How to test a distributor cap

Written by jack hathcoat
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How to test a distributor cap
Testing a distributor cap is primarily a visual inspection. (car engine image by itsallgood from

Some distributor cap designs are conceptually poor, and some distributors are poorly constructed. This is unacceptable, considering that 80,000 volts or more pass through some models. On the other hand, some companies are uncompromising in their products and their caps are superior in every respect. It is vitally important to have a good product installed so the spark plugs receive a strong spark signal from the cap, through the spark plug wire, and finally to the spark plug. You can test distributor with a bit of time and a few tools.

Skill level:

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Things you need

  • Screwdriver set
  • Masking tape
  • Ohmmeter

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  1. 1

    Open the bonnet of the car and follow the spark plug wires to the distributor cap. It is part of the distributor assembly and normally located at the rear of the engine.

  2. 2

    Mark each of the spark plug wires that plug into the cap with a small piece of masking tape with a number on it. Place a similarly marked piece of tape on the cap plug wire towers that correspond to the individual wires. You will unplug these wires, and this helps you to avoid mixing them up during reinstallation.

  3. 3

    Unplug all the wires, and use a screwdriver to remove the cap. Look inside the cap and notice the electrodes that are fitted just under the plug wire towers. These contacts are where the spark is transmitted by the distributor rotor. If they are burnt, oxidised or excessively worn, replace the cap.

  4. 4

    Visually inspect the inside of the cap, paying particular attention to the walls. You are looking for black streaks. These are carbon tracking, and they indicate misfiring. The high-voltage signals are not being effectively sent down the plug wires and they are leaking inside the cap, arcing to ground much like lightning seeks the earth. Replace not only the cap, but the spark plug wires as well.

  5. 5

    Set an ohmmeter to its lowest scale. Touch one lead to the centre tower where the ignition coil wire connects. This is done on the outside of the cap. Touch another lead to the inside contact where the coil connector comes through the cap. Look insdie for the small, round, carbon button. This button is spring-loaded and rides on top of the distributor rotor.

  6. 6

    Read the ohmmeter. The reading should be at or very near zero. This means the spark will encounter little or no resistance as it flows from the ignition coil inside the distributor cap, where it is routed to the individual spark plugs. If there is significant resistance, replace the cap.

  7. 7

    Install a new cap by following the reference marks that were made on the masking tape and placed on the old cap. The new cap indexes on the distributor one way only, usually indicated by a small notch on its base. Reinstall the wires and start the engine to verify the repair.

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