How to Test Ignition Capacitors

Written by john yarbrough
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How to Test Ignition Capacitors
Older engines like these feature ignition systems with a capacitor. (Auto Engine image by Andrew Breeden from Fotolia.com)

Ignition capacitors are more common in older vehicles than modern ones, which are most often equipped with electronically-controlled ignition. With age, an ignition capacitor can lose its ability to hold a charge. A leaking capacitor can prevent an ignition system from working properly. Testing the capacitor is helpful when troubleshooting an ignition system. Minimal equipment is needed to perform this test properly. But, for the sake of safety you should know how to service your vehicle ignition system before attempting this task.

Skill level:
Moderate

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Things you need

  • Ignition capacitor
  • Digital multimeter with probe test leads
  • Capacitor checker with test leads
  • 22 AWG solid wire (optional)
  • Wire cutter (optional)
  • Wire stripper (optional)

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Instructions

  1. 1

    Examine the capacitor to be tested. If the capacitor shows any signs of bulging, then replace it. If it does not appear to bulge, look for any markings that would determine the nominal capacitance. This saves having to look up this value somewhere else -- like in replacement part literature or message boards.

  2. 2

    Find the polarity of the ignition capacitor. Locate the positive and negative terminals of the ignition capacitor, which may be indicated by respective plus and minus markings. Some ignition capacitors resemble a bare metal can with a wire lead protruding from one end. If this is the case for the one you are testing, the metal casing is the negative connection and the protruding wire lead is the positive connection.

  3. 3

    Prepare the digital multimeter for the test. Following the instructions supplied with your digital multimeter, turn on its power and know how to change between ohms and volts. Also, make sure you have plugged in your probe test leads.

  4. 4

    Perform a resistance test. Using the ohmmeter function of your digital multimeter, set it to the highest resistance range it can measure. Hold the positive and negative probe test leads to the respective positive and negative connections for the ignition capacitor. The readout on your digital multimeter should indicate an overload, meaning the resistance is too high to measure. This indicates the capacitor does not leak. Any numerical readout would indicate a leaky capacitor. When finished, remove the leads and turn off the digital multimeter.

  5. 5

    Set up the capacitance meter. Turn it on as per the supplied directions and plug in the test leads. If your test leads are of the alligator clip variety, then you will need to use the wire cutter and stripper to prepare two 3 inch lengths of 22 AWG solid wire with about 3/4 inches insulation stripped from both ends. If you had to prepare wire lengths, clip one end of each to the alligator clip test leads.

  6. 6

    Check the capacitance of the ignition capacitor. Touch the positive and negative leads from the capacitance meter to the respective connections of the ignition capacitor. The readout on the meter display should be close to the nominal value, typically within a 10 per cent range. Remove the meter and test leads from the capacitor and turn off when done.

  7. 7

    Make a proper determination. If the capacitor has failed any of these tests, then it should be replaced. However, if it has passed the testing processes, then it is safe to use. If you are troubleshooting a faulty ignition system and the latter case is true, then you can test another component in the system.

Tips and warnings

  • In order to perform the test properly, the ignition capacitor should be disconnected to the rest of the ignition circuit.
  • Always use caution when working with capacitors that may have a charge stored on them.
  • Carefully discharge capacitors with a simple series high wattage resistor and alligator test lead circuit before testing.

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