START/RUN CAPACITOR MEASUREMENT PROCEDURE
|4. Turn off all power to the unit. Use the capacitor discharge tool (Figure SP-8-6)|
to discharge the start and run capacitors and any other high-voltage circuit
capacitors that may be used in the unit. (Note that most start capacitors have a
bleeder resistor connected across their terminals. However, you should still
bleed the charge off the capacitor with the discharge tool because the resistor
may be open and may not bleed the charge.)
|All high-voltage capacitors used in the equipment are discharged, including the
run and start capacitors.
|5. Locate and disconnect the wires from the start and/or run capacitor to isolate|
them from the remainder of the circuit. (Refer to the unit wiring diagram.) If
testing a start capacitor, it is recommended that one end of the bleeder resistor
also be disconnected. Inspect the capacitor for any visible signs of damage,
such as bulging or leaking.
|The start and/or run capacitor is isolated from the remainder of the circuit and
prepared for measurement. If the visual inspection reveals a bulged or leaking
capacitor, it should be replaced. Start capacitors usually have a relief plug or disc
in their top that will be bulged or missing if the capacitor has overheated or
|6. Set up the VOM/DMM to measure resistance on the R x 1,000 or R x 10,000|
ohm scale. Connect the VOM/DMM across the capacitor terminals and measure
the resistance as shown in Figure SP-9-14.
|toward infinity or some high value of measurable resistance. This indicates that
the capacitor is most likely good. If it is necessary to find the capacitor's exact
capacitance value, test it further using a capacitor tester per step 8.
VOM/DMM reading goes to zero or a low resistance and stays there. This
indicates that the capacitor is shorted. Replace the capacitor.
VOM/DMM reading indicates infinity. This indicates that the capacitor is open.
Replace the capacitor.
|7. If testing a run capacitor enclosed in a metal case, check for a grounded|
capacitor. Set up the VOM/DMM to measure resistance on the R x 1,000 or
R x 10,000 ohm scale. Connect the VOM/DMM between each one of the
capacitor terminals and the metal case and measure the resistance.
|VOM/DMM reading indicates infinity. This indicates that the capacitor is not
grounded to its case.
VOM/DMM reading indicates a measurable resistance. This indicates leakage to
ground. Replace the capacitor.
|8. To measure a capacitor's exact capacitance MFD value, test the capacitor using a|
capacitor tester. Follow the tester manufacturer's instructions to perform the
|For a start capacitor, the measured MFD value should be -0% to +20% of the
value shown on the capacitor. If the value is not within these limits, replace the
For a run capacitor, the measured value should be ±10% of the value shown on
the capacitor. If the value is not within these limits, replace the capacitor.
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