Murphy's Law
by George Murphy
 
Original Breaker Point and Solid-State Ignition Systems
 
Mechanical breaker point ignition systems have been around almost since the dawn of the automobile age and so have failures of its basic components – the points and condenser. Mercedes-Benz recognized these shortcomings and in the late 1960s introduced a “hybrid” electronic ignition system.

The Way It Was
With the original M-B electronic ignition systems, the breaker points were still used, but they connected to a transistorized switching unit, which in turn provided the impulse to the ignition coil to fire the spark plugs.  This arrangement allowed very low current across the ignition points, thereby extending their operating lifetime.

The switchgear comprises several transistors, resistors and other electronic components in a metal housing, depending on type (shown above). The housing protects these components against mechanical damage and moisture and serves simultaneously to dissipate heat. Contact connections on the switchgear are made by means of a 3-point flat plug connector or a 4-point round plug connector with separate coaxial connection for the points. Switching units for model 107 are provided with a firmly attached connecting wire.



As shown in the wiring diagram, the switching transistor is energized when the contact breaker point is closed. When the contact breaker point opens, the transistor locks and the ignition coil current is interrupted. The interruption of the circuit in the primary winding induces the ignition voltage in the secondary winding similar to the conventional coil ignition. To increase the ignition voltage when starting, the 0.4 ohm series resistance is bridged by contact 16 on the starter switching unit (3).

Unfortunately, these early Bosch systems only worked well on new engines. Once aging set in – at 50,000 miles or so – distributor shaft wear allowed oil mist to deposit on the points. The distributor shaft can wear to the extent that the dwell cannot be accurately set and oil mist can migrate up the shaft to deposit on the points – degrading ignition performance.

Of all the systems that enable your older Mercedes-Benz to stay on the road, fewer are more trouble-prone than this old pre-1975 hybrid transistorized ignition system. The main culprits are usually the transistor switching unit (3) and/or the contact breaker points in the distributor. The usual scenarios are either the point arm rubbing block eventually wears down, reducing the gap, or the switching unit begins to fail from heat buildup. (Some owners found that allowing the switching unit to cool down after a sudden engine failure would permit engine starting and operation for a short time, when the switching unit would again quit).

Replacement switching units are generally no longer available or very expensive – and you still have to deal with contact points and associated wear. Fortunately, modern solid-state technology is now available to eliminate these ignition failures.
Three systems are available; they include:

Pertronix Ignitor Electronic Ignition (www.pertronix.com)

The Pertronix conversion uses non-mechanical means to trigger the ignition coil. The points and condenser are replaced by a magnetic pick-up and a “trigger” wheel fitted to the distributor shaft.

The trigger wheel is fitted to the distributor shaft by a self-centering spring clip. There are no points to wear out, no condenser to short out and no expensive transistor switching unit. Also, if the distributor shaft is worn, it will have negligible effect, as the system compensates for it regardless of the shaft’s orbit. Because there are no mechanical parts to wear out with the Pertronix unit installed, once set, you will never again have to adjust your timing or dwell. An added bonus is future tune-ups will take less time and expense. Cost is $120-$150.

Crane Cams FireBall XR-700
(www.cranecams.com)
The Crane XR-700 uses non-mechanical means to trigger the ignition coil. The points and condenser are replaced by an optical pick-up and a “shutter” wheel attached to the distributor shaft. A light-emitting diode (LED) shines a beam of light through the shutter wheel at a photocell immediately opposite.

The shutter wheel has the same number of evenly spaced slots as engine cylinders, and is fixed to the distributor shaft by a self-centering spring clip. As the wheel rotates, the slots pass over the photocell, allowing it to “see” the LED, and in conjunction with the external Crane ignition module, signals the ignition coil to fire. There are no points to wear out, no condenser to short out and no expensive transistor switching unit.  Also, if the distributor shaft is worn, it will have negligible effect, as the shutter wheel is still wide enough to compensate, regardless of the shaft’s orbit. Cost is $120-$150.

123ignition
(www.123ignitionusa.com)
123ignition offers custom-made complete distributors for most 4-and 6-cylinder M-Bs. In addition to the standard advance curves, it offers optimized advance curves for driving on liquid propane gase, or the new (cursed) ethanol fuel most of us are finding in our gasoline. These units include solid state ignition switching also. Prices range from $395-$500.
Where to order: Pertronix and Crane products can be ordered from various STAR advertisers; 123ignition is available at www.123ignitionusa.com.

Next issue: How to install solid state ignition.