Chapter 9 Basic Electrical Tests 179 Copyright by Goodheart-Willcox Co., Inc. Voltage Drop Measurements A voltage drop measurement determines the internal resistance of a wire or component. Instead of disconnecting the power source and using an ohmmeter, you can check voltage drops without disconnecting the circuit. Th is is a much faster way of checking a circuit for high resistance. For example, suppose you suspect a switch of having a poor internal connection and high resistance. To measure the voltage drop, connect the voltmeter across the input and output leads of the switch, as shown in Figure 9-24. If the closed switch voltage drop reading is too high (more than about one volt for most switches), then the switch has a poor internal connection. A good switch, when closed, will has little or no voltage drop. Figure 9-25 further illustrates the principle of voltage drop measurements. If the load resistances are equal, the voltage drop is equal across these components. As you may recall from Chapter 3, the sum of all voltage drops equals the supply voltage in a series circuit. Figure 9-26 shows how voltage drops vary with resis- tance. In this example, if a blower switch is set on low, a small amount of current passes through the resistor pack to the blower motor and the motor runs slowly. Th is shows up as a high voltage drop at the low-speed (high-resistance) ter- minal of the switch. With the switch on medium, less resis- tance allows more current to the motor, resulting in higher blower rpm. With the switch on high, the voltage drop is zero because there is no resistance across the resistor pack. Resistance Measurements Resistance measurements are commonly taken when the circuit or component is disconnected or when very accu- rate resistance readings are needed. Figure 9-27 illustrates some examples of resistance measurements. Infi nite resistance, shown with an ∞ (infi nity) symbol, means the circuit path is not complete. Something is pre- venting current through the tested circuit or component. Th ere is no continuity with an infi nite resistance reading. Zero resistance, or no resistance, means a perfect electri- cal path exists between the two test points. Th is shows that there is continuity and a resistance of 0 in the tested circuit or component. Figure 9-28 shows an example of using an ohmmeter to check a resistor pack. Th e wires from the switch and to the blower motor have to be disconnected so that accurate resistance readings can be taken on the resistors in the pack. By comparing your readings to specifi cations, you can determine the condition of the unit. Amperage Measurements Amperage measurements are needed to check the con- dition of various components. Figure 9-29 illustrates how an ammeter reading might be used to check the condition of a motor. Th e measured values can be compared to known good readings. A high current draw might point to a prob- lem producing a drag on the motor, such as bad armature bearings or a short in a winding. A low current measure- ment might point to a problem that is preventing normal current, such as burned motor brushes, poor terminal con- nections, or a bad ground. Figure 9-24. Voltage drop measurements show resistance without requiring you to disconnect components. If the closed switch has a high voltage drop, the circuit has a high resistance problem. Fuse Block Load Switch Closed High Voltmeter Reading Equals High Resistance ~V OFF V Ω mA mA A ~ 16.3V Figure 9-25. A component’s resistance determines its voltage drop, so components that have equal resistance have equal voltage drops. Total circuit voltage drop equals voltages applied. 12 V 0.5 Ω 0.5 Ω 0.5 Ω Voltage Drop Total Resistance = 0.5 Ω + 0.5 Ω + 0.5 Ω = 1.5 Ω Total Voltage = 4 V + 4 V + 4 V = 12 V ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ 4.00V 4.00V 4.00V