Chapter 9 Basic Electrical Tests 181 Copyright by Goodheart-Willcox Co., Inc. • Place the meter in a secure location. Multimeters are expensive, and they can be ruined if they are dropped. Also keep them away from moving or hot engine parts. • Keep the test leads away from hot or moving parts to ensure that they do not get caught in engine fans, burned on hot exhaust manifolds, or suff er other physi- cal damage. • Use the specifi cations in the service manual to determine whether a circuit or component is faulty. Component and circuit resistance, voltage, and current values vary from model to model. • If needed, zero or calibrate the ohmmeter before use to ensure accuracy. • To help fi nd intermittent problems, wiggle wires and carefully use a heat gun or freeze spray on electronic components. Th is can help change the reading to indi- cate the problem. Caution: A heat gun produces enough heat to damage sensitive electronic components. Use the heat gun sparingly and avoid generating very high temperatures around these components. Figure 9-27. These are some examples of resistance measure- ments. A—A wire conductor should have low resistance. B—A coil is basically a conductor wound in a spiral. It should have very low resistance unless it is shorted to ground. C—The ohm- meter should show a high resistance when connected in one direction across a diode. It should show low resistance in the other direction. This shows that the diode is functioning as a one-way electric check valve. Broken Wire No Continuity Wire Has Continuity A Checking Wire Continuity Diode Forward Bias Diode Reverse Bias C Checking Diode Low Resistance Shows Continuity Infinite Resistance Indicates Open B Checking Coil ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ 0.01Ω ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ Ω ∞ 0.00Ω 0.01Ω Ω Figure 9-28. The ohmmeter also measures internal resistance, but the leads must be disconnected so that other components do not affect the readings. Compare this to Figure 9-26. Disconnect from Blower Motor Disconnect from Switch to Isolate from Power LO OFF MED HI 1 = 4 Ω 2 = 2 Ω 3 = 0 Ω Blower Switch Resistor Pack Blower Motor 1 2 3 ~V OFF V Ω mA mA A ~ 4.00 Ω Figure 9-29. Using an ammeter to check a circuit. If current is too high, a low resistance or high voltage is indicated. If current is low, a high resistance or low supply voltage is indicated. Power Source (12 V) Motor or Load ~V OFF V Ω mA mA A ~ ~V OFF V Ω mA mA A ~ Inductive Ammeter Conventional Ammeter + –