A distributor is an enclosed rotating shaft utilized in spark-ignition internal combustion engines that have mechanically-timed ignition. The distributor’s main aim is to route secondary, or high voltage, current from the ignition coil to the spark plugs in the correct firing order, and for the correct amount of time. Except in magneto systems, the distributor houses a mechanical or inductive breaker switch to open and close the ignition coil’s primary circuit too.
The first reliable battery operated ignition was made by Dayton Engineering Laboratories Co. (Delco) and introduced in the 1910 Cadillac. This ignition was developed by Charles Kettering and was considered like a wonder in its day. Atwater Kent invented his Unisparker ignition system about this time in competition to the Delco system. By the end of the 20th century mechanical ignitions started disappearing from automotive applications in favour of inductive or capacitive electronic ignitions fully controlled by units (ECU), rather than directly timed to the engine’s crankshaft speed.
Direct and distributor less ignition
Modern engine designs have left the high-voltage distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary (low-voltage) pulse to individual coils for each spark plug, or one coil for each of the pair of companion cylinders in an engine (two coils for a four-cylinder, three coils for a six-cylinder, four coils for an eight-cylinder, and so on).
In traditional remote distributor less systems, the coils are placed together in a transformer oil filled ‘coil pack’, or separate coils for each cylinder, which are secured in a specified place in the engine compartment with wires to the spark plugs, similar to a distributor setup. General Motors, Ford, Chrysler, Hyundai, Subaru, Volkswagen and Toyota are the automobile manufacturers known to have utilized the coil packs. Coil packs by Delco for use with General Motors engines help in the removal of the individual coils in case one should fail, but in most other remote distributor less coil pack setups, if a coil were to fail, replacement of the whole pack would be required to fix the problem.
More recent layouts utilize a coil placed very near to (Coil-Near-Plugs) or directly on top of each spark plug (Direct Ignition, DI, coil-on-plug, or COP). This design helps to avoid the need to transmit very high voltages, which is often a source of trouble, especially in damp conditions.
Both direct as well as remote distributor less systems also allow finer levels of ignition control by the engine computer, which helps to increase power output, decrease fuel consumption and emissions, and implement features such as cylinder deactivation. Spark plug wires, which require routine replacement due to wear, are also eliminated when the individual coils are mounted directly on top of each plug, since the power is transported a very short distance from the coil to the plug.
Four-stroke 2-cylinder engines can be built even without a distributor, as in the Citroen 2CV of 1948 and BMW boxer twin motorcycles, and some Honda motorcycles from the 1960s (e.g. the CL160 Scrambler). Both spark plugs of the boxer twin are fired simultaneously to result in a wasted spark on the cylinder currently on its exhaust stroke.
Four-stroke 4-cylinder engines can also be built without a distributor, as in the Citroen ID19. Two coils are utilized with one coil firing two of the spark plugs simultaneously, resulting in a wasted spark on the cylinder currently on its exhaust stroke, and the other coil used for the other two cylinders. This system has been scaled up to the engines through virtually an unlimited number of cylinders.
Four-stroke one-cylinder engines can be built without a distributor too, as in many lawn mowers, and a growing number of spark-ignition, four-stroke model engines. The spark plug is fired on every stroke, leading to a wasted spark in the cylinder when on its exhaust stroke.
Symptoms of a Bad or Failing Distributor Rotor and Cap
Most common signs include engine misfires, car not starting, the Check Engine Light coming on, and excessive or unusual engine noises.
A running engine gives out a large amount of electricity through the ignition coils to the rotor, which turns inside the distributor. The rotor routes the energy through the spark plug wires to the engine’s cylinders in the correct firing order.
The distributor rotor and cap help to keep the distributor’s contents separate from the engine and keep the distributor’s working parts clean and tidy – while supporting the incredibly high volts of energy and delivering all of them to the appropriate spark plugs. The spark plugs utilize the spark from the distributor for igniting the fuel mixture, which is what keeps the engine running.
High voltage runs within this entire distributor system during the operation of your vehicle, but if there’s an issue this voltage won’t get distributed to the correct spark plugs to ensure that your engine will run. Mostly, a faulty distributor rotor and cap will produce a few symptoms that alert the driver that service might be required.
