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Jaguar Technical Articles

Article 4 – Checking the Spark Ignition System

Tech Article 4

Troubleshooting your XK Engine – Checking the Spark Ignition System

In the course of the first 3 articles on how to troubleshoot your XK engine, we have concentrated on the mechanical components that pump fuel and air into cylinders, namely the valves and the pistons. Now that we have established that the cylinders are being filled and exhausted at the correct time, we will now take a look at the spark ignition system.

The spark ignition system has one simple task to perform. It must ignite the fuel/air mixture that has filled your combustion chamber at just the right instant. When the spark occurs, the fuel/air mixture ignites and burns, producing hot gases that force the piston downward, rotating the crankshaft. This is a basic requirement for all gasoline engines. Engines back to the Model T have had some form of spark ignition system. That said, there are some difficult conditions that the ignition system must overcome. First, there are extremely high temperatures in the combustion chamber, over a thousand degrees. Even external to the engine, the under hood temperatures can be several hundred degrees. Second, things may have to happen very quickly. On my 4.2 litre E-Type, the tachometer redline is 5000 rpm. That means that the engine crank shaft is rotating at 5000 revolutions per minute or close to a 80 revolutions per second! 3 of the 6 spark plugs fire for every revolution of the crank shaft, meaning there are 240 spark events each second! It’s really hard to fathom how things can occur that quickly. Third, the ignition system must be able to respond to differing driving conditions, from puttering around town to high speed motoring.

In spite of these challenges, the stock ignition system used for the XK engine up to the 70’s is basic and simple. In order to better understand how to get your ignition system in good working order, we will go over some of the required components and their function. The key parts that will be discussed are the ignition coil, the distributor, the spark plug wires, and the spark plugs.

Each ignition spark event starts at the ignition coil. This is generally a round cylinder about 2″in diameter and 6″ long. The coil is really a small electrical voltage multiplier. Your basic engine electrical system is 6 volts or 12 volts, depending on the vintage of your car. The spark requires a voltage over 1000 times that value in order to function properly. So one job of the coil is to provide this increase. The coil has a primary and a secondary circuit, both which consist of spools of fine wire wound around the central shaft of the coil. One spool of wire has many more turns than the other. When 6 or 12 volts is momentarily introduced through one circuit, a much higher voltage is induced in the other circuit. This effect can only be sustained for an instant but that instant is enough to provide one jolt of high voltage electricity that fires the spark plug. And yes, this effect can be turned on and off up to 240 times a second, producing the rapid series of spark events required to run the XK engine up to 5000 rpm.

The ignition coil has 3 electrical connections, 2 of them light duty and one heavy duty. One light duty connection generally leads back to the ignition key on the dashboard. This connection provides a constant source of 6 or 12 volts to the coil. This connection is usually denoted on the coil with a + sign. It isn’t the intent of my articles to diagnose inoperable ignition systems but suffice it say that if you can’t read voltage at the + terminal of your coil when the ignition key is turned on, your engine will not run! The other light duty connection on your coil is usually denoted with a – sign. This connection will have a light duty wire that leads the short distance to your distributor. This wire provides the On/Off path to ground, through the distributor, that tells the coil when and how fast to fire its high voltage pulse. The final connection to the coil is generally right in the middle of the end and is a large wire called the high tension lead. As its name implies, the wire routes the high voltage pulse from the coil. Its other end in anchored in the middle of the distributor cap.

There are no adjustments for coils. They generally either work or they don’t, although occasionally you can get a flaky coil that may stop working when it gets really hot. If your coil is so hot that you can’t stand to hold your hand on it, its probably not long for this world. There are several electrical tests that you can try on the coil, like checking for the lack of shorts or open circuits on the primary and secondary circuits. But in general, as I am going to recommend for all the ignition system parts (except the distributor) you might as well invest in new parts. In the great scheme of Jaguar ownership, this is will be a small investment of only a few hundred dollars.

This is probably as good as a time as any to speak my mind about high performance ignition systems. In short, I think they are a waste of money for a stock XK engine. There are several ways to look at this but in effect, the spark plugs only need a certain minimum threshold voltage to fire. Any greater voltage than that is not required and will not be used, even if available. Buying good quality parts is a very good idea but don’t waste time or money on upgrades that are always being marketed by various companies interested in separating you from your hard earned money. I will mention a few exceptions to this thinking as we go along.

I’m going to have to spend a entire article or two discussing the distributor and since my article length limits are looming, I’m going to skip over the distributor for now. Suffice it say that the single spark from the coil high tension lead is “distributed” into the 6 individual spark plug leads, each of which lead to one of the spark plugs. All the spark plug wires have to do is carry the high energy voltage pulse from the distributor to the metal end of each spark plug. They need to do this efficiently, without losses or shorts to the metal engine parts, and without disrupting the reception of your radio! If you are going for the original equipment look to your engine, then you will wind up buying OEM or similar wires from one of the usual parts houses for Jaguar parts. In my case, I must admit that I succumbed to some of the marketing hype and went for a set of Magnecor wires. I don’t know, they look sharp and I was convinced by the Magnecor web site that they would be durable. Use your judgement but as I said earlier, a set of good quality brand name spark plug wires should satisfy your engine’s requirements. Just avoid the el cheapo no name brands.

So for now, I’m going to wait until my next article to get into the key element of the ignition system, the distributor. It routes the spark pulse, sends it out at the correct time, and assists in adjusting the spark for differing driving conditions. As we will see, it is a rather elegant electro-mechanical device.




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