180 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Rules for Using Multimeters As you work with multimeters to diagnose electric or electronic circuit problems, keep these rules in mind: • Make sure the test leads are connected properly. An incorrect connection may damage electronic compo- nents or the meter itself. • Set the meter to read a high enough value. If you have the meter set on 0 to 10 amps, for example, and you try to measure 100 amps, you might burn up the meter or blow its fuse. • Use a digital or digital-analog meter to check computer circuits. Figure 9-26. Example of how voltage drops can be used to check internal resistances without disconnecting components from a circuit. A—The branch of the resistor pack for the lowest blower speed shows a high voltage drop and a high resistance. B—The branch of the resistor pack for medium blower speed has a medium voltage drop and a medium resistance. C—The branch of the resistor pack for high blower speed must have low resistance to pass high current. The voltage drop is also low. LO OFF MED HI Lower Blower Speed 8 V Drop High Resistance Blower Switch Resistor Pack Blower Motor LO OFF MED HI Medium Blower Speed 4 V Drop Medium Resistance Blower Switch Resistor Pack Blower Motor LO OFF MED HI High Blower Speed 0 V Drop No Resistance Blower Switch Resistor Pack Blower Motor ~V OFF V Ω mA mA A ~ 8.00V ~V OFF V Ω mA mA A ~ 4.00V ~V OFF V Ω mA mA A ~ 0.00V A B C