Washington, D.C. Centre
the Spark


by Jim Lunson

One of the essentials to the starting of an automobile engine is the ignition system. This element, along with fuel makes the whole thing work. Ignition works on the principle of a coil working as a large storage cell for electrical energy. This element is wired into the car’s electrical system similar to any electrical item, with a positive wire bringing current to the instrument and a negative wire to ground making the connection a complete circuit (the reverse in positive grounded cars). The coil works as an electrical storehouse, building up an enormous charge inside it. When the negative wire is suddenly disconnected: this high voltage charge surges out through a secondary outlet, the fat wire connected to the spark plugs. This current surge to the plugs causes the spark which ignites the fuel, giving our car the power to move us forward. All this happens over and over, very fast, sending out a burst of electrical current at the proper time to the spark plugs each time as the engine runs.

The coil disconnect is done in the distributor. Inside the distributor is a set of points. These points serve as the switch, disconnecting the negative wire from the coil, releasing the surge: on and off, over and over. There is a small condenser connected to these points to eliminate any spark at this switch so the points don’t burn out quickly, but it is a simple circuit; the wire from the coil goes to the points and then is grounded out through the distributor body. When the points "open", this circuit is broken, causing the voltage surge noted above to occur.

A second function of the distributor is to distribute the electrical surge described above to the proper spark plug. This is done by the rotor and cap. The high current from the coil enters the distributor through the top in the center where it connects to the rotor. As the rotor spins in time with the engine, it makes contact with each spark plug wire, one at a time. The rotation of the rotor coincides with the points system so that when the circuit is broken causing the electrical surge, the rotor is connected to the proper spark plug wire, sending the current on its way to the proper plug at just the right moment to cause the spark in that cylinder. This happens very fast and continuously.

So to trouble shoot a car that will not start requires an evaluation of this ignition system. If you have gas to the carburetor, this is usually the problem. Begin by removing the wire from the coil to the distributor at the distributor end, push back the rubber boot, and wedge the exposed metal connector about 1/8" away from metal on the engine. Then, have someone crank the starter and see if a spark jumps from this connector to the engine. If the coil and points system are working, there will be a visible blue/white spark jumping very rapidly. I recommend wedging the wire to check it because there is high voltage going through this wire which will shock you if you hold it in your bare hand. It can be held, but only with thick, well insulated gloves. This is the first test to see if you have spark.

If there is no spark here, then the problem is in the points and coil system which will require more electrical testing. I’ll cover this testing in the next issue. If there is spark at this spot, then the problem with the car not starting lies in the distribution i.e. in the rotor, distributor cap, or the spark plug wires. You now know the electrical surge is occurring and getting to the distributor.

At this point, pull off the distributor cap, using the two snap clips on either side where it hooks onto the distributor body. First check the rotor which sits on the very top of the distributor shaft. This part takes the current entering through the center of the cap and spreads it out in a circle to the various spark plug wires. Pull it off and look for cracks where it doesn't sit properly or wobbles, and for pitting on the outboard connector where it may not make proper contact with the spark plug wire terminals. I have heard of rotors shorting out, sending the current down through the shaft instead of to the plugs too, so look inside it to see if it is clean or shows signs of sparking there. Often, either this shorting, or the loss of contact with the plug wires starts very small and only when the engine is hot, but this action creates additional heat on the plastic housing and the rotor will soon split or warp from this heat. There is a lot of wear and tear on this small plastic part as it spins and makes lots of electrical contact and is often the source of no spark. I have found it a good idea to carry a spare rotor cap, as they are inexpensive and small. And at the first sign of trouble, it is very easy to pull out the old one and stick on a new one. Then you know this is not the problem.

The next check is the distributor cap to see if the spark plug wires are secured into it and the prongs inside where the rotor touches each wire are clean and not burned. It is essential that the rotor contact be centered on the spark plug lead when the coil fires the voltage. If not, there is first a loss of voltage to the plugs (causing weaker spark and power loss), and second, there begins the cycle of sparking inside the cap, causing pitting on the connectors, further diminishing the current passage, and causing heat buildup. Like the rotor, the cap is plastic, and this heat causes it to distort, moving the connecting points and increasing the potential for the connection to be broken. Remember, this is all happening very rapidly, but must be solid for the car to start and run. These caps wear out after years of use.

I carry a spare cap as well as rotor for my specific distributor, so if the car fails to start, I simply replace it with new one. Neither part is very expensive, but hard to locate when you need one. The rotor will only go onto the distributor shaft one way, and the cap onto the distributor body one way so it is impossible to mess this up. The only tricky part is when replacing the cap; make sure the spark plug wires are reinserted in the new cap in the proper holes. I have made a small sketch of my distributor cap showing which plug wire goes into which hole, using 12 o'clock toward the engine as a reference point. I keep this sketch in the car, because it is easy to forget the sequence when you pull it all apart, especially if your car won’t start when you are in a vulnerable spot, it's dark, or in a hurry and try to replace the cap rapidly.

The last items to check are the spark plug wires and the plugs themselves. These elements are generally not critical for the car to start as usually only one wire or plug goes bad at a time and the car will start and run with only three connected. I did have a problem with this once, which occurred on a very damp, foggy night. It seems my wires were many years old; the insulation was soft and saturated from the humidity. Then, all four wires shorted out to the engine block rather than at the plugs. Upon trying to start the car in the dark, there was a big blue glow as sparks jumped all over the area around the top of the plugs. I wiped down the wires with a towel to remove a lot of the moisture, and the car then started. Needless to say, the next day, those wires were all replaced.

Next issue, I will talk about what to do if there is no spark coming out of the ignition coil, and how to troubleshoot that element of the ignition system to get the car to start.


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