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A distributor cap is a component of the ignition system on many older engines.
For your engine to run, a mix of air and fuel are sprayed or injected into the cylinders. There, the air/fuel mixture is compressed by the pistons and ignited by the spark plugs. The rapidly expanding gasses that result from combustion force the pistons downward in their cylinders. The pistons then rotate the engine crankshaft in a way similar to how you might pedal a bike. But these operations do not happen all at the same time. Each cylinder must operate in succession. That means, each spark plug has to fire at a specific time so that the pistons work as a team to pedal the crankshaft.
Spark plugs, to do their job effectively, require a lot of voltage. Therefore, an ignition coil is used to increase the voltage coming from the battery or alternator. On many modern engines, the engine control module (ECM) sends a signal to a coil placed directly on each spark plug. But on other engines, especially older ones, there is but one coil, and it sends power to the spark plugs by way of a set of wires.
Between the coil and the spark plug wires lives the distributor that “distributes” electricity to each spark plug in its turn. On top of the distributor is a plastic rotor that spins in time with the crankshaft.
The distributor cap covers the rotor on top of the distributor assembly. The spark plug wires are attached to the cap. As the rotor spins inside the cap, electricity from the ignition coil is passed from the rotor to contacts placed all around the inside of the distributor cap, corresponding with each plug wire. Therefore, as the rotor spins, it energizes each spark plug through the distributor cap with power from the coil.
HOW IS THIS SERVICE PERFORMED?
How is a a Distributor Cap Replacement done?
The replacement procedure for a bad distributor cap depends on the vehicle make and model, as well as the type of distributor featured on the engine. To replace the distributor cap on a mechanical or “conventional” distributor, a technician will take the following steps:
Disconnect the battery and remove any shields, guards, or covers needed to gain access
Remove the ignition wires connected to the distributor
Remove the distributor cap from the distributor, taking care to avoid removing the spark plug wires
Carefully remove the spark plug wires one by one and either marking their location or placing them in the same location on the new distributor cap
Remove and replace the rotor (in most cases)
Install the new distributor cap and reconnect the battery
Start the engine and set the ignition timing according to the manufacturer’s specifications
Check the timing advance
Replace all other components that were removed for access
Test drive the vehicle
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How do I know if my vehicle needs a new distributor cap?
Most newer vehicles do not have a distributor. Instead, they feature what is known as a distributor-less ignition. If your engine does have a distributor, it will fall into one of two categories - conventional and electronic. While the way each of these distributors does its job might differ a bit, they both rely on a distributor cap and show similar symptoms when the distributor cap goes bad. You might need a new distributor cap if you notice one or more of the following:
Your engine lacks power, sputters, and/or stalls
Your vehicle will not start (or it stalls and will not restart)
You hear backfiring
The check engine light comes on
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Other questions customers ask
Can you just replace the distributor cap?
While it is possible to replace only the distributor cap, it is usually recommended to replace the rotor at the same time. It is also common to change the points (on older distributor models) and even the condenser. Spark plug and coil wires are also inspected when this service is performed.
Should I replace the distributor or just the cap and rotor?
Distributor cap and rotor replacement are common service items on older engines. Replacement of the distributor assembly, however, is far less common. It is also more complex and costly. There is no need to replace the distributor if only the cap and rotor are worn out.
What causes distributor cap failure?
The most likely cause of a bad distributor cap is carbon that builds up inside the component. The high voltage electricity that is passed between the rotor and the contacts inside the distributor cap can cause erosion and carbon buildup.
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