Washington, D.C. Centre
the Spark


By Jim Lunson, tech committee


I talked last time about troubleshooting the electrical ignition system of a car that won't start. I went into what to do if you have current into the big wire from the coil to the distributor but no spark at the plugs. Now let's look at what to do if there is no juice coming out of the coil. Remember, to get a car started, you need the spark in addition to the gasoline in the cylinder and the spark is caused by electrical current emanating from the wire in the center of the coil.

The ignition coil providing the juice works just like any electrical device in the car as it requires voltage to the hot side and a ground wire on the other side. So the first check is to look at all the connections to the coil (both sides) to make sure the electric current is flowing. Corroded terminals and wires broken right at the junction with the connector are usually the culprit. Plus the coil terminals have either a thumb screw or spade connector that can also work loose very easily. A disconnect anywhere here will disable the coil resulting in no spark at the plugs.

Next, see if the 12 volt DC current from the cars electrical system is reaching the coil. This is done by checking the positive lead on the coil (or negative lead if your car has positive ground) with either a volt meter or test light (most coils have a + and - marked on the plastic top). Check between the incoming wire to the coil and suitable ground. If there is no power here, the problem is somewhere back up the line toward the dashboard ignition switch or relay. A quick way to remedy this temporarily is to use a jumper wire with alligator clips on both ends and hook it up directly from a good power source such as in the fuse box directly to the ignition coil terminal. This bypasses everything that could go wrong and insures proper power is reaching the coil. Then see if it starts.

One note on using this jumper wire: the last 4 years of the MGB lifespan used a six volt coil instead of the customary twelve. The jumper wire noted above will work and get a car started, but is not recommended for a long duration as it will shorten the life of this coil. So be careful if this jumper works to find the source of the power gap quickly so the coil can be reconnected as required by the factory.

If this jumper wire does not do the trick, the problem has to be in the points. The points form the switch inside the distributor that rapidly connects and disconnects the coil from its ground, sending the power surge to the plugs. These points open and close as the distributor shaft rotates. The fault with the points is that they have to be set a proper distance apart to work properly. If they are too close, the switch stays open too long and there is not enough time connected for the coil to build up the voltage required for the plugs to fire. If they are too far apart, they don't open long enough to let the coil send electrical power go to the plugs. Generally, these points erode away as they open and close so many times during driving plus the rub point against the distributor shaft wears down. Wear in either spot changes the gap, reducing the power to the plugs, eventually to the point where there is not enough voltage for the plugs to spark. This generally shows itself as the car becomes harder and harder to start over time to where it eventually won't start at all, although they could slip on the screw and change overnight.

The goal here is to check the gap between the points and make sure it matches the specification for the car. To do this, remove the distributor cap and rotor, rotate the engine until the points open on the cam and then check the gap with a feeler gauge. The gap varies for each car, but is available by checking the specs for the individual car. The gap is set by loosening the screw that holds it and rotating the assembly slightly along the slot provided. Reset this gap correctly and see if the car now starts. Another check is to make sure the points are connected properly to the distributor body. Often grease or dust can get under the points and destroy the ground linkage connection for them to work. There is also a condenser attached to the points, but this device only serves to eliminate any spark between the points and has little effect other than helping to extend their lifespan.

If still no starting, it is necessary to test if the points are working at all. Connect a test light between both the coil connections. This light should flash on and off when the car is cranked, indicating the points are connected and opening as they should. If this light does not flash, then there is a bad wire or connection between the coil and the points inside the distributor. Check this and the car should now start.

Another check to make is if the points have been replaced by an electronic type point system such as Petronix or Crane. These systems do away with the points and make the connection electronically so there are no moving parts. The best way to test these is to disconnect this system completely at the distributor so as to not damage the sensitive electronics inside it. Then set the big wire from the coil center to allow it to spark against the engine block. Set up a jumper wire connected to the side of the coil going to the distributor. Turn on the ignition and tap this wire to a ground such as the engine block. Each tap (or tap release) should produce a spark. If this test gives a spark, then the electronic system has failed and needs to be replaced. There is no service that can be done to it other than to insure all the wires are connected properly.

The last thing that could be eliminating electrical power to the spark plugs is a bad coil. I saved this until last because coils are very simple, sturdily made, and very rarely fail. They tend to get hot during the running of the engine and most have an oil jacket on them to dissipate this heat. It is only very gradually that the coil windings inside short out internally reducing the voltage sent out to the plugs. This is so slow that it is hard to notice, and only by replacing the coil with a new one, will you feel any performance difference due to a hotter spark (from a higher voltage). The coil's lifespan is very long and should only be replaced as a last resort if everything else checks out.

I hope these testing procedures will get your car started. Remember, it's fuel and spark and your baby should run. There may be timing, valve and carburetor problems that prevent the car from running very well, but it should start and sputter along with these two elements. One last note: lots of people ask me about the conversion from the points system to the electronic type I mentioned above. This system change is pretty complicated to explain so I will discuss the reasons and methods, as well as the pros and cons in a later article.


Back to Technical Tips