Main.TrailIndexPage | Electrical Systems | Ignition system | Electrical.Distributor | Electrical.Points

Points

The points are a key part of the Distributor. Points time when the IgnitionCoil energizes and releases energy through the cap, rotor and into the spark plug wires and into the Spark Plug, igniting the fuel mixture in the Cylinder.

Function

Points provide the mechanical switching of power to the ignition coil which produces the high voltage spark for the spark plug to ignite the compressed air fuel mixture and produce power.

How point ignitions work

Point ignitions have been around forever (Bosch first used them in 1902 in a magneto), and are fairly simple. As the engine spins the Distributor shaft, there is a 6 lobed cam mounted to the distributor shaft. Just off of the cam is a single set of breaker points that open and close by the distributor cam's contact with the point set(s). When the points are closed, they allow the coil to energize and store energy. When the points open, the stored energy is released as high voltage energy through the rotor, cap, the high tension wires to the spark plugs. How long the points stay closed is the 'dwell' or dwell angle.


Dwell Angle

At ordinary engine operating speeds, the points open and close a couple of hundred times per second, the exact number depending on the number of cylinders and the engine RPM. The points need to be closed for a appreciable time in order to build up the maximum magnetic flux in the ignition coil core.

The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation. In a six cylinder engine, the angle between each ignition cam lobe is 60 and the period of points closure or "DWELL" is usually a 30 to 35.

Dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse in the coil before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum.

Both conditions give a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds. The dwell, as well as spark plug gap, do have an effect on ignition timing. The later the points open, the later the spark comes and retards the timing. The earlier the points open the sooner the spark comes and advances the timing. That is why TIMING IS THE LAST THING TO BE SET IN A TUNE UP.


Other Cause for Point Failure

When the ignition switch turned to the ON/RUN position (before the car started or to listen to the radio) it is possible that the points in the distributor are closed. Thus, we would have about 8 Amperes of current being drained from the battery and the points would begin to heat from the current. In order to avoid this, a "radio" or "accessory" position was added to the ignition switch of most vehicles. In the radio position, no power is applied to the ignition system, battery drain is significantly lowered and the points do not wear due to heat.

Solving the Points Problem

The most common ignition failures are points and spark plugs, followed closely by the distributor rotor, cap and spark plug wires. The points, however, are the "Achilles Heel" of the system. If we could find a way to extend the life of the points, there would be fewer failures and tune-ups. Automobile manufacturers began to employ transistorized ignition systems in the late 1960s and early 1970s to reduce point failure.

But (yes, there's always a "but"), we still have the points. Points are mechanical, subject to bounce and vibration and must be actuated - mechanically - by the camshaft in the distributor. Whereas the wear on the point contacts has been reduced, the fiber pawl, which rides between the points and the distributor camshaft, can still wear out or break.

The susceptibility for engine failure due to the points has been reduced, but not eliminated. A method needs to be found whereby the mechanical points can be replaced with some other type of trigger for the ignition. This is achieved by the use of Electronic Ignitions.

Maintenance

Points work very hard. They open and close three times for each revolution of the engine. So, at 800 rpm idling speed,they open 2400 times and at say 4000 rpm, 12000 times or 200 times a second. Each time they open there is a little spark across the points and eventually, they burn out. Also, there is a fiber pad which rubs against the lobed cam and this will wear too, reducing the points gap and also changing the spark timing.

Regular maintenance involves a visual examination and checking/resetting the points gap / dwell angle to bring the dwell angle within specifications. A small blob of distributor grease is used behind the points pad to keep it lubricated to minimise wear. A damaged condenser will cause a build-up of material on one of the points and a crater in the other. Any adjustment to the points changes the relationship between the fiber pad and the cam and therefore the ignition timing will change as well.

Points burning

With the W113 series of cars, having the ignition on for the radio but not running the engine may burn the points.

There is no ignition 'accessory' position for radio playing that does not actually fire off the ignition. At the first notch that plays the radio the fuel pump also starts running. The next notch starts the car.

At this position the points can be burned, if the engine stops when the points just happen to be slightly open, arcing and burning across the contacts can occur. It's not likely but possible. The ignition coil gets warm when the ignition is left on most of the time. On the later transistorized ignition cars the effects may be different.


Points Gap Adjustment

Dwell trouble shooting tips and tricks

What are correct point gaps and dwell settings?

 Point Gap 0.3 to 0.4 mm
 Dwell 38.

These settings can vary among distributors. See Chart under Distributor Settings.

How to set gaps and dwell settings

For single point distributors it is very simple. Just pre-set the point "gap" by using a feeler gauge of the correct thickness and placing it between the contacts on the point set. You must have the rub point of the point set touching the highest part of one of the cam lobes when you do this The easiest way is to spin the rotor by hand slightly (you should be able to do this since there is play from the mechanical advance curve in the distributor) so that the rub pad on the point set opens the point set. Snug the screws holding the point set and you are ready to spin the engine.

Loosen the screw holding the stationary point just enough to be able to move the plate to set correct gap. A flat screwdriver can be used between the two dimples and the slot in the plate to get the correct gap. Then retighten the screw.

Now, with the (Safety Note) ignition coil wire removed, the cap and rotor off the distributor, and a dwell meter properly connected to the coil, you can spin the engine. The best way to do this is with a remote starter button. The only other way to do this is with someone assisting you. Crank the engine and watch the reading on the dwell meter.

If the dwell reading is not within the specifications, you must adjust the point set (the procedure above is the easiest) and securely tighten the point set mounting screws. The () in your specification settings means "plus or minus", so if the dwell is supposed to be 38 2, this means that 38 is the optimum setting, but between 36 - 40 is acceptable.

Troubleshooting

There is really not much that can go wrong with a breaker point ignition system. But, I've listed below some common problems, what the possible causes are, and hopefully how to fix it.

Points going out of adjustment.

Cause:

  1. Normal Wear
  2. Mounting screws not holding
  3. Excessive shaft/bushing play
  4. Incompatible points
  5. Early point wear



Points set ready for a change

Cure:

  1. Replace the point hold down screws
  2. Rebuild the Distributor
  3. Use the correct point set
  4. Replace the points and Condensor

Condensor Failures

Cause:

  1. Outer shell coating is too thick, therefore does not ground properly

Cure:

  1. Lightly sand your new Condensor for better grounding

Loss of Performance

Cause:

  1. Incorrect point settings
  2. Worn Points
  3. Low supply voltage

Cure:

  1. Re-Set the points
  2. Replace the points
  3. Verify supply voltage at 8-10 volts (measured at the coil at idle speed)

Early Point Wear

Cause:

  1. Incorrect point settings
  2. Excessive voltage
  3. Condensor failure

Cure:

  1. Re-Set the points
  2. Make sure a ballast or resistance wire is installed
  3. Replace the condenser


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