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Capacitor question

Discussion in 'General Electronics Discussion' started by the_0utsider1, Jun 12, 2015.

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  1. the_0utsider1


    May 2, 2013
    This is small engine question where it still has the old points with a capacitor (they call it condenser).
    My problem is that i replaced it and the motor works but upon testing the original with a simple voltmeter, they appear to be the same.
    Any way to accurately test that it works without specific equipment?
  2. Martaine2005


    May 12, 2015
    Not really. An ESR meter is probably the only real method.
    Gryd3 likes this.
  3. Gryd3


    Jun 25, 2014
    Special meter is best, but how are you currently testing it?
    You measuring resistance and watching it climb, or are you measuring the voltage it's retaining?
  4. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Not really. These things were routinely replaced, bad or not, along with the points and the distributor rotor because (1) it is easy to do so and (2) if it is bad it would save time not having to remove the distributor cap again. Those particular capacitors are damned reliable because they have to be, considering where they are located under the distributor cap and right next to the cam and ignition-coil switch-points. That's a very hot environment after the engine has been running for awhile.

    Most of them fail from excessive leakage, rarely by outright shorting, although that is a typical failure mode for capacitors. To test leakage, you need a power supply, a sensitive current meter, and a current-limiting resistor (in case the capacitor is shorted) all connected in series with the capacitor. There should be at most a few microamperes of leakage with a hundred volts or so applied with the power supply. Ideally, there would be zero measurable leakage current, but hey! This is the real world. These capacitors absorb and release amperes of current when operating. A few microamperes of leakage won't hurt anything.

    The voltage "spike" that occurs in the primary of the ignition coil when the points open (collapsing the magnetic field in the ignition coil and creating a high voltage in the secondary winding connected to the center post of the distributor) must be suppressed (absorbed) by the capacitor for proper operation of the coil and preservation of the point contacts. Without the capacitor connected, this "back EMF" created in the primary can reach hundreds of volts and cause serious arcing and erosion of the points as well as timing problems.

    The best "test" that I know of is "if it works don't fix it." But most mechanics usually do, replacing a good capacitor with a who-knows-what capacitor, imported from a who-knows-where country, with unknown quality control. Keep the old one as a spare in case the replacement prematurely fails. The old one will probably work just fine.
    Gryd3 and Arouse1973 like this.
  5. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Condenser capacitors range from 0.25 to 0.29 micro farads or (uF) Having the same result for both using a DMM should tell you something.... These are high voltage capacitors and you can't test them properly with your bog standard meters.
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