How to Detect a Bad Distributor Cap & Rotor

Written by gary proulx
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How to Detect a Bad Distributor Cap & Rotor
To get the hottest spark to the plugs, be certain that the distributor cap and rotor are in perfect shape. (spark image by Zbigniew Nowak from

The distributor cap and rotor work together to distribute the energy from the coil to each individual spark plug. As the rotor is turned by the distributor shaft, its metal contact passes each individual metal contact on the inside of the distributor cap. This allows the spark energy to travel through the spark plug wires until it reaches the spark plugs. The energy then jumps the plug gap, producing a spark. This spark is what ignites the fuel mixture inside each cylinder.

Skill level:

Things you need

  • Flat blade screwdriver

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  1. 1

    Raise the hood of your vehicle and locate the distributor cap. Inspect it for any signs of cracking or discolouration.

  2. 2

    Start the engine and listen for any unusual clicking noises. These can indicate a cracked cap, or possibly a bad fitting of a plug wire to the distributor cap. The clicking sound is caused by a gap being created in the system and electricity jumping this gap. Badly worn plug wires will also do this.

  3. 3

    Park the vehicle in a shaded area and leave the engine running. Look for any sparks around the distributor cap and wires. When the electricity jumps the gap in the connection, you can often see and hear it.

  4. 4

    Turn the engine off and remove the distributor cap. This is done by either turning the cap screws counterclockwise, or by prying the mounting clips from the side, depending on the type of distributor.

  5. 5

    Inspect the rotor for any signs of corrosion or breakage. Inspect the inside of the distributor cap for signs of cracking. Look for signs of " carbon tracks' inside the distributor cap. These are streaks of carbon which will funnel the electricity away from the metal contact. The presence of any of these indicators requires the replacement of the distributor cap and the rotor.

Tips and warnings

  • Always replace the distributor cap and rotor together.
  • Remember, electricity always follows the path of least resistance. This means that any corrosion or carbon tracks will bleed energy from the system.
  • Use caution around the vehicle's coil, as it produces enough energy to give a painful shock when the engine is running.

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