systems is the voltage drop. This is where voltage is consumed by resistance. 

Figure 1

Voltage is electric energy or electric pressure. This electric energy is the potential difference between two points with a charge present between them. This charge arises when an atom becomes imbalanced by losing an electron as seen in Figure 1. In most atoms, there are an equal number of protons (positive charge) as electrons (negative charge) and the opposite charges of these two kinds of particles balance out or are in equilibrium. It is possible to break electrons free from their orbits about the nucleus, causing an imbalance in charge. In order for the atom to lose an electron the atom must have an outside force acting on it. This force must exert more energy on the electron than the bond between the nucleus of the atom and the electron. This force will be a large energy source that can be caused by light, magnetism, heat or chemical energy. As this energy is applied to a number of atoms, more atoms become imbalanced causing a greater difference of charge. When this occurs a potential energy is created. This energy is referred to as electrostatic potential, which is the difference of charge in an electric field. This charge is typically measured in units called voltage. The greater the potential difference or voltage the greater the work can be done by the electrical energy. In order to determine how great the potential difference is, or how much voltage is present, a measurement must be taken.

Figure 2

In order to make a voltage measurement a reference must be provided. The voltage measurement is based on two probes: positive and negative. The potential difference between these probes is all that is measured. This potential difference is converted into units of voltage. In order for the total voltage in a system to be measured a fixed reference point such as the earth (ground) must be used so the total potential can be calculated. When measuring the voltage in a vehicle, the lowest point of potential will be the negative post of the battery. Therefore the ground probe will be connected to the battery negative post (ground), while the positive probe will be connected to the circuit to be tested. The difference between the probes, or voltage units measured, will then be displayed on the measuring device as shown in Figure 2. The entire test is based on where the two probes are placed. This is very important because the only results displayed on the measuring instrument is the difference between the two test points.

Ohms, amps and volts

When testing a circuit several methods of measurement can be used; Ohms, amperage and voltage. Ohms is the measurement of the resistance that is within a circuit. The problem with using this type of measurement is that the circuit under test will need to be open. This means the ohms can only be measured in a circuit that is not under load; therefore the resistance in ohms can measure low (good) but can be too high under a loaded condition. One such example is if an 18-gauge wire were used as the battery cable on an automotive starter. This wire would ohm with very little resistance but under the load of the starter motor draw would create a large amount of resistance to the current flow. This is caused by a limited area of the conductor that can flow electrons. With a low electron flow rate such as with an OHM meter, this limited area would not be seen. However, this limited conductor area when exposed to high current flow will allow resistance to occur to electron flow.

In the case of the starter motor, the resistance would be so high to a high electron flow rate that the electric energy (voltage) consumed by the wire would release enough heat to melt the wire, thus breaking the wire and the electrical circuit. This could also be illustrated with a highway in a large city. If one was to watch the flow of vehicles at two o’clock at night, there would be very little resistance to the vehicles moving on the highway. This is due to very few vehicles occupying the highway. But if one was to watch the same highway at the same location at seven o’clock Monday morning, you would find a totally different outcome. The vehicles would be backed up on the highway due to high resistance. The highway area and number of lanes did not change between these two times. The change that occurred was the number of vehicle occupying the highway. Now think of the highway as the electrical conductor and the vehicles as the electrons. In order to see resistance, the load must be high. So the use of an OHM meter to measure vehicle electrical circuits is not a good choice for most vehicle diagnostics.  

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