More About Ignition Systems for 1970s Classics
Quincy Leslie, Michael Salemi
We weren’t able to cover the whole range of ignition systems available for classic Mercedes Benz vehicles in our May-June 2017 issue. Two of our readers took the time to write in with detailed descriptions of both a system used during the transition from points-based to an electronic system and an interesting alternative to installing a modern aftermarket system.
Early transistorized ignition systems
My 1971 250/8 with its 2.8-liter 7-main-bearing straight-6 with two Zenith carburetors had an early transistorized ignition system manufactured by Bosch, as did the Mercedes-Benz Pagoda models of the same period. As I remember, the model had an off-red rotor and distributor cap, and a blue coil with a silver-metal transistorized ignition control module under the battery tray. It was essentially a transition technology, using points in the distributor to activate a solid-state control module connected to the coil.
When the points opened and closed in response to cam motion, they activated the control module to provide the actual voltage from the battery to the ignition coil. As in a standard step-up transformer, when the points closed, the charge from the module to the coil rose to about four volts. Then when the points opened and the module cut off the power, the charge collapsed, creating the 50-60,000 step-up voltage to the distributor. The distributor then routed the lighning-like high-voltage output to the spark plug to cause combustion. The specific blue Bosch coil in this system may have had a higher voltage step-up output value.
This initial system had three shortcomings. First, the correct points were chrome-plated, manufactured by Bosch, but non-chrome-plated points also would fit; and if the mechanic didn’t use the correct points, the system would soon begin to malfunction. Second, due to the low amperage going through the points, the oil and corrosion on the points didn’t burn off as with normal points carrying the full ignition current. As the points became oily or corroded, the resulting higher impedance reduced current flow to the relay box, causing the engine to run rough. Third, the transistorized relay boxes were susceptible to heat and so could often fail prematurely due to ambient under-hood heat. If the system failed, the ignition system would fail, often leaving the driver stranded on the side of the road.
Because of its limitations and additional complexity, many owners eventually replaced the system with a previous generation points-based system. However, given the mechanical issues associated with the traditional systems, nowadays it is more typical to install a modern aftermarket electronic ignition system, such as Ignition123, Pertronix or Crane. However, the owner or mechanic must be very sure to remove all of the original components when replacing the points-over-electronic ignition. Stories are told of owners who replaced the internal mechanism in the distributor with a Pertronix or Crane system without removing the relay module; thousands of volts through the relay module on the first revolution of the engine when starting can produce a nasty fire.
For the owner who values total originality, these issues can be mitigated by making sure that the correct Bosch chrome points and other components are always used when replacements are made. The challenge is that replacements for the original ignition control modules are difficult to find, so the original or rebuilt ignition control module should be tested and verified to be in good working order during regular scheduled maintenance. Quincy Leslie
For a more detailed article by Pierre Hedary, visit www.mbca.org/star-article/may-june-2011/transistorized-ignition-systems.
Rebuilding Bosch distributor systems
In Richard Simonds’s ignition article (The Star, May-June 2017), he mentions that wear of the distributor shaft can be a source of ignition-system failure. He noted that repair would require a replacement system with an unworn distributor shaft, but that new replacements are not available from standard parts sources.
However, in my experience with my 1969 280SL, I have found that Bosch distributors are mechanically very robust, and a large number of them can be rebuilt. While my next distributor might be the all-electronic replacement 123ignition as mentioned by Simonds, owners might prefer to rebuild the original system if they know the option is available and they know where to go for the service.
I’ve had distributors rebuilt by both Dan Caron of SL Barn in Ontario and Glenn Ring, who rebuilds distributors as a sideline in his workshop on Long Island. In both cases, the distributors were returned in near flawless condition. The Star advertiser Bud’s Benz in Georgia (BudsBenz.com) also offers a distributor rebuilding service.
Ring and Caron explained that in early models from the 1950s, the main shaft spins in a cast-iron bore; later models have bronze bushings. The later bushings can be replaced and early distributors can be machined to accept bushings. The control pins also wear where they contact the weights and cam surface. The control pins can be rotated 90 degrees so the worn part is not in contact. Replacement pins can also be salvaged from other cores.
Typically the main shaft, made of hardened steel, doesn’t wear very much, but moisture can cause pitting on the shaft. The much more typical source of wear in these distributors is between the point lobe plate and the main shaft. The point plate itself can wear. In either case, the result is erratic timing as the points open and close, one of the main diagnostic symptoms that those classic Sun distributor machines with their oscilloscope screen were designed to detect.
The most common problems with the cam surface are gouge marks due to abrasive contact between the cam surface and the points block, because the casual owner or uninformed mechanic neglected to put oil in the little oil cup under the knurled cap in the distributor or apply grease to the cam lobes. After the disassembly, rebuild and restoration process, both of these individuals use salvaged Sun distributor testers for final testing and setup.
Certainly there are distributors that can't be rebuilt, but most can; and even if they aren’t brand-new, they are close enough to restore ignition performance and maintain originality. Michael Salemi
Resources
Dan Caron’s SL Barn: [email protected], 519.677.5939.
Glenn Ring: [email protected]. www.glenn-ring.com
Ring’s website has may photos of the rebuild process
Bud’s Benz: BudsBenz.com