Cables act as highways through which electricity flows to the spark plugs. In time, they can fail. When they do, voltage needed by spark plugs to ignite the fuel mixture won’t reach the plugs, resulting in hard starting, missing, and/or rough idle. If the problem is severe, the engine won’t start.
As time passes, the integrity of the distributor cap can also be compromised. The distributor cap contains metal pickup terminals that parcel current to spark plug cables. When one or more of these fail, the result is the same as with faulty spark cables.
A distributor cap can also crack, allowing moisture into the distributor, where it can suppress the production of electricity. The result will be an engine that won’t start when it is raining or snowing, or when the relative humidity is high.
Follow this procedure to replace spark plug cables and the distributor cap:
- On self-adhering labels, write consecutive numbers that correspond to the number of cylinders in your engine. Make two sets. For example, if your engine has four cylinders, you should have two labels marked “1,” two labels marked “2,” two labels marked “3,” and two labels marked “4.”
- Look to see whether the ignition system uses an external ignition coil. If so, a cable will extend from the ignition coil to the distributor cap. It, too, should be replaced.
- Buy a replacement set of cables and a new distributor cap from an auto parts supplier.
- Stick the first label marked with a “1” on one of the cables. Trace the cable to where it connects to the distributor and place the other label marked “1” on the distributor tower. Follow the same procedure to identify the other cables with their respective towers.
- Align the new distributor cap with the old one so that distinguishing features and markings on the caps coincide. Then, unlatch the old cap. Some types are held by screws, others by clamps. Turn the screws to release the cap. If the cap is held by clamps, place a screwdriver between the backside of the clamp and the distributor housing and pry against the clamp until it snaps open. Remove the old cap and install the new one in exactly the same position.
- From the new set of cables, select the one that most closely matches the length of the cable marked “1” on the engine. Disconnect that cable from the spark plug and replace it with the new cable. Make sure the new cable is routed to the new distributor cap exactly like the old one. Push the cable into the same slot in the cable holder that the old cable occupied. Attach the cable securely to the spark plug and the distributor. Follow this procedure until all cables have been replaced.
- When you are finished, inspect the setup to make sure cables crisscross. They should not be parallel to one another, or a condition called engine crossfire will result. Crossfire occurs when a cable serving a cylinder draws current from a cable serving another cylinder because the two cables run parallel and close to each other. The spark plug with the induced current can then fire prematurely, which will damage parts in the cylinder. If cables are parallel, disconnect and cross them.