1. Engine misfires
Engine misfires can happen for a number of reasons. Checking your distributor rotor and cap to see if they require to be replaced is one way to ensure that everything is in solid working order.
2. Car doesn’t start
When the distributor cap isn’t on tightly or is malfunctioning, the engine is not able to send the spark through the entire circuit required to move the cylinders – which ultimately make the car run.
3. Check Engine Light comes on
Your Check Engine light can have different meanings, but when you see this light on along with some of the other symptoms listed here, it’s time to call a professional to find what code is from your car’s computer.
4. Excessive or unusual engine noises
Your vehicle might make some very strange noises if the distributor rotor and cap are malfunctioning – specifically because the cylinders will try to fire but fail. You may be hearing a tapping, clicking, or sputtering sound when the distributor rotor and cap are failing.
Removing and refitting the distributor
Do not crank the engine or move the car in gear when the distributor is out of the engine. If the engine is turned, the timing needs to be reset.
With the distributor withdrawn from the engine and the cap removed, the points can be easily adjusted.
On some of the cars, because of limited space, the only way of replacing the contact-breaker points or condenser is with the distributor removed.
Before removing the distributor, inspect which of the high-tension leads is connected to the cylinder used for timing. It is always nearly the best cylinder, but on some cars another cylinder is used.
Mark the lead using a tag, and pencil its position on the distributor body.
There are variations in the procedure for removing and refitting the distributor on some of the cars. Check the details in the car service manual.
Do not crank the engine or move the car in gear whenever the distributor is out of the engine. If the engine is turned, the timing needs to be reset.
Removing the distributor cap
Before removing the distributor cap, just tag the lead to the cylinder used for timing and mark its position on the distributor body.
Ensure that the battery is disconnected. Prise apart the two retaining clips in order to free the distributor cap or remove the screws if the cap is held on by screws. Remove the rotor arm for taking out the dust shield fitted to some cars, then replace it.
Inspect that the position of the timing-cylinder sparkplug lead is clearly marked on the distributor body.
Remove the distributor cap with help of its HT leads.
Preparing to remove the distributor body
Remove the cap and turn the engine till the contact-breaker points are just opening.
Turn the engine with the help of a spanner found on the crankshaft-pulley nut. Alternatively engage top gear, release the handbrake, and then, push or pull the car.
Turn the engine till the contact-breaker points are just starting to open, and the rotor arm points to the position of the timing cylinder HT lead.
To save time while refitting, scribe a mark on the distributor body and a corresponding mark on the drive housing or engine block. See that this will be useless if the engine is turned and the rotor arm position altered after the distributor is removed.
Disconnect the vacuum tube as well as the low-tension lead.
Removing a pinch-bolt assembly
The most used method of fixing the distributor to the engine is a clamp-plate and pinch-bolt assembly. Unscrew the pinch bolt for loosening the clamp.
The most popular fixing is a clamp-plate and pinch-bolt assembly; the pinch bolt is the same one that is loosened while turning the distributor to set the ignition timing.
Loosen the bolt or bolts while holding the clamp plate to the engine. Undo the pinch bolt and safely withdraw the distributor.
If the distributor is tight, just insert a screwdriver blade between the open ends of the clamp plate and prise them slightly apart.
As you withdraw the distributor, the rotor arm occasionally shifts slightly through the movement of the driving gear. If this takes place, mark the final position of the arm to simplify replacement.
Removing a flange-and-stud assembly
When removing a distributor with a flange-and-stud assembly, twist it gently if it is sticking. Do not try to lever it with a screwdriver.
On some overhead-camshaft engines, the distributor has a slotted flange placed at the base that fits over two or more studs on the engine. The distributor is secured through nuts and washers.
The slots in the flange allow the distributor to turn when the nuts are slacked while timing the engine. Mark the position of the distributor through scribing lines on the flange and on the stud plate so that it can be replaced in the exact same position.
Remove the nuts and just pull the unit away from its mounting. Twist it gently if it is sticks but do not try to lever it from the engine block with a screwdriver.
Replacing the distributor
The distributor gear might be an offset dog – a tongue that fits into a slot.
Slide the distributor driving gear back into the engine block, roughly aligning all the scribed marks.
