182 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Basic Component Tests Th is section of the chapter briefl y explains how to test basic electric and electronic components. Understanding these test procedures will help you later when you study other, more specifi c tests. Testing Connectors A good connector has little or no resistance across its terminals. A bad connector has resistance that decreases current through the connector. Th is reduced current can upset or stop the operation of the circuit that contains the connector. A test light can be used to check for power on both sides of a connector. See Figure 9-30. Th is test might be used when a circuit is completely dead, indicating that power is not reaching a component or section of a circuit. First, check for power going into the connector, as shown in Figure 9-31A. Th en check for power on the other side of the connector. Make sure you check the same wire or terminal. Unless you are testing a weather-pack connector, you can touch the test probe on the terminals through the outside of the connector. If the test light does not glow on both sides of the connector, you have found a high resistance or poor electrical connection. A voltage drop test can also be used to check a connec- tor, as shown in Figure 9-31B. A good connector has little voltage drop. A high voltage drop indicates poor terminal contact. Testing Resistors Various types of resistors are used on the modern automobile. Aft er a period of use, resistors can break down and change resistance. Th is can upset the operation of the circuit. An ohmmeter can be used to test actual resistance. See Figure 9-32. A voltmeter can also be used to check the resistor’s voltage drop. If the resistor is not within specifi ca- tions, it should be replaced. Testing Switches A good switch blocks current in one position and passes current freely in the other position. A bad switch will not turn on and off , or it may have high resistance when on. A switch can be tested using a test light, voltmeter, or ohmmeter. If the switch is still installed, a test light can be used to check for power in the two positions. A voltmeter can also be connected across the switch it should show full voltage drop when the switch is off and little or no voltage drop when it is on. An ohmmeter can be used if the switch is removed or isolated from the circuit. Th e switch should have zero resistance when on and infi nite resistance when off . See Figure 9-33. Figure 9-32. The resistance of all resistors in a vehicle must be within specifications. After a period of service, resistor values can change enough to upset circuit operation. ~V OFF V Ω mA mA A ~ 4.3 kΩ Figure 9-31. Testing a connector. A—Using a test light, you can quickly check for power on both sides of connector. If the test light only glows on one side, a bad terminal connection is indi- cated. B—Use a voltmeter to check the voltage drop across a connector. A high drop shows a high resistance problem. Bad Connector 0 V +12 V 0 V +12 V A B ~V OFF V Ω mA mA A ~ 12.0V Voltage Drop Shows Bad Connection Figure 9-30. A good set of test leads makes it possible to probe all types of electrical connectors without damaging them. To avoid damage, never insert a large test probe into a small con- nector terminal. (OTC/SPX Corp.)