We don't believe our potential customers are totally ignorant on the subject of their car's functioning. Nevertheless we realize that better informed customers can tell us clearly what they want and what they should expect but that it often difficult for them to get objective information. By making this article freely available to you we hope to give you some extra knowledge on the functioning and importance of ignition systems of gasoline powered engines so that you know which questions to ask and what to do when you want to improve your car's performance and economy by upgrading the ignition system. Where necessary we have changed some words to make the article more understandable for Europeans with regards to expressions for fuel consumption and engineering units.

Article copied with permission of Jacobs Electronics@ from C.Jacobs recent book, "The Doctor's Step-By-Step Guide to Optimizing your ignition" available at CDC.

HOW TO CHECK FOR A COIL FAILURE

The most common tip-off to coil deterioration is difficult, unreliable starting. To test the coil, remove the ignition cable that connects the coil to the distributor at the distributor terminal. Hold the end of the cable about 3/16 in. away from a grounded component and crank over the engine. A bright blue spark should jump the gap. If the spark is weak and yellow (and the points, condenser and battery are in good shape), the coil is producing weak spark voltage. First, do this one morning when your engine is cold. Then recheck the coil the same way when the engine (coil) is very hot, like after a hard trip. Remember, a coil, like any other metal component, expands with heat and the windings may be making contact when the coil is expanded but not when cold, or vice versa. If there is no spark at all, but the headlights go on and the motor cranks over, then the problem must be either: (a) key switch not making contact to ignition wire; (b) ballast resistor or wires to and from key have opencircuited; (c) coil is now history; or (d) the switching mechanism (points or amplifier) malfunctioned.

To determine which part is defective, look at "The Basic Wiring Diagram of Virtually All OEM Ignitions."

Step 1: Put a test light between point A (+ coil terminal) and ground. The light should go on and off when the key goes on and off (see note). If it does go on, proceed to Step 2. If it doesn't, try this. Take a jumper wire from the + battery to the + coil terminal. Now have a friend crank the engine over while you hold the high voltage coil wire (point C) about 3/16 in. from ground. If you get a spark with the jumper wire on, then plug the high voltage coil wire back into the distributor and the motor probably will start. If it does, the problem is probably, first: (a) burned-out ballast resistor; (b) bad or broken connection (usually as the wire passes through the firewall); (c) bad contact at the key; or (d) broken or cut wire (in the middle of the wire, as opposed to (b), which is a bad connection at the end).

Step 2: Move the test light to voltage point B; that is, negative coil terminal, and ground. Have a friend crank the motor over. The light should blink on and off, or at least waiver as the points or amplifier open and close.

1. If the light does not go on at all, either the coil primary is open-circuited (remember, there was voltage on the (+) coil terminal), or there is a short circuit in the points/ amplifier or the wires going from the (-) coil terminal to the points/amplifier.

To determine which, if the light did not go on, disconnect the wires on the negative coil terminal and reconnect the test light to this negative terminal. If the light goes on, there was a short in the points/amplifier or wires to the amplifier/points, and the coil is probably okay.

2. if, on the other hand, the light stays on, without blinking on and off, then the problem is probably not in the coil but rather there is an open circuit in the points/amplifier or the wires leading from the negative coil terminal to the points/amplifier.

3. If the light does blink on and off, but you only get a little wimpy spark, or no spark at all, then the problem is in the coil secondary. If you have an ohmmeter, put it between the coil high voltage output and the negative coil terminal. You would expect to see between 1,000 and 15,000 ohms. The higher the quality coil, the lower the resistance.

However, if the test light blinks and you only have a weak or no spark, change the coil and see if the engine runs better. Never forget that coils have many modes of failure, not all of which are easy to spot.

HOW TO SELECT THE RIGHT COIL FOR YOUR APPLICATION

Physical Things to Look for: If you've diagnosed your problem as a bad coil, or more important, you just want to improve the way your engine runs, there are a number of considerations that must be addressed when looking for a replacement coil. To begin with, the core of a coil will only hold a certain quantity of electrical energy. The amount of energy that is pumped through the distributor into the sparkplugs is designed by the factors of voltage current and duration. (See Chapter 4, Electricity and Wiring Made Easy, for an explanation of these electrical terms.) High-voltage coils, commonly known as high-performance coils, can often produce a load of voltage as high as 48,000 volts or more. A coil that produces such ultra-high voltage (48,000 volts) must (if it runs off the same battery) also reduce its spark duration and current strength proportionately. The problem is that the energy from a highvoltage coil generates shorter duration sparks. At idle and normal cruising rpm levels, the plugs need a long duration burn as well as a solid punch of spark power to burn off the contaminants and ignite the air/fuel mixture. The weak, low current, high-voltage coil has too low a current to burn off the contaminants.

The result is misfire, prematurely fouled plugs and poor gas mileage.

There are only five items you need to look at to tell a good coil from a weak one.