If the drive fitting found on the shaft is an offset dog – a tongue that fits into a slot – turn the contact-breaker cam till the dog engages, then push the distributor home.
Some distributors have a skew-gear drive fitting.
If the drive fitting is a skew gear, it might be difficult to align the scribe marks without turning the distributor slightly.
You might be needing to point the rotor arm roughly 30 degrees before or after the cylinder-lead mark on the distributor body, depending on the direction of rotation of the rotor arm. As the gear meshes, the rotor arm is required turn to line up exactly.
With the scribe marks properly aligned, just tighten the fixing and reconnect the vacuum tube and low-tension lead.
If the engine has been turned while the distributor was removed, the ignition timing needs to be reset. If the engine has not been turned, inspect the accuracy of the timing and reset if necessary.
Refitting the cap
Before refitting the cap, just check whether it is marked or cracked. If it is, renew it.
Inspect the cap for any cracks or racking marks, fine lines that look Ike forked lightning, before fitting it to place. If it is marked or cracked, it needs to be renewed, changing over the HT leads in identical order. Wipe inside the cap through a clean, dry rag and then reconnect the battery.
Variations on some cars
Vernier timing adjuster
If the distributor has a vernier timing control, just set it midway between advance and retard. Turn the knob fully anti-clockwise and then fully clockwise while counting the number of turns. Divide the number by two to get the midway position.
The distributor might be fitted with a vernier timing adjuster – a graduated scale which allows fine adjustment of the timing. Turning the knurled knob clockwise retards the ignition while turning it anti-clockwise advances it.
While replacing the distributor after the engine has been turned, set the adjuster to midway between advance and retard.
Slide the distributor into place, and place it so that when the driving gear is engaged the rotor arm is pointing to the timing-cylinder electrode.
On cars with electronic ignition, removing and refitting the distributor is the same but the timing needs to be set with a stroboscope.
Other types of distributor
When you remove some types of distributor, the driving gear might stay in the engine. Hook it out with your finger or through a suitable piece of wood.
When refitting, oil the driving gear using a clean engine oil and replace it in the block before positioning the distributor.
There might be a gasket between the distributor mounting and the cylinder head. Remove and discard it, and fit a new one while replacing the distributor.
On another type of distributor, the first job is to remove a TDC sensor from the clutch housing, like on a VW so that the timing marks can be seen. Make use of a plug spanner sawn off to about 2 in. (50mm).
Turn the engine through hand until the 0 mark on the flywheel is opposite a reference mark on the clutch housing.
In that position, the timing mark on the camshaft sprocket needs to be level with the cylinder-head cover, and the rotor arm should be in line with the best electrode in the distributor cap.
If the engine is turned while the distributor is out of the car, just turn the engine until the flywheel and clutch-housing marks are aligned.
If the camshaft-sprocket mark is not then level with the cylinder-head cover, just turn the crankshaft through another complete revolution.
Ensure that the plug on the end of the oil-pump drive shaft is in line with the crankshaft axis.
Resetting the distributor after the engine had been turned
For resetting the firing position, the distributor must be refitted so that when the contact-breaker points are just opening, the timing mark on the crankshaft pulley is in line with its pointer and the piston of the timing cylinder is approaching towards the top dead centre on its compression stroke.
Remove the spark plugs to make it easy for the engine to turn.
Refit the distributor so that the timing marks align to the piston of the timing cylinder approaches TDC on its compression stroke.
To inspect when the piston is approaching top dead centre on the compression stroke, ask a helper to place a thumb over the timing cylinder plug hole.
Turn the engine with the help of a spanner on the crankshaft pulley nut. As the piston rises, your helper will feel the pressure building up, as the piston reaches the required position. The pressure is sufficient to force the helpers thumb off the sparkplug hole.
Use a test lamp to inspect that the contact-breaker points are just opening.
Alternatively, take off the rocker cover and see when both valves of the cylinder are fully closed.
Turn the engine a little till the timing mark and pointer are in line with the appropriate number of degrees before TDC.
Slide the distributor into position so that when the driving gear is engaged, the rotor arm is always pointing to the position of the timing-cylinder lead. After that, reconnect the battery, and connect a test lamp across the contact-breaker points to check that they are just opening.
Check the piston’s position through removing the rocker cover and noting when both valves of the cylinder are fully closed.