The distributor cap is not always highlighted as a major part of the vehicle, but this part has a very important function when it comes to the ignition. If your car has trouble starting and you've checked things like the battery and the alternator, it's possible the distributor cap is at fault.
This part gets its name because its job is to distribute voltage across the vehicle's ignition system. Spark plugs are tasked with firing at different times and in a certain order, and it's up to the distributor cap to ensure this goes off without a hitch.
Underneath the cap, a rotor spins past a series of contacts, with one contact corresponding to each spark plug. As the rotor passes the contact, voltage is sent down the connected wire to the corresponding spark plug. In this way, the cap "distributes" voltage to each plug.
Obviously, the critical tasks trusted to the distributor cap means that a faulty one can cause major problems for a vehicle. Distributor caps can suffer from a number of different problems. Due to the fact that it's handling high amounts of voltage, it's easy for power surges to burn or damage the inside of the distributor cap. Moisture can also become trapped inside the cap, which may be why your car doesn't start in the rain.
The cap can be easily located by following the spark plug wires. Before you do any work with the distributor cap, get some masking tape and mark each wire with a number. This way, you know how to plug all of the wires back in when you're done. Unplug the wires once you've marked them.
Once the wires are unplugged, use a screwdriver to remove the distributor cap. Inspect each electrode, which should be located where the wires connect to the cap. Look for signs of burning or oxidation, as this will indicate that the corresponding spark plug is not getting voltage from the distributor cap. Having one of these contacts misfiring will not necessarily prevent your car from starting, but it will reduce your performance. Unfortunately, you cannot replace a single contact - it requires replacing the entire cap.
In addition, be sure to look inside the cap. If there are black streaks on the interior walls of the cap, there is likely additional misfiring going on. To be on the safe side, you should replace both the distributor cap and spark plug wires in this instance.
Besides misfiring, the distributor cap may also simply be encountering electrical resistance due to wear and tear. To test this, use an ohmmeter, touching one lead to the center tower on the top of the cap and the other to the inside center of the cap. The ohmmeter should read zero, or close to it, to signify little resistance. Otherwise, the cap will need replacing.
Replacing the cap is relatively simple once you buy a new one. The cap should fit exactly where the old one went, and then it's simply a matter of putting the screws in place. Then, attach the spark plug wires back in their previous place, according to the numbering on the masking tape.
Finally, condensation inside the cap can contribute to car troubles on rainy days. If you see signs of moisture in the cap, use mechanic's solvent, which is available at any auto parts store. The solvent will absorb the moisture. Dry the cap out with a clean rag and you should be good to go.