A distributor is an integral part of a typical ignition system found in older vehicles. It handles an important job, distributing high voltage from the ignition coil to spark plugs with the help of the cap and rotor placed inside the distributor.

A failing ignition distributor may cause an array of problems to the engine and significantly reduce its performance and fuel efficiency. Here are the most common signs of a bad distributor:

Your check engine light turns on

An improperly operating ignition distributor cannot deliver high voltage to all spark plugs, resulting in engine misfires and, therefore, abnormal emissions. If this is monitored by your engine control module, it will illuminate the check engine light.

The engine is hard to start

To keep the combustion process running your engine needs fuel, air and spark. The absence of spark due to a faulty distributor will make it impossible to run and even start the engine. The engine will crank, but won’t start.

The engine stalls 

A faulty distributor will cause misfiring, so one or more of your engine cylinders won’t work, making the engine run harder and stall when idling.

Your engine misfires

A failing distributor can’t provide enough voltage to every spark plug, resulting in incomplete combustion and engine misfires. As a general rule, engine misfires caused by a bad ignition distributor are noticeable in all cylinders.

Ignition distributor operation basics

An ignition distributor is a complex device with several moving parts inside it. When the engine runs its camshaft spins the distributor’s shaft that features a rotor on its end. As the rotor spins it distributes high voltage supplied by the ignition coil to the spark plugs via spark plug wiring. 

Today’s vehicles rely on ignition control modules and individual coil packs to supply spark plugs with high voltage, however, distributors can be found on most cars produced till the end of the 1990s.

When a distributor needs high voltage to be generated, it applies battery voltage to the ignition coil that increases it and returns to the distributor via a central ignition wire, so the distributor can deliver it to the correct spark plug. 

As it was mentioned above, there is a rotor on the end of the distributor’s shaft. When it spins, the rotor completes the electrical path with one of the contacts under the cap, sending high voltage to the corresponding spark plug via its individual wiring. Some distributors also feature a vacuum or centrifugal advance to provide some advance in ignition timing for better engine performance and fuel efficiency. 

How to fix a bad ignition distributor

Some distributor components such as the rotor and cap can be replaced if the other distributor’s parts are in perfect condition. If they are not, a failed distributor should be replaced. When reassembling your distributor, make sure to note the engine firing order. As a rule, the first cylinder must be set in its top dead center and then matched to the rotor. It is also a good idea to check ignition timing after the installation.

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