there, the capacitor is shorted. See Figure 25‑38. An open or shorted capacitor is faulty and must be replaced. An ohmmeter test will not reveal a weak capacitor. Low mfd capacity can only be discov- ered by using a tester specifically designed to test ac capacitors. See Figure 25‑39. These meters are accurate, inexpensive, and simple to operate. Most multimeters now have a capacitor checker built into them, which is used to get the actual mfd reading. meter battery (9 Vdc) will slightly charge one side of the capacitor. When the leads are reversed from one terminal to the other, one side of the capacitor discharges into the meter and the other absorbs electrons from the battery. A digital ohmmeter shows a slight resistance reading, then returns to zero. The resistance reading is slight because the meter battery is weak compared to normal line voltage. Increasing meter resistance can increase resistance for easier readings. See Figure 25‑37. If the ohmmeter reads infinity and stays there, the capacitor is open. If the meter reads 0 and stays OL HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 1010 2 100m 20m 2m 200m 0 200 2k 20k 200k 2M 20M DIGITAL MULTIMETER OL HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 1010 2 100m 20m 2m 200m 0 200 2k 20k 200k 2M 20M DIGITAL MULTIMETER Select multiplier scale A B Goodheart-Willcox Publisher Figure 25‑37. Checking capacitors with an ohmmeter. A—Reversing leads should cause the needle of an analog meter to deflect slightly, then return to zero. A digital meter will briefly show a low resistance reading, then return to OL (overload). B—The very slight needle deflection (or digital meter reading) can be increased for easier viewing by selecting the ×10,000 resistance scale on the meter. 0.00 HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 1010 2 100m 20m 2m 200m 0 200 2k 20k 200k2M 20M DIGITAL MULTIMETER OL HOLD ON OFF OHM DCV DCA A C V ACA COM VΩ A mAμA m 20 m 2 200 1k 750 20 200 2 200 200m 2m 20m 2 1010 2 100m 20m 2m 200m 0 200 2k 20k 200k2M 20M DIGITAL MULTIMETER Zero resistance (continuity) Infinite resistance (overload) Open Short A B Goodheart-Willcox Publisher Figure 25‑38. Capacitor defects. A—An open (burned out) capacitor causes the meter to show an infinite resistance reading. B—A shorted capacitor shows zero resistance (continuity). 506 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.