1. Metal (not plastic) case is best because it gets rid of heat. However, that does not mean to say there arerA any good plastic coils. When operating, all coils generate internal heat. Metal conducts heat so much better than plastic that the same coi in a metal case will last twice as long and also, when driven hard, a cool-running metal case coil will put out 35% more punch than it would in plastic. The very best coils are aluminum cased (aluminum conducts heat much better than steel) with cooling fins, much like on an amplifier. You can tell a really good coil simply by feeling it after the engine has been running for about an hour or more. The better the coil, the less its temperature should have risen above that of all the surrounding metal parts. Since a coil generates heat internally, it can never be colder than the surrounding (ambient) area, but the objective is to have it no hotter than the surrounding environment.
   
   On the other hand, due to their longer voltage escape paths, plastic-cased coils can often put out more voltage than metal before insulation breakdown, but you will pay the price of higher internal temperature.
   
   
2. For cylindrical (round type) coils, look for those that are fluid filled-the sloshing action of the fluid pulls heat off the windings and splashes against the case, thereby keeping the windings cool by drawing heat into the metal case. For extruded aluminum coils, look for highthermal conductivity epoxy. This type of thermal conductivity is achieved by filling mechanically rugged epoxy with a thermal transferring material.
   
   
3. About 28,000 volts out is the right balance for general application, as well as racing, as long as the gap doesn't get too large. Don't sacrifice punch and duration for high voltage. Always remember that it's current, not voltage, that ignites fuel.
   
4. Variable permeability core (usually advertised as automatically controlled, or words to that effect). THIS MAY BE THE MOST IMPORTANT FEATURE TO LOOK FOR. This causes the coil output to change upon engine demand so that when a real hot spark is needed, such as starting, and when a plug starts to foul, the core automatically increases output. On the other hand, when running hot, or when detonation is a problem, it calms the output and gives smooth power with no punch through (see Chapter 1, page 19). There are some test results that indicate production ignition systems don't have the power required to initiate punch through or cavitating. There is also significant evidence that it does. (Remember, in ignition work it usually isn't that simple.)
   
5. High-amp/bigger plug gap-Look for a coil from a manufacturer who emphasizes its ability to fire through fouling contaminants and recommends a larger plug gap. Stay away from a coil manufactured to operate with a reduced plug gap.

The Poor Man's Ignition and Coil Evaluator: If you want to compare the useful output-spark quality of various coils, and don't want to spend megabucks for a tester, here's what you do. Go down to your electronics store and ask for a 2 watt-one million ohm resistor, 2 alligator clips, and 2-3 feet of 20-gage stranded wire.

We are going to use this to simulate the load of an average running sparkplug. (NOTE: The heavier the load on the plug, or the worse shape the plug is in, the lower the resistance, so if you want to check how a coil will run under deep load, or weak plug conditions, use a 500,000 ohm-2-watt resistor.)

There are some pre-packaged/calibrated ignition/coil testers available. Most are okay; some are better than others. Always use the ones with a load in parallel with the spark gap because they show you how your ignition is doing in a real life engine running situation. Many ignitions can show a really hot looking spark into an inexpensive unloaded gap but then fall on their face when asked to fire a real engine's plug that may be a little fuel wet, has some miles or laps on it, or even moisture. The answer is to buy a gap that either has one million ohms (new plug); 500,000 ohms (moderately fouled), or 250,000 ohms (fouled) load in parallel with the gap.

If you are building your own, connect the resistor to the wire and alligator clips as shown in "Poor Man's Coil Evaluator."

Select any vehicle, except one with a coil inside the distributor cap, such as a General Motors HEI, which have their coil buried so deep inside the distributor that you can't get to it. Connect the coil to be tested as if you were going to actually run the vehicle on it. Also connect up a battery charger or jumper off another running vehicle; you are going to crank over the engine and if the battery runs down, then the coils tested later would appear weaker, even if they were good, because coil output goes down as battery voltage runs down. Take the high voltage coil wire out of the center of the distributor, attach your poor man's tester, and hold the end of the high voltage coil wire against a good ground, like a valve cover.

Have a friend crank over the engine as you slowly pull the high voltage wire away from ground. (Turn gap size adjustment knob on the purchased ones.) You will reach a point where the SPARK DISTANCE is barely too big for a spark to jump. Measure this DISTANCE. Now do the same thing with the next coil. Whichever coil has the largest SPARK DISTANCE with this resistor load is the best coil to buy for your application and vehicle.

Coil Mounting and Installation:

When mounting the new coil, if it will fit, use the original mounting location, which in most cases was designated by OEM engineers as a relatively cool spot. Moving the mounting location as little as 12 in. can often make a 200 degrees F difference in operating environment.

If the new upgraded coil won't fit, feel the metal surface at any possible alternate mounting locations after you've run the vehicle for, say about a half hour drive. Whichever spot is coolest is the best place to mount the coil. Other good locations are anywhere the coil will be bathed in cool air, but with reduced chance of water splashing all over it.

Notes CDC. Some manufacturers have a system that no voltage will be applied to the primary when the engine sensor is not giving a signal that it is turning to the engine management computer, this has misled a lot of technicians into thinking that there was something wrong with the primary supply.

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Last Update: September 15, 2000