What Does a Distributor Do in a Car?

Your car’s distributor is responsible for sending the high-voltage current from the ignition coil to the spark plugs, allowing the engine to ignite.
Written by John Davis
Reviewed by Bellina Gaskey
An automotive distributor’s primary function is to send a high-voltage current from the ignition coil to the engine’s spark plugs in the correct firing order. The distributor’s other main job is to control the on-off function of the ignition coil by sending electronic signals to the ignition module. 
While modern cars are built without distributors, many older vehicles with distributors are still on the road. In these cars, the distributor is a vital component of the ignition system and is essential for controlling ignition timing. If you drive an older vehicle, a faulty distributor can cause various problems, and if it fails entirely, you might be left stranded. 
This article will cover everything you need to know about automotive distributors so you can experience a new level of confidence behind the wheel.
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What is a car distributor?

A car distributor is essentially a rotating switch used in vehicles with mechanically-timed ignition
The distributor serves two main purposes. The first and primary function of the distributor is to send a high-voltage current from the ignition coil to the engine's spark plugs in the correct firing order
As its name implies, the distributor distributes electrical signals to the spark plugs, allowing them to provide the spark necessary for ignition and internal combustion engine function.
The secondary job of the distributor is to control the opening and closing of the ignition coil's circuit. 
On some engines, the distributor is also responsible for controlling the oil pump.
Automotive manufacturers have replaced distributors in modern vehicles with "distributorless" electronic ignition systems. Cars with these systems use individual ignition coils on top of the spark plugs or coil packs with multiple connections. In these vehicles, the engine control unit (ECU) detects the crankshaft's position and sends signals to the ignition coils. 

How does a distributor work?

In vehicles that use mechanically-timed ignition, the ignition system contains an ignition coil, rotor, spark plug wires, spark plugs, and the distributor. The distributor's job is to control the opening and closing of the ignition coil's primary circuit and then send the electrical signals to the spark plugs in the proper firing order. 
The top of the distributor—known as the distributor cap—houses a rotating arm (sometimes called a rotor button) at the top of the distributor shaft. On most vehicles with overhead valve engines, the distributor shaft's movement is controlled by the camshaft
Inside the distributor cap are brass contact points, which are sometimes referred to as contact breaker points. Each point connects to a terminal on the side or top of the distributor cap. The terminals, in turn, connect to the spark plug wires.
As the rotating arm spins inside the distributor cap, a spark from the ignition coil travels up the shaft and down the rotor. When it reaches the cap, the spark jumps from the distributor rotor to one of the brass contact points. The ignition spark then travels down the spark plug wire to the corresponding spark plug. 
Because the camshaft controls the distributor shaft, the rotating arm moves in time with the engine—this causes the ignition spark to reach the spark plugs in the correct firing order. 
While this process takes place at the top of the distributor, the bottom portion controls the opening and closing of the ignition coil's circuit
Key Takeaway An ignition distributor spins in time with the engine because it is connected to the camshaft. It allows the ignition spark to reach the spark plug and ignite the air-fuel mixture in your engine.

Symptoms of a bad distributor 

A bad distributor can cause a variety of problems for your car. Unfortunately, it's often difficult to differentiate between a faulty distributor and other ignition system problems. Because of this, one of the best things to do if you experience any of the issues below is visually inspect ignition system components
Ignition problems—including bad spark plugs, faulty plug wires, and a malfunctioning distributor—can cause similar problems. Despite this, understanding the potential issues caused by defective ignition components can help you locate the source of the issue. 
Let's take a look at the most common symptoms of a bad distributor. 

Poor engine performance

A bad distributor can lead to various problems with your car's engine performance. The most common issues are misfires, backfires, and vehicle stalling
Misfires occur when the fuel/air mixture doesn't combust properly in the engine and can feel like a jerking sensation. Backfires are similar and happen when unburned fuel exits the cylinder and is ignited by the next spark plug. Backfires often cause the car to stall. 

A check engine light

Your car's check engine light is designed to turn on when the computer detects a problem with the engine. If a bad distributor disrupts your vehicle's firing cycle, you'll likely notice a check engine light on the dash. 

The engine cranks but won't start

A complex series of events occurs when you turn your ignition key to start your car. If one step of the process fails, the car won't start.
The distributor controls your vehicle's spark timing. If it's malfunctioning or faulty, then those signals won't reach the spark plugs. If the spark plugs don't receive the electrical current necessary to ignite the fuel/air mixture, then the engine will not start.  

Increased vehicle emissions 

A faulty distributor can affect the way your car's engine cylinders fire. Sometimes, it can result in unburned fuel escaping your engine through the exhaust system. When this happens, you will most likely fail your emissions test. 

Unusual noises

Misfires and backfires can both result in loud noises. However, a lousy distributor can also cause high-pitched squealing noises. This often occurs if the distributor is clogged with debris or grease. 

The car is shaking

While shaking is often a symptom of other car problems, including issues with the suspension, it can be a sign of distributor problems. The shaking may be caused by incorrect engine firing or a faulty distributor rotor's erratic spinning. 

How much does a replacement distributor cost?

If you have a faulty distributor, the cost to replace it will vary depending on your car's make and model. You can generally expect a new distributor assembly to run you between $250 and $500, while the labor cost will typically fall between $75 and $150
Distributors are replaceable maintenance items, and as such, replacements are not covered by insurance. However, your insurance plan may pay for the repairs if your distributor is damaged in a covered accident. Because of this, it's crucial to have a car insurance plan that includes
collision coverage
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A car's distributor controls the spark timing and ensures that the engine's spark plugs receive the high-voltage current in the correct order and time. Distributors also control the on/off function of the ignition coil.
If your car uses a mechanically-timed ignition system—meaning a distributor routes the electric currents to the spark plugs instead of the ECU—then no, you cannot drive without the distributor
However, modern vehicles are built without distributors. In these vehicles, the ECU controls the engine timing with the help of coil packs and crankshaft position sensors.
A bad distributor can cause a variety of problems. The most common signs of a faulty distributor include poor engine performance, an inability to start the engine, unusual noises, a shaky ride, increased emissions, and a check engine light.
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