What is a distributor and how does it work?
The distributor is an electro-mechanical component installed in a car’s ignition system to provide, time, and “distribute” the spark that occurs at each spark plug in an engine’s combustion cylinders. The distributor contains a rotating shaft with a gear at the bottom that engages a gear on the engine’s camshaft. Atop the shaft is a plastic rotor that has electrical contacts molded in. As the plastic rotor turns, it completes a high voltage circuit to each electrical contact, one per engine cylinder, that is molded into a plastic distributor cap. That high voltage then flows along the spark plug wire to the spark plug, creating the spark that ignites the air-fuel mixture in an engine cylinder.
Distributors vary in design: some may have the ignition coil built in, some have vacuum advance mechanisms attached for engine timing, and really old distributors may even have mechanical points. Vintage cars, that is cars older than 25 years, will more commonly be equipped with a distributor. Most modern car designs have replaced the distributor with fully electronic systems controlled by engine sensors for more precise, and more powerful, ignition system operation.
When to consider replacing the distributor:
- Worn or damaged integral parts. Worn shaft bushings can result in wobbling or eccentric rotation of the rotor, resulting in misfires, loss of power, rough idle and poor mileage. If the shear pin breaks that secures the distributor shaft drive gear, complete loss of ignition spark distribution is the result. A failed ignition module that is mounted to the distributor housing may also be more cost-performance efficient to replace the entire distributor.
- Visible oil leaks. Distributors are attached to the engine and consequently, there are seals, both internal and external, that keep oil from leaking at the point of attachment. External O-rings are serviceable but if an internal oil leak occurs, due to a worn shaft, for example, it might necessitate rebuilding or replacing the distributor.
How do mechanics replace the distributor?
Distributors are bolted to the engine block or the cylinder head. The replacement procedure is as follows:
- Once all diagnostic tests are complete and the distributor is confirmed to be faulty, disconnect the battery negative cable.
- Set engine at top dead center on the compression stroke for cylinder no. 1. Mark location of distributor housing and shaft relative to the engine mounting surface.
- Remove primary and secondary ignition system wiring connected to the distributor. Unbolt and remove the distributor. Temporarily cover the hole in the head or block so no foreign matter drops into the engine.
- Install new distributor and new cap and rotor, with the rotor pointing to the cylinder no. 1 firing-order electrode of the distributor cap. Reconnect all wiring and reconnect battery negative cable.
- Start engine and set basic engine ignition timing per original equipment manager specification. Then check timing advance for proper function. Road test vehicle.
Is it safe to drive with a distributor problem?
Yes. A faulty distributor will cause increased fuel consumption, increased emissions and poor engine operation but does not generally create a safety hazard.
When replacing a distributor keep in mind:
- Many ignition system failures will mimic a distributor failure, including faulty spark plugs, faulty wiring, and a worn cap and rotor. Mechanics will always test the distributor directly to ensure it is not needlessly replaced.
- If a distributor is removed for testing and then re-installed, be sure that new o-rings are installed. Re-using old o-rings often results in oil leaks.
- If the distributor is replaced, and the spark plugs and spark plug wires have considerable mileage on them, the best results will be obtained by replacing those components at the same time the new distributor is installed.