Continuity, Open, Short, and High Resistance (Voltage Drop)

The terms continuity, open, short, and high resistance are often used to describe a circuit’s or component’s condition. For example, the wiring harness could be described as having a short or the connector as having an open circuit. Continuity is achieved when an electrical circuit has a continuous and uninterrupted electrical connection and is thereby capable of conducting current and working as designed. No continuity means there is a break in the circuit and current cannot flow past the break. It can be measured with a digital volt-ohmmeter (DVOM) in ohms between two points in a circuit. For example, if a technician suspects that a circuit has a break or bad connections in the wiring, the continuity could be measured between two points. A faulty circuit has high resistance or no continuity. A circuit with continuity has very low resistance. But be careful, checking the circuit for continuity cannot determine all electrical faults. If the continuity test determines that there is a break in the circuit, you can bank on that and track it down to determine the fault. However, if the continuity test determines that there is continuity, you still don’t know for sure that the circuit is good. It is possible that while the circuit has continuity, it has excessive resistance that could affect the operation of the electrical device under normal current flow. Thus, continuity testing is limited in what it can indicate. It is a great test for determining whether a wire or component is open or whether two circuits are shorted together.

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FIGURE 36-9
Short circuit.
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FIGURE 36-10
Short to ground.

The term open describes a low-voltage circuit that does not have a complete circuit and therefore cannot conduct current. In other words, the circuit does not have continuity. For example, when a switch is turned off, the circuit is open and no current can flow. Open can also be used to describe a fault in a circuit. For example, if the fuse is open-circuited, which describes a blown fuse, no current will be able to flow. A multimeter or test lamp can be used to test for an open circuit. An open circuit has infinite resistance—so much resistance that it is not measurable.

The term short, in its purest definition, describes a circuit fault in which current takes a shorter path, resistance-wise, through an accidental or unintended route. The low-resistance fault causes abnormally high current flow in the circuit and may cause the circuit protection devices, such as fuses or circuit breakers, to open the circuit. An example of a pure short would be insulation on the windings within a relay coil that has worn through and is allowing current to bypass many of the windings Figure 36-9. In this case, resistance decreases, amperage increases, and the magnetic field created by the winding becomes weaker, potentially causing the relay to not operate.

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FIGURE 36-11
Short to power.
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FIGURE 36-12
High-resistance fault.

A pure short circuit is not the only type of short circuit. There are three additional types of short circuits that you need to understand. The first is a short to ground. In this case, the circuit has an unintended path directly to ground. For example, if the wire from the brake switch to the brake lights rubs through the wire insulation on a sharp edge of a body panel, the bare wire may make contact with the metal panel and cause a short to ground when the brake pedal is pressed Figure 36-10. A short to ground causes increased current flow and will typically blow the circuit fuse.

Another type of short is a short to power. In this case, the circuit has an unintended path directly to a power source. An example would be two wires in a harness that have melted together, one that supplies power to the blower motor and one that feeds power from the brake light switch to the brake lights Figure 36-11. In this example, turning on the ignition switch would cause the brake lights to come on because power would be sent to the blower fuse and then, due to the short in the wiring, to the brake lights.

Unintended high resistance in a circuit causes a reduction in current flow in the circuit as well as a drop in voltage at the resistance. Both of these cause the intended circuit device to not operate effectively or at all. Unintended high resistance can also cause an overheating condition at the area of resistance, which can melt wire insulation or plastic connectors. This condition can be caused by a number of faults, including corroded or loose harness connectors, wire that is too thin for the circuit current flow, incorrectly connected terminals, and poorly soldered joints Figure 36-12.

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FIGURE 36-13
Voltage drop testing the positive battery cable.

Voltage drop is another name for high resistance, and voltage drop testing is the best way of finding high resistance in the feed side or ground side of the circuit. For a voltage drop to occur, two conditions must present: resistance and current flow. Thus, a voltage drop in a circuit indicates that there is resistance present and current is flowing, or trying to flow, through the resistance. If the high-resistance fault is excessive, it will reduce both the voltage and the current flow in the circuit, affecting its performance. For example, a voltage drop in the headlight circuit will cause the lights to be dim, reducing their performance. A high resistance in the main battery lead to the starter motor will cause the starter to crank the engine over slowly or not at all. The voltage drop can be measured in a circuit by placing a voltmeter across two different points in a circuit while the circuit is being operated. For example, to measure the voltage drop in the main positive battery lead to the starter motor, the voltmeter’s black lead would be placed on the positive battery lead and the red lead would be placed on the main battery lead of the starter motor, and the voltage would then be read on the meter while the engine is cranked Figure 36-13.

 
Technician Tip
Since current flowing through a resistance causes heat, you can sometimes locate the high resistance in the circuit just by feeling the wires and connections. For example, if one of the battery posts is corroded between the post and the battery cable, a voltage drop will be present when the starter is engaged. If you operate the starter for several seconds and then feel each battery terminal, the one with excessive resistance will be warmer than the other terminal.