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Distributor rotor button and cap replacement
Distributor rotor button and cap faulty?
At the end of the 20th century, the distributor started to disappear from new vehicles as mechanical ignitions were replaced with electronic systems controlled by the engine control unit (ECU). If you have a classic older vehicle it will probably have a distributor.
A distributor directs current from the coil to the spark plugs. This is done in the correct firing order to coincide with the timing required for optimal engine combustion. When this system isn’t working correctly the timing will be noticeably out, the engine will run rough, backfire and be hard to start, if it starts at all.
Distributor caps and rotors can wear over time or be subject to damage. They will need to be replaced periodically over the life of the vehicle. If the distributor rotor or cap are worn or damaged it can cause problems with engine performance and they should be replaced as soon as possible otherwise engine damage could occur.
What is a distributor rotor and cap?
A distributor has a cap on its top and a shaft at its base. The shaft sits inside the engine while the top of the distributor is located on the outside. An o-ring on the distributor shaft seals the unit, preventing oil and pressure escaping the engine. On the top of the distributor cap are ports where spark plug leads attach to the cap.
The distributor uses the crankshaft to spin the shaft. This motion spins a rotor (rotor button) which is situated at the top of the shaft, inside the distributor cap. There are metal contact points on the edge of the rotor and inside the top edge of the distributor cap there are metal contact points. The number of contact points will match the number of spark plugs in the vehicle.
Power from the coil is controlled by distributor points located within the enclosed cap section. These open and close based on a mechanical cam operation on the distributor. When the points are open it produces the high voltage pulse of power.
When the rotor spins past the contact points in the distributor cap it creates a spark. The spark travels up to the leads and on to the spark plugs. This creates the spark plug ‘spark’ required to ignite the air/fuel mixture.
The distributor rotor and cap are made from plastic. If the cap breaks or becomes damaged this can allow water to get and/or cause the distributor to fail. Also, the rotor and cap contact points will wear over time. If the gap between these two points gets too large, electricity won’t be able to jump the gap and no spark will be created.
This will mean the spark plug won’t spark at the ideal time for optimal combustion. Heat will eventually ignite the air/fuel mixture but this is when engine misfires, and deposits and engine hot spots can occur.
Symptoms a distributor rotor and cap needs replacing
- Engine misfire
- Problems starting the car
- Rough idle or odd engine noise
- Overall poor engine performance
How is a distributor rotor and cap replaced?
- Distributor located
- Ignition leads removed and lead configuration identified
- Distributor cap removed
- Distributor rotor removed
- New distributor rotor fitted
- New distributor cap fitted
- Leads fitted onto new distributor cap in the correct configuration
- Car started and timing checked
Tips to remember
The leads need to be fitted in the correct firing order - the same as they were on the old cap. If the leads are mixed up, the firing order and which lead goes into which distributor port will need to determined. Most distributors and leads are numbered to assist in placing it with the associated distributor port.
If the engine doesn’t sound right on start up, the engine needs to be turned off immediately and the leads checked. If they seem correct, then the distributor may have moved. If this is the case, the timing will need to be reset by a professional. The vehicle should not be driven. An AutoGuru mobile mechanic can fix any distributor issue including the timing.
How important is replacing a distributor rotor and cap?
Whenever the engine firing timing is out, you are putting strain on the engine and causing possible long-term damage. The car will not be working at its best and could be difficult to drive, if it starts at all. Best practice is to get the distributor rotor and cap replaced as soon as possible.