Chapter 9 Basic Electrical Tests 183 Copyright by Goodheart-Willcox Co., Inc. Testing Fuses A fuse can be checked quickly using a test light. See Figure 9-34. Th e test light should glow when connected to either side of the fuse. If the light glows only on the feed side, the fuse is blown. If you fi nd that the fuse is blown, you should check for a short that may have blown the fuse. However, sometimes fuses fail aft er prolonged service, even if there is no short in the circuit. A voltmeter can also be used to check a fuse, but it is not as handy as a test light. If the meter is digital and makes an audible signal when the voltage changes, you can check the fuse without too much trouble. Again, voltage should be present on both sides of the fuse. Testing Variable Resistors A good variable resistor should exhibit a gradual change in resistance as you turn its shaft or move its slide. A bad variable resistor might be open (infi nite resistance) or shorted (zero resistance). It may also be stuck at a specifi c resistance value, or its resistance may not vary properly with shaft or slide movement. In most conditions, an ohmmeter is the most accurate way of checking a variable resistor. Th e service manual usually gives resistance specifi cations for diff erent variable resistor positions. If tests show diff erent values, the variable resistor is bad. See Figure 9-35. You can also check a vari- able resistor by testing voltage drops, but specifi cations are not given this way. Figure 9-33. A bad switch may be shorted closed or burned open. Resistance should be zero when the switch is closed and infinite when the switch is open. ~V OFF V Ω mA mA A ~ ∞ Ohmmeter Reading Changes from Zero to Infinite Move Switch On and Off Ohmmeter Leads Figure 9-34. Using a test light is the easiest way to check for a blown fuse. A blown fuse will light the tester on the feed side, but it will not light the tester on the output side. Test Light Ground Test Probe Figure 9-35. A variable resistor or potentiometer should have a specific resistance reading at each position. Compare your readings with those given in the service manual. A—A variable resistor at a low-resistance position. B—A variable resistor at a high-resistance position. (Union Electronics) B A