Ignition Systems: A Technical Brief

Article by Richard Simonds
Photography Richard SImonds
Traditional points-based ignition systems are found in many old classic cars. As discussed in recent articles, aftermarket companies now make replacement electronics-based ignition systems. Both systems have pros and cons.

Classic gasoline-engine Mercedes-Benz cars used a standard points and condenser system to time the build-up and release of the charge in the coil and transmit the ignition spark to the spark plug in each cylinder in turn. As discussed in recent articles, aftermarket companies now make replacement electronics-based ignition systems. Both systems have pros and cons.

Traditional points, condenser and coil systems

In the traditional system, as the points wear over time, the distance between them when open changes, and they begin to develop pits, which affect the dwell angle – the time needed for the condenser to store the high voltage spark – and the engine can misfire. The points should be checked every few months, lightly filing the surface to smooth out surface pitting and then checking and adjusting the gap.

As the points are pushed open and released by the cam lobe at higher engine speeds, if the spring that closes them is not strong enough, the points “float,” so no spark is transferred to the cylinder on that rotation. If this occurs, the points system must be replaced. Also, crankcase oil vapor can seep up the distributor shaft into the distributor, leaving oil mist on the points, which can cause misfiring.

At a minimum, the points and condenser should be replaced once a year – when the engine is tuned. Timing and dwell angle must be checked and the gap adjusted when new points are installed as the gap of the points affects the dwell angle. When replacing the points and condenser, be sure to put a fine film of grease on the cam lobes.

Under normal use and maintenance, a points and condenser system works just fine, and is quite inexpensive to maintain. However, as mileage increases, the shaft of the distributor can start to wear, which can cause erratic timing behavior. That can only be corrected with installation of a replacement distributor, which can be costly and hard to find. According to Tom Hanson of the Mercedes-Benz Classic Center in Irvine, California, replacement distributors are no longer available from Bosch, and the Classic Center no longer has any in stock in either the U.S. or Germany. Although there may still be replacement stock available elsewhere in the supply chain, ultimately anyone with an older Mercedes-Benz is eventually going to have to install an electronic-ignition conversion system if the original fails.

Electronic-ignition conversion systems

Typical electronic-ignition conversions keep the distributor, but replace the points with either a light or a magnetic sensor to send spark to each cylinder; kits cost $100-$150. These kits have an electronic control module to be mounted near the distributor and coil, offering better spark and more accurate timing. The PerTronix Ignitor is perhaps the best-known brand, but Crane Cams, Accel, Proform, and others are available. They control dwell angle, are not affected by oil mist, and can run at higher engine speeds without misfiring.

A self-contained distributor with all the components inside has been introduced by 123 Ignition (Netherlands) and sold at http://123Ignitionusa.com in the United States. The basic system can read the timing of each cylinder’s spark and adjust the distributor accordingly to improve operation, fuel economy, and performance. The system costs approximately $450 plus shipping and installation.Advantages and disadvantagesMany people who have made the conversion report easier starting, smoother engine performance, and increased gas mileage, with more accurate spark timing due to a consistent dwell angle, and no misfires at higher speeds due to points float. Certainly, there is no need to adjust and clean the points on a regular basis or replace pitted points with every tune-up.

The disadvantages of these replacement systems are that the replacement system changes the appearance of your engine compartment, which can be an issue if you enter your classic in a concours d’elegance to be judged, can fail without warning and can’t be repaired; it must be replaced. For owners touring far from home, it is a good practice to carry an original points and condenser system in the event of failure of the replacement system.

My experience

As replacement points and condensers became more unreliable in the early 2000s, and needed replacement every 2,000-3,000 miles, I purchased a Crane XR700 electronic system for two 1960s-era Mercedes-Benz cars. One failed without warning and I replaced it with a 123 Ignition system; the other still has the Crane XR700, which is now discontinued. I appreciate the reliable operation from the electronic ignition systems and not having to replace points and condenser every several thousand miles. I no longer enter concours events, so the benefits exceed the disadvantages for me.

 

A. Original equipment Mercedes-Benz points and condenser system.

 

B. An electronic ignition conversion: Crane XR700 disc and light system.

 

C. Another electronic ignition conversion: Self-contained 123 Ignition system.

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