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How to Spot a Failing Distributor

By Mia Bevacqua, June 21, 2018

On older vehicles, the distributor is the center of the ignition system. It triggers the firing of the ignition coil, helps control ignition timing and routes high voltage to the spark plugs. It contains a cap and a rotor.

distributor

Distributor / Image source

The distributor cap protects the device's internal parts and holds contacts between the distributor's rotor and the spark plug wires. The rotor spins within the distributor, passing current from the ignition coil to the spark plugs in each cylinder of the engine.

If you have a newer car, there’s a good chance it doesn’t have a distributor. Individual coil packs began taking their place on most vehicles in the late 1990s. 

There are a handful of signs that a distributor might be failing. Let’s look at how it works, and how you can tell it’s time to replace one.

How a distributor works

Your distributor applies battery voltage to the ignition coil, where the voltage is increased. High voltage then flows from the coil to the distributor via an ignition wire. As its name implies, the distributor then distributes that high voltage through the spark plug wires to the spark plugs. 

Inside the distributor, there’s a rotor that attaches to the distributor shaft. As the rotor turns, it completes the electrical path between the coil and individual spark plugs. This causes each spark plug to fire as needed. On some vehicles, the distributor can also control ignition timing.

Symptoms of a failing distributor

Here are some signs you might have a faulty distributor:

  • Your check engine light is on: If your car’s computer detects a problem that could affect emissions, such as a misfire caused by a bad distributor, it turns on the check engine light.
  • Your vehicle won’t start: An engine needs three basic things to start up: spark, fuel and mechanical compression. A faulty distributor can keep the spark from happening, resulting in an engine that cranks but doesn’t start.
  • Your car keeps stalling: A failing distributor can prevent adequate voltage from reaching the spark plugs. Since the engine needs that spark to run, a bad distributor can result your car stalling when it idles.
  • Your engine is misfiring: If the distributor isn’t providing enough juice to the spark plugs, it can cause your engine to misfire, which typically feels like the engine is stumbling. It’s usually caused by incomplete combustion. Typically, a failing distributor will cause a misfire on all cylinders.
Get it diagnosed by a professional

 

How to fix the problem

A faulty distributor should be replaced. Prior to replacement, your or your mechanic will need to note the engine firing order. If you don’t, the No. 1 cylinder will have to be set to top dead center on the compression stroke and then matched to the distributor rotor. Also, after replacing the distributor, it’s good to check the base ignition timing.

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