Classic Velocity

Classic Cars, Classic Bikes, Classic Commentary

"A good traveler has no fixed plans, and is not intent on arriving."  -- Lao Tzu

To Spark, Or Not To Spark

Given enough time working on classic vehicles, you will encounter all kinds of experiences. There are long journeys through some aspect of a rebuild. There are endless components of a restoration due to the search for parts. There are surprisingly quick and easy solutions. There are rewarding outcomes for hard work. There are low-cost solutions that work flawlessly, and there are expensive solutions that do not. There is the joy of victory, and the agony of defeat. And then there are the mysteries. ah yes, the mysteries. There are some puzzles and riddles that take inordinate amounts of time and effort to solve, but the mysteries are different. The particular issue at hand can be remedied, but they are either never solved or solved a very long time later. A few have been chronicled in this blog (see on getting grounded), but a recent mystery is again worthy of sharing.

The Mercedes 280SE was running beautifully. I had been out for about 45 minutes just cruising around the sparsely populated country roads. The interior smelled like only 40-year-old leatherette from Germany can smell. The acceleration was steady and sweet. That is, if you accept a liberal definition of acceleration. Rather like a locomotive, the 280SE builds speed in a way that does not disturb the occupants. But I digress. The sky was blue, the sun was shining, and all was right with the world. And then, five minutes from home, the engine lost all power and I coasted to the shoulder of the road. The car simply would not restart. It cranked over fine but would not fire. Eventually the battery began to get a little weak, and I accepted the inevitable. I was able to walk back easily from that location (once again proving the wisdom of the Concentric Circle theory). The 280SE is a tank, and it would have taken four adults to push it any reasonable distance. It occurred to me that there is something particularly embarrassing about needing to walk back to the garage from a half a mile away as opposed to getting towed from 100 miles away. It is equally embarrassing to tow-rope a car home from that short distance away, because it would be even more embarrassing to get a trailer for that distance, and it would be mortifying to call the flatbed service from the insurance company. 

Back at the garage, I was hoping for something heat related, like a coil or perhaps some kind of vapor lock. Well let's get to work checking the obvious, since all you need is fuel, air, and fire. It was soon apparent that there was no spark. That might have supported the coil theory or even just a bad condenser as had happened to me along time ago. I grab the multimeter and measure ohms across the coil. Not a complete fail, but suspect. A few hours rummaging through the old parts bins did not result in me producing a new condenser or a new coil, although I had plenty of used and suspect versions of each (see Hoarding for Gearheads). Calling and visiting the few parts places that were still open resulted in smiles and chuckles. I tried one of the used coils and used condensers from my parts stash. No spark. I headed for the computer to track down some parts....

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Days later, I had a new coil and new condenser. I also had a new set of points and a new cap and rotor which I needed to do anyway. they all got installed in a few minutes. No spark. I look through the manual and find no help there. It seems that there were three systems in use on the early electronic ignition cars, and my car looked perfectly original, but was none of the above. I checked a few forums and a couple of posts matched the symptoms; bad resistor(s), and bad CDI module. Of course I went the resistor route first. A week later they arrive and I get them installed. No spark. "Ssssunnuvva....". It is at moments like these that you start to get creative in your thinking. I remember that I have a Pertronix unit new in the box along with a coil to match it. I look up the correct unit for the six cylinder that is in the 280SE and it is a match since the unit was intended for my long gone 230SL straight six. Surely this is a good omen. I wire everything up, bypass my lovely new resistors, and triple check all of the connections. There is juice at the coil, and everything looks good. No spark. I head to the computer to locate a CDI unit....

A week later, I have a new CDI unit on the work bench. The old one is an absolute pain to remove as it is located down near the bottom of the radiator and has bolts that are difficult to access. I get the new one in and connected. No spark. Now we are a few hundred dollars into this issue, and about 3 weeks between parts and ability to work on the car. I take the Pertronix out and go back to stock. No spark. I try a few condenser and coil combos. No spark. I walk away. Days later, a forum post suggests a different wiring configuration to bypass a harness issue. I try it. No spark.

A few days later, I decide to get professional help. I am always reluctant to do this until I have done everything I can, but I was clearly not making progress. I trailered the car over to a shop that I knew, and simply told Rolf (not his real name) that I had no spark. Rolf is a former certified Mercedes mechanic who trained on these cars when they were new. It was in good hands. A few days after dropping it off I called to see how they were progressing. "I haven't gotten to it yet, but by the end of the week...". At the end of the week we got a massive dumping of snow. And then another 10 inches 4 days later. And then another. For two weeks no progress was made. When they finally got to it, they were stumped by the Pertronix and the shiny new CDI. "You know, ze original from za factory iz alvays best", said Rolf. They did not want to touch it. I suggested they put it back to stock, but they were unsure how since it matched none of the electrical diagrams. Another week passed, and they simply  replaced the cap and rotor. No spark.

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Finally, I went over one day and worked in their shop to get the car back to stock points and condenser and coil. I told Rolf that I would charge him his own shop rate for the work. He laughed. Then we combined forces. Juice was clearly not making it to the positive terminal of the coil. We by-passed the resistors and got 11.6 volts at the new non-resistor coil. Clearly something going on somewhere in that small section of harness. We opened the points and got a circuit. Great. We got excited and cranked the car over. No spark. Rolf scratched his head and declared this a mystery that required more strong coffee. I was actually pleased, as the problem was stumping the Pros as well. After a while of testing and cranking, I discovered that we lost the circuit whenever we cranked the car. But why ? Rolf returned and suggested that we remove the ground from the negative terminal of the coil. "Why ?" I asked. That connection from the chassis to the coil was stock, and solid. "Why do ve need za coil to be grounded?" asked Rolf. "It has always been grounded" I said. "Just try it, what do we have to lose?". "How about this car and your shop set on fire?" I retorted. "Nonsense, i vill pay you if zis happens" said Rolf, and he removed the ground. I reluctantly returned to driver's seat, and paused taking a deep breath. I cranked it, and cheers and laughter erupted from the front of the car. Spark.

I reinstalled the spark plug that had now been out of its hole for a month. I let the car run for a while, and looked for smoke and felt the coil. Everything seemed fine. I shut it off, and started it again. It fired right up. I went around the block, returned, and let it idle some more. Everything was fine. Miles later, all remains well. You would think that I am happy the mystery is solved, and I am, but with a certain uneasiness. Mystery #1 has just been replaced by a greater mystery. How did it ever run in the first place ?