Distributor caps and rotors: Your FAQs answered
Every time you get into your car and hit the ignition, a complex chain of events unfolds under the bonnet. In a matter of seconds, the ignition system generates a colossal amount of electrical energy, which produces a spark and ignites the fuel and air in the combustion chamber. The engine then roars into action, and you’re ready to go.
The distributor — including the rotor arm and a cap which covers it — is one of the many parts in this system. So, naturally, you’ll experience all kinds of problems when something is wrong with it — in fact, your car may not start at all. To help you get back on the road as quickly as possible, we’ve put together this handy guide to distributor caps and rotors, include a step-by-step guide on how to replace them. We’ll cover:
- What does a distributor do?
- Signs of a wet or bad distributor
- When to replace a distributor
- How to replace a distributor
The purpose of the distributor is to direct high-tension voltage (also known as the secondary current) from the ignition coil to the spark plugs at exactly the right time and in exactly the right order to create a spark. This then ignites the engine, firing up the cylinders and powering the car.
The distributor is comprised of a number of different components, the most important of which are the rotor arm, ignition points, and a distributor cap with output contacts that connect to the spark plug cables. There are multiple contact points arranged in a ring around the rotor arm, and each connects to a different spark plug. The distributor rotor arm is connected to the ignition coil by a cable and a spring-loaded carbon brush. This sealed unit is mounted on the distributor shaft, which is connected to — and driven by — the engine camshaft.
When the engine turns the camshaft, the distributor shaft turns as well, spinning the rotor arm inside the distributor. As it turns, the metal part of the rotor arm passes very close to the output contacts. A very high voltage (the secondary current) is then delivered from the ignition coil into the distributor rotor arm. Because the voltage is very high, it is strong enough to leap across the small gap between the spinning rotor arm and the output contacts, which then direct the charge to the appropriate spark plugs. This allows the distributor to deliver the high-voltage current to the right spark plugs at the right time, which keeps the engine cylinders firing in the correct order.
These days, car manufacturers are moving away from traditional distributor ignition systems, and most modern cars are now fitted with electronically controlled systems instead. However, many vehicles produced before the mid-noughties still use distributor ignition systems.
Just like any other serviceable part, the distributor can wear down over time, and it can also be susceptible to water damage if any moisture seeps inside the cap. If there's a problem, you may notice the following signs of a wet or bad distributor:
The car doesn't start
If the engine won't turn over, the culprit could be a faulty distributor. When the distributor cap is loose or damaged, then the rotor arm may not be able to generate the spark needed to direct voltage to the spark plugs. Without this, the spark plugs can't fire and ignite the engine.
If your engine has died suddenly, then remember that there may be any number of other causes for this: the problem isn't necessarily with the distributor. If you can’t find any faults with the distributor, then take a look at our car-starting problems guide to figure out what may be causing the issue.
The engine misfires
If any part of the distributor is faulty or damaged, then the ignition timing will be affected, which can cause the engine to misfire. So, you'll want to check the distributor for signs of water damage or wear and tear — we've outlined what to check for below.
The check engine light comes on
While there may be many reasons for the Check Engine light to come on, one of the possible causes could be a bad distributor. If you own a fault code scanner, you can use this to work out whether or not the distributor is causing this.
You hear strange noises coming from the engine bay
When the distributor rotor and cap are malfunctioning or dirty, the cylinders will try to fire but fail to ignite. This means a surplus of air starts to circulate through the engine, resulting in a loud squealing or whistling noise. The engine may also splutter loudly or make tapping and clicking noises.
How can I tell if the distributor is bad?
If you suspect that the distributor has developed a fault, you'll want to get under the bonnet and check to see if it is damaged, worn, or loose. Visually inspect the cap and rotor carefully: the internal and external surfaces should be clean and free from corrosion or rust. The rotor arm in particular needs to be in very good condition, as the spark may not be able to pass from one terminal to another if it is dirty or damaged. So, check it carefully for any build-up of carbon or pitting.
You should also check that there are no loose parts and that the cap and all cables are tightly secured. If you find a problem, you will need to replace the affected parts or possibly the entire distributor.
As a rule of thumb, the distributor should be replaced every 50,000 miles or 5 years, depending on which milestone you reach first. However, service intervals for the distributor cap and other distributor parts can vary, so check the owner's manual or Haynes manual to find out exactly when to replace them. You will also need to replace your distributor cap earlier than the recommend interval if it develops a fault or becomes damaged by water, as this could prevent your car from starting at all.
If your distributor is overdue a replacement, don't put this job off. The distributor rotor arm is exposed to high voltages every time the engine runs, and if they aren't maintained or replaced regularly, it can gradually reduce the engine efficiency of your vehicle. So, be sure to service your distributor at the recommended intervals and replace it when necessary.
Replacing a distributor cap is an intermediate job that most amateur mechanics with a bit of experience and a solid understanding of the ignition system should be able to carry out. However, the complexity of the job can vary depending on the make and model of car. So, we would always recommend checking your car owner’s guidebook or reading the relevant section in your Haynes manual. If the job is beyond what you feel comfortable with, be sure to consult a professional mechanic.
Before you begin, you will need the following tools and parts:
- A replacement distributor. This should include the rotor and distributor cap. These must be the correct type for your vehicle — check your handbook for more information or use our product-finding tool to view the right parts for your car.
- Assorted hand tools including a wrench, screwdriverset, and socket set.
- A marker pen for marking the correct position of the rotor and the order of the spark plug cables. Make sure this is a bright colour that will clearly stand out against dark materials and won't rub off.
- The timing specifications for your vehicle. In order to correctly install the replacement distributor, you'll need to set the timing of the engine once the new part is installed. All vehicles have their own unique timing specifications, so be sure to find these before you begin. They can normally be found on a sticker under the bonnet or in the engine compartment. They may also be detailed in the owner's manual. This information is crucial, so if you can't find the specification, don't attempt to carry out this job yourself: take your car to a professional.
Now, let’s take look at how to replace a distributor step by step:
- Get set up: Park your car on an even surface, engage the handbrake, and switch off the engine. Gather everything you'll need for the job (listed above). Ground the battery by removing the negative battery cable.
- Find the distributor: Open the bonnet and locate the distributor. In most cars, it's positioned directly above or to one side of the engine, but check the Haynes manual or owner's guide if you can't see it. The distributor will be cylindrical in shape and should have a plastic cap with several thick cables attached to it — these are attached to the spark plugs. There's also another wire which attaches to the ignition coil.
- Remove the distributor cap: The cap will normally be held on using hinged clips or bolts or a combination of both. Pop the clips off by hand or with a flat-head screwdriver and unscrew any bolts with a socket wrench or screwdriver. Once all fastenings are removed, pull the cap away from the body of the distributor.
- Mark the spark plug cables and output contacts: Take your marker pen and number all of the spark plug cables and output contacts so you can reconnect the cables with the matching outputs on the new cap. This will ensure you don't accidentally interfere with the firing order.
- Mark the engine mounting point: To make it easier to fit the new distributor correctly, mark the point on the outside of the distributor housing where the distributor is mounted to the engine. This will make it easier to accurately line up the new housing the with engine mounting point. The replacement rotor may have an indentation on it to indicate where the mount should line up with, but it never hurts to be too careful!
- Mark the rotor: Mark the distributor housing to indicate the position of the rotor. It's important that this is as accurate as you can make it, as the car won't start if the new rotor isn't in exactly the same position as the old one.
- Remove the old distributor: Unscrew any bolts or screws that are holding the distributor in place and carefully lift it away from the distributor shaft.
- Mark the new distributor: Take the replacement distributor and make the same marks on the housing and rotor that you made on your old distributor, taking care to put them in exactly the same places. This will ensure the part is in the same position as the old version.
- Line up the rotor and re-install the replacement: Re-install the replacement distributor. The position of the new rotor must match the position of the old one or the car won't start. So, make sure the new rotor is lined up with the marking you made earlier. Be careful not to accidentally nudge the rotor out of alignment when placing it onto the distributor shaft.
- Secure the distributor using the mark on the engine mount: Secure the new distributor in place. To make sure it's in exactly the right position, make sure that the mark on the distributor housing lines up with the mark you made on the engine mounting point. When it’s in right position, replace any bolts or screws. Don't fully tighten them, however — you need to be able to adjust the position very slightly later on when checking the ignition timing.
- Replace all the cables: Replace each cable using the markings you made earlier as a reference point to make sure that each one corresponds to the original connection on the old rotor.
- Check your connections are sound: Check the connections between the cables and the distributor are sound. Reconnect the negative battery terminal.
- Attempt to start the engine: Try to switch on the engine. If it starts on the first attempt, let it idle for a minute or two to warm up the engine. If the engine won't quite turn over, you'll need to re-adjust the rotor arm by turning it very slightly and try again.
- Check the ignition timing: Once you've successfully started the engine and left it to idle, switch it off again. Place a timing light device on the first spark plug and then re-start the engine. Following the timing instructions for your vehicle, adjust the timing by rotating the distributor housing by very small amounts. When the timing is exactly right, you can finish tightening the bolts or screws that hold the distributor housing in place and lower bonnet.
Well done — you've now successfully replaced a distributor! If it's been a while since you last serviced your ignition, you could also take this opportunity to clean and change the spark plugs.
Here at GSF Car Parts, we have all the parts and tools you need to get your ignition system back up and running, including ignition cables, rotor arms, ignition points, ignition condensers, and distributor caps for all makes and models of car, all with free UK delivery when you spend over £25. Or, if you need to get your distributor fixed in a hurry, take advantage of our speedy Click and Collect service, which allows you to pick up your new parts as quickly as an hour after ordering. While you’re here, be sure to check out our car advice centre to find helpful how-to guides on all things automotive.