I had an interesting problem to troubleshoot on an old Ford ignition system from the 1960s. As problems go, this was a rather tricky one to diagnose. When the car ran, it ran fine. When it cut out, there was no restarting it. You really did have to be an engineer (and one who'd sat through a few courses in physical metallurgy totally unrelated to his EE degree) for the notion of A+B phase PbSn (lead-tin) grain boundary migration to occur to you.
As luck would have it, I had taken one of those classes on a lark and briefly shared my grad school office with a guy being paid by IBM's Essex Junction wafer fab to research hypereutectic PbSn alloys for use in soldering bondout wires. At the time, the fab was having package failures due to the degree to which it had shrunk transistor geometries so rapidly that it had a lot more little heaters in the same space.
The tachometer from the 1960s Fords.
Those 1960s Fords were shipped with in-dash tachometers that actually routed battery power through the tach to the ballast resistor that fed the primary side of the ignition coil. The whole setup was a series circuit, where one of the elements was the tach. Inside the tach there was a small transformer with a handful of windings on one side and many thousands on the other, so that every time a pulse went through it as points opened and closed, there'd be a higher-voltage, lower-current pulse on the transformer windings on the other side.
From the ignition coil's point of view, this small theft of energy didn't affect sparking in the least. But it did provide just enough energy that when a diode clamped and integrated with a capacitor, it could drive a d'Arsonval movement that moved the tach needle on the car's instrument panel.
A closeup of the 1960s Ford tachometer.
The problem was, if any of the solder joints were open on the primary side of that transformer in the tach, the entire car died, and it became tiresome to chase back the source of the problem. Ford used plenty of solder, but nobody expected the cars to last long enough for grain-boundary migration of PbSn alloy to open up cracks and Kirkendall voids in the joint. Nobody anticipated that body styles like the Mustang, Cougar, and Thunderbird would become such classics that the cars would still be out there running around the streets three or four decades later -- which, when you calculate e-KT for grain boundary migration times, is about what you get.
"What other mechanisms have been used for tachs?"
The other method, vs. counting/integrating primary current or voltage pulses was to run a speedometer type cable, typically from the distributor shaft IIRC, to the tachometer.
At least you solved your problem, Eric. Some of these stories about ignition problems are only figured out much later when the car is long gone. These are maddening problems. I've experienced them as well. Sometimes everything works fine, other times it doesn't work at all.
The culprets we have seen include old solder joints and a weird switch on the steering column of the Ford Fairlane.
I had a problem with a 1959 MBZ that would run fine, then not run at all. It had the characterists expected as if the timing chain or cams had lost synchronization...cranking strongly with pops and backfires, but not starting.
It was obviously getting fuel, and spark, and a careful investigation of the cams and chain and distributor shaft-gear showed them to be fine.
This had me flummoxed, and every once in a while the engined purred like new...mocking my best problem-solving efforts.
Finally, I peeked inside the rotor housing where I saw that the key had nearly broken off, leaving only a small nub to engage the distributor shaft. Sometimes it would catch in the slot, sometimes not.
That's a VERY HARD question to answer!!! IF I kept the MACH I, knowing the young person I was, I could likely be "worm food" now. The change to the DATSUN, although a major psychological & philosophical shift, was more than likely one of my better decisions. IF I had the MACH I now, I might be tempted to act as I did all those many decades ago, but w/ reflexes of a 70 year old! Considering that one cannot purchase SUNOCO 260 anymore, the DATSUN would probably be the mature road to travel.
You must understand that the MACH I wasn't just another "go fast" of that era. I can still remember going to the dealer on Oct 10, 1969 to take delivery of that vehicle. I drove it home, used it through the first winter, and then in the Spring of 1970, it underwent a major transformation into a barely street-legal rocketship. Having the benefit on engineering & physics & mathematics college degrees only exacerbated the situation because now I was more wont to apply all those neat equations into something that I could propel down the street @ the "c" (the speed of light!)
But, I survived...... maybe only for the Grace of GOD!!!!!
Grain boundry contact failures are unique in that they are quite sensitive to very small movements between the sides of the crack. Examining a failed part under an adequate microscope will make the reasons obvious.
The explanations of why using the current pulses works is fairly easy. Each pulse is held to a constant length, so that the total energy transferred is dependant on the pulse rate. The integration of that variable energy into a useful display motion is due to inertia. These are all freshman physics class principles, the analagy to calculus functions is just part of the explanation, although it would also work for a mathmatical explanation.
The editing of this story could use a bit of improvement. I still do not understand quite why the car failed or why the failure was intermittent rather than permanent. A picture of the damaged/cracked solder joint might have been helpful.
In trying to reconstruct the logic of the problem, it sounds like the 12V side of the ignition coil was routed from the battery through the instrument cluster (with the transformer in the tach) to breaker switch in the distributor ... so when the crack opens in the solder joint in the transformer in the tach, there is no power at the distributor/ignotion coil.
But I am VERY surprised that the current derived from the pulses from that transformer would be a reliable function of the RPMs of the engine. I would think that variations in the primary current caused by deficiencies in the wiring and the charge state of the battery would lead to variations of up to 25% of the currents.
Nowadays we would of course want a purely digital measurement of that frequency.
How normal was this tach mechanism at the time?
What other mechanisms have been used for tachs? I would have expected something like a very small alternator; maybe a magnet pole attached to the crankshaft passing by a coil pickup affixed to the end of the engine block.
Well, FINALLY, after 40+ years I know why my 1970 MUSTANG MACH I failed when drag racing on city streets @ 150 MPH!!!! I can't imagine all the trophies I would have had cluttering up the basement IF I had the presence of mind to take the dashboard apart to check the primary ignition circuit. I'm sure glad I traded it for a DATSUN........ that was the start of a very satisfied 100K + miles.
There's nothing more annoying or time consuming than having a problem in your dash cluster. At least in this example, you could diagnose and fix it. These days, you have to remove the whole instrument panel -- a massive task in and of itself -- and send it out or get a rebuild. And don't even mention trying to replace bulbs; that also necessitates removing the dash cluster. That's almost as bad as an earlier Made By Monkeys about having to remove the whole front bumper to change a headlight.
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