How to check and replace HT Leads on a Classic Car

How to check and replace HT Leads on a Classic Car

This guide captures ‘bygone’ advice on how to test and replace your HT circuit on a Classic Car. It is part of a series I am building that describes what to do on a Classic Car Basic Service.

Useful information for Classic Car and Retro Car enthusiasts, as many of these tasks are no longer required on modern day cars.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





How to check your Coil Connections

One matter that is easily overlooked on a Classic Car is making sure that the LT wires to the coil are connected to the correct terminals. Reversing the coil connections can reduce the voltage by a massive 30 per cent.

So its a good idea to check this before starting to change your HT Leads.

On the coil is a heavy lead going to the distributor cap. There are also two thinner leads, connected to the and – symbols on the coil. One of these wires goes to the side of the distributor and the other to the ignition . switch, but which goes where depends on the car.

Make sure that the LT wires to the coil are connected to the correct terminal. These coil terminals are marked + and -
Make sure that the LT wires to the coil are connected to the correct terminal.
These coil terminals are marked + and –

On a negative-earthed car, the wire from the distributor must be connected to the – symbol on the coil. On a positive-earthed car, the distributor wire goes to the + terminal on the coil. If they have been reversed, you simply swap them around at the coil.

On older cars, the coil may be marked SW and CB, and you need not check the car’s polarity. The CB wire goes to the distributor (contact breaker) and the SW wire to the switch.


How to check HT Leads on a Classic Car

Spark-plug leads are subjected to heavy stresses from heat, oil and mechanical effects. For high tension electricity of around 10,000 volts to flow back to earth via the air gap of the spark-plug calls for an effort.

If faults in the insulation of the lead allow an easier path to be taken then the electricity will accept the opportunity. Sparks jumping from the lead to the engine are a good example of this.

A temporary cure is to spray the lead in question with a silicone-based de-watering fluid from an aerosol, such as WD 40. Although this will-literally-fill the gaps, the only proper cure is to fit new leads.

lf the insulation of the HT leads is faulty, you can fill the gaps temporarily by spraying the leads with a fluid like WD 40
lf the insulation of the HT leads is faulty, you can fill the gaps temporarily by spraying the leads with a fluid like WD 40

If one lead is giving trouble, it is reasonable to assume that the others will soon be doing the same.


When to change the HT Leads on a Classic Car

New leads come in two types. You can buy the ready­ fitted sort, cut to length and fitted with terminals. Or you can make up your own from a length of cable.

Whichever you buy, it is important to have the correct type of lead. Carbon-cored leads for example are designed for radio interference suppression; copper­ cored ones, by themselves are not.

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The length of the new leads should match that of the old ones. Too short a lead will result in acute bends, throwing an extra mechanical load on the leads especially when the engine rocks in its frame at low speeds. Over­long leads are untidy and may touch the hotter parts of the engine.

Do not be tempted to tape or bind plug leads together, as this may result in ‘crossfire’ from one lead to another. Crossfire occurs when one plug lead’s close proximity to another causes its plug to fire out of sequence and possibly at the same time as another plug.

At the plug-cap end, fitting is usually made by screwing the cap on to the lead. This simple arrangement gives a high degree of mechanical support besides making a good electrical connection. Chipped or cracked plug caps should be renewed to avoid the possibility of voltage-leak (or ‘tracking’ as it is usually known). Wipe off any dirt with a rag.

Testing the security of the connection by pulling the lead is not a good idea, the internal electrical connection could be strained. If the cap is screwed fully home the connection must be sound.


How to remove the HT leads on a Classic Car

The first stage in servicing spark-plugs is to remove their HT leads. But first you need to number them (see below) since, if you connect the wrong lead to any plug the engine could fire in the wrong order and perform very badly indeed.

To make sure that you return each of the HT leads to the right spark-plug, number each lead with a piece of adhesive tape
To make sure that you return each of the HT leads to the right spark-plug, number each lead with a piece of adhesive tape

For changing spark plugs, it does not matter which order you use in numbering the leads as long as you remember it.

But for some other tasks about the car you will need to know which cylinder is No. 1, which No. 2 and so on. So if the correct order is given in your owner’s manual, use it now to save confusion later.

Always remove the plug caps by pulling the caps themselves not the leads. The caps are not difficult to remove and using the leads to tug them off could damage the leads enough to interfere with the flow of current.

Be very careful about touching the plug leads or pulling off a plug cap when the engine is running. The high voltage running through the leads can give you a nasty jolt and when a plug cap is removed the voltage tries to jump the gap and rises in consequence.

The ‘bite’ from a plug lead is not dangerous in itself because, even though the voltage is high, the current is correspondingly low, so there is no danger of electrocution. But whipping your hand violently away from the source of the shock could send it into the fan or onto the hot exhaust manifold.


How to connect HT Leads to a distributor on a Classic Car

At the distributor end, the plug leads are sometimes held by pointed screws reached from inside the cap. A slim screwdriver is needed to remove the old leads and fit the replacements. Be careful here, for a screw­driver whose blade is a tight fit inside the screw holders ­might crack the plastic, thus ruining the cap.

These HT leads are located in the distributor cap by pointed screws. You will need a slim screwdriver to remove the leads
These HT leads are located in the distributor cap by pointed screws. You will need a slim screwdriver to remove the leads

As the old leads are removed, clean the insides of the distributor cap sockets with a twisted-up piece of clean, dry rag. Then push the new lead inside the socket as far as it will go, and tighten the screw.

It will take a certain amount of effort to pierce the plastic insulation but there will be no doubt when it is fully home. Do not over­ tighten the screws, ordinary wrist pressure is enough.

Other types of distributor have knurled plastic thimbles on top of the lead sockets which hold the leads in place. With these, you undo the thimble, remove the copper washer and slide it off the old lead.

On other distributor caps, the HT leads are held in place by knurled plastic thimbles. Undo the thimbles to pull out the leads
On other distributor caps, the HT leads are held in place by knurled plastic thimbles. Undo the thimbles to pull out the leads

The end of the replacement lead should have about 3 mm (3/8th inch) of the insulation carefully cut round with a sharp knife, taking care not to cut through the con­ductor wire.

Carefully unravel the strands of the conductor wire, then spread them out and bend them back on themselves to cover the cut end of the wire and copper washer.

They will then make firm contact with a metal insert in the socket in the distributor cap, and the contact will be maintained by the physical pressure of the plastic thimble onto the copper washer.

Before pushing the end of the lead home look inside the socket to make sure that the metal is free from corrosion. Any green-coloured deposits or black burning traces can be easily removed by careful scraping with a screwdriver tip or a piece of sandpaper.

Finally, refit the leads in the position in which the manufacturer originally fitted them. This avoids the risk of crossfire between the leads.


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