How To: Care for the Distributor Rotor and Cap

Common on vehicles up until the end of the 20th century, the distributor rotor and cap are the most simple and most consistent mechanical means to distribute and feed current to each cylinder. That said, ignition is controlled electronically on newer models, making it more difficult to solve distributor problems if you don't have an electronic scanner at home.

Photo: Marie-Andrée Ayotte

How do you know if you have a defective distributor? The engine will hesitate more than usual at stops, or feel unstable when running, or back-fire. In short, look for any sign that the engine is not running properly.

The distributor cap is subject to wear and tear, so it's good to recondition it when you are tuning-up your car. Give it about 60,000 km (about the same as four spark-plugs change), and then look it over - a well-invested ten minutes of your time.

Take notice of how the high tension wires are arranged before you unplug them. If you have any doubts, you can look up diagrams on the internet. Nothing will suddenly blow up if you reconnect them in the wrong order, just no power output or whatsoever. Don't keep the engine running for nothing and double-check your connections.

There will be three or four screws on the plastic cap. Unscrew them and remove the cap. First, check the plastic casing. Is it cracked? Dried out? Faded? A damaged cap must absolutely be replaced, as it could let humidity in, which is exactly what you don't want, given all the electricity that flows through it. The same goes for the seal, which must also be in pristine condition.

For some cars, the next step is to examine the anode in the centre of the cap. It's usually a graphite anode with a little coil. Does the coil stick out of the plastic? If you press on it lightly, does it spring back? This little anode transmits the current to the distributor cap. Since graphite is brittle, it wears out with time.

Next, we have to change the cap.

On other models, you must check the ignition coil to make sure it is not cracked or if it "leaks". If so, it should be replaced.

When inspecting the underside of the cap, check for worn-out terminals. These connect to the spark plugs wires on the outside of the cap, and there is one for each cylinder. Are they thinner than usual? Do they show signs of wear? If the answer is yes, they have to be changed.

So far, these repairs will cost you between $20 and $150, depending on the vehicle brand, the number of parts that need to be replaced and where you purchase them.

Next, check the rotor. This small plastic piece holds a blade on one end. Made of tightly wrapped metal wires on jobber models, it rubs up against the terminals when it spins, distributing the spark which is then conducted by the high tension wires to ignite the spark plug in the combustion chamber.

If your distributor cap looks fine, your spark plugs have been put in properly, and your high tension wires are in good condition, but if you still have an ignition problem, it may be an issue with your electronic parts. The condenser or the coil might be malfunctioning.

Check for "verdigris" and carefully clean with steel wool. These parts may burn if there is a surge in current.

Once you've replaced these parts, don't forget to close the cap and seal it properly.

Happy tinkering!

Photo: Marie-Andrée Ayotte

Photo: Marie-Andrée Ayotte