Re: no spark

Posted by Tom on January 26, 2004 at 06:22:14 from (152.163.253.66):

In Reply to: no spark posted by smark on January 25, 2004 at 13:28:33:

Smark,

If you have enough battery capacity to turn the starter motor, you have enough battery capacity to generate a spark in the ignition system. It would be good to get a new battery, but I seriously doubt that is what is causing your lack of spark.

How about a little review of what generates the spark?

The ignition system is comprised of the primary side and the secondary side. The primary side consists of the battery, ignition switch, primary winding in the coil, points, and the condensor. One of the little posts on the coil will be connected to your battery via the ignition switch. The other little post on the coil will have a wire that goes into your distributor.

When the ignition switch is turned on, battery voltage will be applied to one side of the coil. If the points are closed, current will flow from the battery through the primary winding of the coil, resulting in the build up of an elecromagnetic field in the coil. When the points BREAK (open), the magnetic field collapses and induces a large voltage into the secondary winding of the coil. This large voltage is presented to the large center connector on the coil. The only purpose of the condenser is to reduce the arc that occurs when the points open after having been closed. The condensor does not create any of the spark itself. Condensors seldom fail, but if one did short out, it would be just like the points were staying closed all of the time. If one opened (which virtually never happens with a condensor), the only effect would be that the points would arc and burn more quickly than normal.

The secondary side of the system is comprised of the secondary winding on the coil (large center post), the distributor cap, rotor, spark plug wires, and spark plugs. When the points OPEN and the large voltage is presented at the center connector on the coil, it is directed into the center post on the distributor via the wire that goes from the center post on the coil to the center post on the distributor cap. It then is applied to the top of the rotor which presents it to a post on the distributor cap. It is then delivered to the spark plug via the spark plug wire.

So, having said all of that, if you have no spark, you need to insure that the you have battery voltage applied to one post of the coil when you turn on the ignition switch, that the points really are closing, that the points really are opening, and the coil is okay. If you have an ohm meter, you can disconnect the wires from the coil and check for continuity between the two little posts on the coil. You should have a few ohms of resistance in the coil. If it is open, the coil is bad. If it is zero ohms, the coil is shorted. If it is a few ohms, the coil is good.

By far, the most frequent cause of failure to generate a spark is that the points are incorrectly adjusted and therefore they are not closing when they should or opening when they should. That's the first thing I'd check if I were you. Good luck.

Tom

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