George Murphy/Murphy's Law – Installing the Pertronix Ignition System
Murphy’s Law – George Murphy
Installing the Pertronix Ignition System
Of all the systems that enable your older Mercedes-Benz to stay on the road, few are more trouble-prone than pre-1975 hybrid transistorized ignition systems. The main culprits are usually the transistor switching unit, contact breaker points and condenser. Typically, the rubbing block eventually wears down, reducing the gap, requiring that the points be replaced or that the condenser shorts out and takes the points with it by frying the contacts – and the (expensive) switching unit.
What can go wrong
Points and condenser ignition systems have been around since the dawn of the automobile age, and so have failures of their basic components: the points and condenser. Further, points get pits and deposits on the contact surfaces that shorten their usable life. The quality of replacement points has degraded over the past 10 years and it is not uncommon to have to change them every 2,000-3,000 miles instead of the former 5,000-10,000 miles between tune-ups. Mercedes-Benz Cars recognized these shortcomings of the point-type ignition systems and in the late 1960s worked with Bosch to develop a “hybrid” electronic ignition system.
Unfortunately, these early Bosch systems only worked well on new engines. Once aging set in around 50,000 miles or so, distributor-shaft wear allowed oil mist to deposit on the points. The distributor shaft also 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.
The Pertronix system
The Pertronix conversion unit available from several suppliers uses non-mechanical means to trigger the ignition coil: A magnetic pick-up and a "trigger" wheel fitted to the distributor shaft replace the points and condenser. 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 because the system compensates for it, regardless of the shaft’s orbit. And because there are no mechanical parts to wear out with the Pertronix unit installed, once set, you will never again have to adjust timing or dwell. In 85,000 miles, the ignition timing on my 1971 250 sedan timing hasn’t changed one degree. An added bonus is that future tune-ups will be quicker and less expensive. If you own a timing gun, you can do this job yourself in under an hour; if you’ve never adjusted your timing, a shop can do the swap in about the same time.
Installation is easy on a Mercedes-Benz
To install the magnetic pick-up, first set the engine timing mark on Top Dead Center (TDC) for cylinder No. 1. If in doubt as how to achieve this, consult your workshop manual. Now follow the spark plug wire from No. 1 cylinder back to the cap, and mark both the cap and the side of the distributor body with a felt tip pen. On earlier Bosch distributors such as on M130 engines, it may be easier to install the pick-up in the distributor if you first remove the distributor from the engine.
Remove the points, condenser and wires to the transistor-switching unit. Next install the bracket for the magnetic pick-up with its adjusting arm and mounting foot on the breaker plate of the distributor, using the screws that originally mounted the points.
Now fit the trigger wheel to the distributor shaft and slide the magnetic pick-up into place. Don’t tighten all the screws completely until you reinstall the distributor, and make sure that there is at least 0.030 inches clearance between the trigger wheel and magnetic pick-up so that they don't rub anywhere.
Be sure to leave enough extra cable inside the distributor body to allow movement of the breaker plate for the vacuum advance. If you have removed the distributor, now is the time to reinstall it in the engine. Slacken the clamp bolt and rotate the distributor body until the rotor points to the line you marked earlier. Connect the Pertronix unit to the ignition coil as shown in the Pertronix instructions.
Now set the timing to Mercedes-Benz specifications with a timing light. In your engine manual, find the amount of spark advance specified for high-rpm operation. For example, if your ignition is specified to advance to say, 30 degrees at 3,200 rpm, then adjust the timing to show 30 degrees advance with the engine at 3,200 rpm. In this manner, your engine timing is optimum for operating engine rpm. Never mind what the idle advance is – you don’t drive the car at idle rpm.
It is a good idea to check the timing once a year to see if there is any change due to timing-chain wear or wear in any other components.
While we are in the neighborhood of the ignition system, it’s a good idea to examine the rest of the components. On a 35-year old Mercedes-Benz with original components, the distributor cap, rotor and ignition wires will need replacing. Last but not least, you'll need a set of new and properly gapped spark plugs – do not use platinum spark plugs! Mercedes-Benz specifically advises against these for all M-B engines up into the 1990s. The correct spark plug gap is 0.8 mm (.031 inch) for all engines of this vintage.
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