The distributor cap should be removed for visual inspection. Check the cap for:
Inspect the distributor cap and rotor. Courtesy of American Honda Motor Co., Inc.
- Distributor cap locating tab. This tab assures proper location of the cap onto the distributor housing.
- Distributor cap holddown. The distributor cap must be securely attached to the distributor housing to prevent contamination and to assure its location.
- Dirt and water in the cap. These contaminants will require additional voltage in order to fire the spark plug. If available voltage is not enough to overcome this resistance then no spark will occur at the spark plug.
- Center carbon insert. All of the high voltage coming through the coil on its way to the spark plug must pass through this insert. Any additional resistance caused by wear or contamination can cause the engine not to start or to misfire.
- Tower inserts. The inserts are made from aluminum or bass so it is possible for them to oxidize. Aluminum oxide is an abrasive that can cause excessive damage to the distributor. If the insert is made from brass then the oxidation will appear green in color.
- Cracks. If any cracks are present on the distributor cap then replacement of the part is required.
- Carbon tracking. Carbon tracking indicates that the high-voltage of electricity has found a low-resistance conductive path over or through the plastic. The result is a cylinder that fires at the wrong time, or a misfire.
Also check for:
- Physical or electrical damage is easily recognizable.
- Electrical damage from high voltage can include corroded or burned metal terminals and carbon tracking inside distributor caps.
Things to look for when inspecting a distributor cap.
- If the distributor cap has a mild buildup of dirt or corrosion, it should be cleaned. If it cannot be cleaned up, it should be replaced. Small round brushes are available to clean cap terminals.
- Wipe the cap and with a clean shop towel, but avoid cleaning these components in solvent or blowing them off with compressed air, which may contain moisture. Cleaning these components with solvent or compressed air may result in high-voltage leaks.
- Check the distributor cap and housing vents. Make sure they are not blocked or clogged. If they are, the internal ignition module will overheat. It is good practice to check these vents whenever a module is replaced.
Distributor cap replacement instructions vary, so always follow the manufacturer's instructions.
General replacement procedures are as follows:
- For spring-loaded caps, unsnap the clips and remove the cap. When installing the new cap, make sure the clips are snapped back into place.
- For spring-loaded j-hook caps, push down and turn. It will unlock. Lift up to remove it. Position the new cap in place, then push down and turn to lock.
- For caps with screws, use a screwdriver to loosen the two screws. Install the new cap and use the screwdriver to tighten the screws. Don't overtighten.
It is a good idea when replacing the distributor cap to remove the cap with the ignition wires attached. After you have installed the new cap, transfer the wires one by one onto the cap in their correct position. This way the firing order is accurate. If, for some reason after you replace the distributor cap, your vehicle does not function normally, check your firing sequence.
The distributor cap is mounted on top of the distributor assembly and an alignment notch in the cap fits over a matching lug on the housing. Therefore the cap can only be installed in one position, which assures the correct firing sequence. There is a tab or notch somewhere on the distributor body that must align somewhere on the distributor body that must align with a corresponding tab or notch in the cap. Once again, do not force something.
The rotor should be inspected and/or replaced when the distributor cap is replaced.
Each cylinder of an engine must produce power once in every 720 degrees of crankshaft rotation. Each cylinder must have a power stroke at its own appropriate time during the rotation. To make this possible, the pistons and rods are arranged in a precise fashion. This is called the engine's firing order. The firing order is arranged to reduce rocking and imbalance problems. Because the potential for this rocking is determined by the design and construction of the engine, the firing order varies from engine to engine. Vehicle manufacturers simplify cylinder identification by numbering each cylinder.
Examples of typical firing order.
Regardless of the particular firing order used, the #1 cylinder always starts the firing order, with the rest of the cylinders following in a fixed sequence. The ignition system must be able to monitor the rotation of the crankshaft and the relative position of each piston to determine which piston is on its compression stroke. It must also be able to deliver a high-voltage surge to each cylinder at the proper time during its compression stroke. How the ignition system does these things depends on the design of the system.