184 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Testing a Transducer Switch Th e internal resistance of a transducer switch changes with a change in a condition. Examples include temperature switches, position switches, and pressure switches. When a transducer switch fails, it stays in the same position (on or off ) even though conditions change. An ohmmeter is normally used to check a temperature switch because the leads to the switch can be disconnected easily. See Figure 9-36. Measure the unit’s internal resis- tance when cold and hot (two conditions). You may have to remove the unit so that you can control its temperature using a hot plate or with a heat gun. You can use a thermometer to measure the temperature of the switch while it is installed in the engine. If the resistance values are not within specifi ca- tions for specifi c temperatures, the temperature switch has failed. A quick way of checking a temperature switch is to bypass it. Use a jumper to connect the two leads that go to the temperature switch. Th is should make the temperature light glow or temperature gauge read fully hot or cold, depending on gauge design. Caution: Never bypass a sensor switch unless you are sure the switch is designed to have almost zero resistance when closed. If a normally operating sensor is designed to have some resistance, even when closed, you could short out the circuit and cause circuit damage when testing. Note: Testing sensors and pressure switches is discussed further in Chapters 21 and 22. Refer to these chapters if you need more information. Testing Capacitors Capacitors are commonly used to absorb fl uctuat- ing voltage or current in a vehicle’s electrical system. Th ey can be used to prevent interference in the radio caused by unsmooth DC current or switches opening and closing. Capacitors were also used in older contact point ignition systems to prevent contact point burning. Warning: A capacitor can store an electric charge. You should always ground the lead on a capacitor before testing it. A capacitor cannot hurt you, but it can startle you enough to make you jump and be injured. You also do not want a capacitor to discharge into and damage your ohmmeter. To test a capacitor, you can use either an ohmmeter or a capacitor tester. To use a capacitor tester, follow the manufacturer’s directions. To use an ohmmeter, disconnect and ground the capacitor’s lead. Connect the ohmmeter leads across the capacitor as shown in Figure 9-37. At fi rst, the ohmmeter should show low resistance as the capacitor takes a charge from the ohmmeter batteries. As the capaci- tor approaches its full charge, the ohmmeter reading should increase to infi nite resistance. Th is shows that it is blocking DC current but storing voltage. Testing Diodes An ohmmeter is normally used to check diodes. A bad diode may have an internal open (PN junction burned and separated), or it may be shorted (PN junction burned and fused). Th e diode should have infi nite resistance in one direction and low resistance in the other. If it does not, the diode should be replaced. Testing Transistors A bad transistor will no longer amplify its base signal because one of the internal junctions has been damaged. Special transistor testers are available, but an ohmmeter can also be used to test transistors. Note: Most transistors in complex circuits require the expertise of a specialized circuit technician. However, a few power transistors in various devices frequently fail and can be checked and replaced in the field. Figure 9-36. A transducer switch may be open or shorted closed, or its resistance may be higher than normal. It is nor- mally easy to remove the wires and measure the transducer’s resistance. You can also use a jumper wire to bypass the trans- ducer switch to see if the circuit begins to function. Transducer Switch Make Sure Resistance Is within Specs for Specific Condition Could Bypass Switch to See If Circuit Begins to Function ~V OFF V Ω mA mA A ~