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In an internal combustion engine, the distributor cap serves to keep the wires in place between the electrodes of a distributor and the spark plugs.

It resembles the lid of a spray paint can, with one central hole on the top, and eight nodes circumscribing the the top, through which the wires pass.

"Down the line, coming down the line
A V6 Merc in blue
Without the sparks or distributor cap
I'll slap them on as it slides through"
-Moxy Früvous

Only older petrol powered cars without electronic ignition have these. The distributor cap passes high voltage current from the coil to the spark plugs of the individual cylinders. For an n. cylinder car there will be n. wires coming out of the top of the distributor cap.
Diesel engines don't have distributors so dont look for one, as the fuel is ignited not by spark plugs but by the compression of the cylinder.

Watch out if detaching the leads from the cap to the spark plugs as they must be put back in the right order. A car trying to fire its cylinders in the wrong order is not a pretty sight or sound.

A non functional distributor can be the prime cause for a non-starting car as if there are dirty points or moisture inside the cap then the spark will short to the body of the cap or not even make it to the lead from the rotor. Water can make it in by steam evaporating from a wet or snowy car or by driving too fast through a deep puddle.

If your car is turning over but not firing check here first. Take the cap off the body of the distributor. Some caps have screws and some have clips.

Check for moisture and use a clean dry rag or loo roll to dry it out if there is any. A small squirt of a water displacer like WD-40 might help if you have some. Gently scrape the metal nodes with a knife or a screwdriver to clean any corrosion off them then replace the cap making sure it is sitting straight and you haven't knocked the rotor arm out of place.
Be gentle when working on the distributor as it can be easy to break the rotor or knock it a little out of place and compound your starting problem.

Try starting your car. If its still turning over but not firing you may want to investigate the condition of the coil and whether the spark is even making to the cap. Sometimes there can be a crack in the coil body which shorts out the spark before it even reaches the cap. The coil might be integrated into the distributor body (common in Japanese cars) or separate and connected by a thick wire

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