Electrical Troubleshooting Tips

High Resistance

Sometimes an electric motor runs too slow, or a lamp is dim, or a starter barely cranks the engine. These symptoms most often due to high resistance in the circuit. High resistance slows the current to a level too low for proper operation of a particular circuit. In Session #7 we discussed Series Circuits. We learned that as we add more resistance in series with a load, the current decreases. Every circuit is designed to operate at a specific current. If unwanted resistance makes its way into that circuit, it will affect the performance of that circuit.

You might be asking yourself how unwanted resistance can get into the circuit. Usually it is due to loose, dirty, or corroded connections. The circuit ground is the most likely place where this will occur. This is because most circuits are grounded to the body of the vehicle, and many of those grounds points are exposed to the weather. As corrosion builds up, resistance increases and current decreases.

In Fig. 13-5, the ground for the right tail lamp has high resistance due to corrosion. The complaint would be that the right tail lamp is dimmer than the left. The only thing that could cause one lamp to be dimmer than the other (assuming the correct lamp is used) is lower current through that lamp. The only thing that can cause lower current is high resistance in that circuit. The resistance could be anywhere between the splice, where the right lamp circuit branches, to the right lamp ground. However, the ground is the most likely problem area, so that is where we would begin our testing. We can see in this example that the voltage drop across the ground connection is 6-volts. If the ground was good we would measure 0-volts. With the ground resistance dropping (using up) 6-volts of energy, there is only 6-volts for operating the lamp, so it is dim.  By disconnecting the ground connection, cleaning it and reconnecting, we would fix this problem.

This concludes Part 1 of this course. As I mentioned on my intro page, this will be an on-going project. Part 2 of this course will delve into semiconductor devices and some of the basics of electronics and computers as it applies to the automobile.

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