Point Type Ignition Systems

Point Type Ignition Systems

   I've gotten a few letters in the past few weeks asking me how to set up standard ignition points. It seems there are enough antique cars out there that still use them and need replacing from time to time. So I'm going to explain how they work and how to set them up. For you young guys who weren't around when points were in use, this will let you see all the fun you missed.

   All cars up until 1975 or so used this type of ignition system. After 1975 most cars went to Electronic Ignition systems. Basically electronic ignitions were "improved points." The principles were the same and it simplified the ignition system.

   The basic ignition system consists of the ignition coil, points, condenser, distributor and spark plugs. A ballast resistor may also be included in this system. When all of these parts are connected and working properly, we will get the spark the engine needs to run. Now, what are these parts and what so they do?

The Parts

   Ignition Coil: This is the part that makes high voltage, up to 40,000 volts, for the spark plugs from the low voltage that is supplied to it by the battery. The reason an ignition coil works lies in the physical properties of electrical current. When a current flows through a conductor it generates a magnetic field around the conductor.

Conversely, when a conductor is moved through a magnetic field, a voltage will be induced in the conductor. The coil takes advantage of these principles of inductance by winding one coil over the top of another around an iron core. The changing voltage in the primary winding serves as the 'movement' needed to induce a voltage in the secondary winding.

The voltage in either winding is proportional to the number of coils in the inductor; if there are more turns in the secondary, its induced voltage will be higher than the voltage in the primary.

   When the points close, current through the coil primary increases from zero to maximum in an exponential manner, rapidly at first, then slowing as the current reaches it's maximum value. At low engine speeds, the points are closed long enough to allow the current to reach a higher current level. At higher speeds, the points open before the current has time to reach this maximum level. In fact, at very high speeds, the current may not reach a level high enough to provide sufficient spark, and the engine will begin to miss. This current through the coil builds a magnetic field around the coil. When the points open, the current through the coil is disrupted, and the field collapses. The collapsing field tries to maintain the current through the coil. Without the Condenser, the voltage will rise to a very high value at the points, and arcing will occur.

   Points: Ignition points are a set of electrical contacts that switch the coil on and off at the proper time. The points are opened and closed by the mechanical action of the distributor shaft lobes pushing on them.

The points have a tough job, switching up to eight amps of current many times per second at highway speed. Indeed, as engine speed increases the efficiency of your ignition system decreases, thanks to heating problems and fundamental electrical laws. This declining efficiency has a serious effect on your spark voltage and results in poor high-speed performance, incomplete combustion and other drivability problems.

   Condenser: Those same principles of inductance create a kind of paradox, because when the points open and the magnetic field collapses it also induces a current in the primary as well. It's not very much because there are only a few windings in the primary, but it's enough to jump a small air-gap, such as the one between the just-opening points in the distributor. That tiny spark is enough to erode metal away from the points and you'll 'burn' the points.

It prevents the points from arcing and prevents coil insulation breakdown by limiting the rate of voltage rise at the points.

   Ballast Resistor: This is an electrical resistor that is switched in and out of the supply voltage to the ignition coil. The ballast resistor lowers voltage after the engine is started to reduce wear on ignition components. It also makes the engine much easier to start by effectively doubling the voltage provided to the ignition coil when the engine is being cranked. Not all car manufacturers used a ballast resistor in their ignition systems So you should check to see if yours does.

   Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

Point Type Ignition Systems

Replacing The Points

   Now that we know what the parts are and what they do, lets talk about replacing them. Replacing the points and condenser are very easy and you should always put in a new condenser with new points. I always took the old points and condenser and put them in a zip lock bag and kept them in my car. If I had a problem I always had a set that I knew would work and get me going again.

   All you need to replace the points are some basic tools, a magnetic screwdriver, feeler gauges and a dwell meter.

   First remove the old points and condenser. Use a magnetic screwdriver to remove the screws. I think every mechanic has dropped those little screws inside the distributor at one time or another. I know I have. Once you have them out, install the new ones but don't tighten the points completely, just snug them up. Most new points come with a little vial of grease. Make sure you clean the distributor cam and apply this grease. If it didn't come with grease, use a dab, a small dab, of white lithium grease. This will keep the rubbing block from wearing out in a week and a half.

   Setting the Point Gap: Getting the best gap between the points is essential for proper engine performance and reliability. Set the points too wide and the spark plugs don't get enough juice. Set them too close and the engine works fine for a few miles… until the points are burnt beyond use.

   Most cars had a point gap of about 0.019", or the thickness of a matchbook. Some were set higher or lower so check your manual to be sure. To measure the point gap, you need a set of feeler gauges. Adjusting the point gap is a simple process, but it takes some practice to get the hang of doing it properly.

First, make sure the rubbing block is on the high point of one of the cam lobes. If it isn't, you will have to turn the engine a little bit in order to turn the cam.

   Once you have the rubbing block on top of a lobe, you can measure the point gap. Loosen the screw that holds the stationary point bracket to the base plate. Not completely, just enough so that you can move the bracket by inserting a screwdriver tip and twisting it Adjustment is a matter of trial and error. Move the stationary point out a bit if it was too close, tighten the holding screw (not too tight), and measure the gap. If it still isn't right, try again. The feeler gauge should have a light drag when the points are properly adjusted. This is where practice and patience comes in handy.

   Dwell Angle: The dwell angle is the number of degrees of rotation of the cam/distributor during which the points are closed. During each rotation of the cam/distributor, the points must open and close once for each cylinder. The points must stay closed long enough to allow the coil primary current to reach an acceptable value, and open long enough to discharge and produce a spark.

   Many mechanics like to check the dwell measurement with a dwell meter after setting the points.

I know I do. There are some who say you don't have to. But it is a good way to check the point gap and make sure it is right.

   I know many mechanics, myself included, that set the points by dwell alone. It is a perfectly acceptable and accurate way of adjusting the points. In fact, most all GM distributor caps have a little door that allows access to the points so the dwell can be adjusted while the engine is running. On engines that don't have that access you need to be a little more creative. What I do is remove all the spark plugs from the engine, set up the points, turn the key on and crank the engine while adjusting the point dwell. Once it's set, I lock them down and finish the tune-up.

   When I set the dwell, the spec is given as a range. I always set the dwell to the low end of the range.

This way as the points wear, the dwell stays in range.

   Well, that's it. It's not that difficult to do. And if your car has dual points, don't be scared. Just treat them as individual points when setting them up and you'll be fine.

   Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved