Resistance Measurement Quiz

Measure the resistance of the following components: Piece of wire = ______ ohms Piece of rubber hose = ______ ohms

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Using what you learned in the textbook about conductors and insulators, explain your ohmmeter readings from the previous step.

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Many sensors and actuators have a resistance specification. If the internal resistance of the component is too high or low, you have found a problem with the component. Measure the resistance of the primary windings of a direct ignition coil. What does your meter read in ohms?

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Compare your meter readings to factory specifications. What did your meter reading tell you about the component?

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Testing Spark Plug Wires: A good spark plug wire should not have excessive resistance (depending on manufacturer, below 25,000–100,000 ohms). When the carbon strands in the center of a spark plug wire fail, the resistance of the plug wire will increase tremendously. A defective plug wire will often have an almost infinite amount of resistance, and current will not flow through the wire. The engine cylinder with the bad plug wire will develop a “dead miss,” and the engine will idle roughly. Locate the numbered spark plug wires. Measure the resistance of the wires to determine if they are defective. Record your results in the following chart.

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How many spark plug wires were good?

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How many spark plug wires were defective?

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Testing Diodes: A diode is an electrical check valve. It allows current to flow in one direction only. The condition of a diode can be tested with a multimeter set to measure resistance. A good diode will show high resistance with the meter leads connected in one direction and low resistance with the meter leads reversed. A bad diode may be either open or shortened. An open diode will exhibit an extremely high resistance when tested in both directions. A shorted diode will have low resistance in both directions. Measure the resistance of three diodes. Connect the test leads in both directions and record your readings in the chart below. Unless instructed otherwise, set your multimeter to measure resistance. Some meters have test settings specifically designed for testing diodes.

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Explain the condition of the diodes tested in the previous step.

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Connect an ohmmeter to the pickup coil as shown in Figure 3-1. Wiggle the pickup coil wires. If the ohmmeter reading does not remain steady and within specifications, the sensor has failed its test and you would need to replace the sensor.

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