The Kettering ignition system of points, condenser and coil was never more than a compromise. (The Model T, with one coil per cylinder was more dependable but also more complex and costly.) The component most likely to fail in the Kettering system was the points, since they had to carry up to ten amps of current, a difficult task for any switch turned on and off fifty times per second. Until they found something better, Detroit designed their way around the problem with dual points and even dual ignition systems. The cure wasn’t found until the advent of the transistor in 1948, and didn’t gain commercial popularity for another twenty years.
The cure for the Kettering system is to use the points not as a current-carrying switch but merely as a signaling device to tell the ignition system when to fire the spark. The actual switching is done with a specialized form of transistor.
And instead of feeding the coil a diet of 12 volt current, it is fed 400 volts, allowing it to build up the maximum charge in the shortest time. The 400 volt energy is developed using a tiny switching transformer inside the ignition module, the energy stored in a large capacitor which is discharged into the coil at the appropriate time. This capacitance-discharge gave such systems their name, often abbreviated to CD.
‘Point-switched’ CD ignition systems require periodic maintenance as the rubbing block wore down, typically about every 10,000 miles. The points required replacement at about 50,000 miles, due to rubbing block wear. The points themselves are still in perfect condition at that time. Designed to switch a 120 watt load fifty times per second, they hardly notice the microampere load needed to signal the CD module.
The biggest advantage in CD ignition is improved efficiency. Spark voltage remains constant at all speeds, and constantly high. With a dependable 40,000 volt spark at all engine speeds the spark plugs may be gapped much wider, providing better flame-front ignition. The wider plug gap fouls less readily, giving even worn engines a boost in efficiency. With electronic ignition regular sparking plugs last about 25,000 miles, the new platinum jobbies up to 120,000. Over all, CD ignition insures better combustion, resulting in better fuel efficiency and lower emissions. The engine even runs cooler, thanks to less after-burning in the exhaust manifolds.
A point-switched CD ignition module is the wisest ignition modification you can make to an early VW since it supplants rather than replaces any of the stock ignition components. Indeed, should the electronic module fail you may return to the stock Kettering ignition by simply reversing a plug on the ignition module. Such redundancy is lacking on all modern electronic ignition systems, a potentially fatal flaw since they give no warning of incipient failure.
The most practical CD ignition module for Volkswagens was available from J. C. Whitney until March of 1995 (although it was never advertised specifically for VW’s). Cost was about $50 and installation took thirty minutes for a clumsy mechanic with few tools and no electrical background. J. C. Whitney’s current ‘VW-specific’ CD offerings cost about $100 and requires replacement of the distributor. This newer system, and all others presently offered by J.C.Whitney, use optical triggering. Properly installed, such systems need no adjustment for the life of the distributor, about 70,000 miles in most cases. But as with all optical or magnetically triggered systems, you cannot revert to Kettering ignition without reinstalling a set of points or replacing the entire distributor.
As to the dependability of such aftermarket CD modules, I have installed more than 100 (possibly twice that) with but a single failure, and that one was bad out of the box (and promptly replaced by the manufacturer).
I’ve used CD ignition modules since 1961, building the first few. (I am a ham radio operator.) I have more than 20 years experience with one unit, a Heathkit on my 1973 Datsun the week it was purchased (new). The Datsun has since accumulated 230,000 (on two engines).
-Bob Hoover
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