Interesting problem to troubleshoot -- and one that reminds me of one encountered in the old Ford ignition systems of the 1960's. Back then those cars that were shipped with in-dash tachometers 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 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 impact sparking in the least. But it did provide just enough energy that when diode clamped and integrated with a capacitor could drive a d'Arsonval movemen that moved the tach needle on the car's instrument panel.
The problem was that if any of the solder joints were open on the primary side of that transformer in the tachometer, the entire car died, and it became a real pill 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 SnPb alloy to open up cracks and Kirkendall voids in the joint. And 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.
In the early 1990's there was a rash of failures of these systems, and I always told people diagnosing their problems to look for the words "Sun-X" in the corners of the automotive glass as a tipoff that this could be a bad tachometer. When asked why, I'd explain that this was the indicator of "tinted glass," which was an option (not standard offering) in the 1960's that'd tip you off that the car was sold in a southern state -- where summertime interior temperatures in the passenger compartment drove up the "T" portion of that e-KT equation. After a few years, failures became less common, and then they became more common again -- only this time among cars without tinted glass. These were the "northern" cars where interior temperatures drove the PbSn grain boundary migration reaction more slowly.
Nowadays the occasional tach failure does show up, but most cars that have ever had it have had it fixed by now -- and with the mystery solved, car collectors either know what's happening or know somebody who can help troubleshoot. The problem is always, however, that a solder joint opening up is an intermittant open, which can always be a pill do to diagnose under any circumstances.
Smart collectors of these cars often just open the tachometer and reheat those solder joints as a matter of course the minute they buy one of these cars, anyway.
The story reminds me of my first computer, a Commordore PET. After about a month of operation it began to loose the sync to the display. I used a pencil eraser to gently push on the ICs on the circuit board and one popped out of the board. Commodore probably could not get the IC on time and built the board with plugs in the IC lead holes. When they finally got the part, they completely forgot about soldering it into the board. At least my problem was easily repaired without having to remove potting compound.
This calls to mind assembly techniques used in a number of modern, miniaturized gadgets, where if you take them apart, you sometimes see that the rechargeable power cell isn't solidly soldered in place. Usually there's that thin, slowly corroding tab, attached with poorly fluxes solder. But more times than you'd think, the battery is just press-fit. Hey, don't drop that thing.
How, pray tell, could the described module with only power, ground, and an OUTPUT possibly do ANYTHING, much less adjust timing???? Even with input from the points (a 4th wire not described, or the "coil wire" served as I/O in parallel with the points), I don't see how the "computer" is getting any useful input for determining optimum timing based on engine load. I can imagine the best it could do was RETARD timing based on RPM (assuming the "parallel points" scenario). P.S. I had a 1972 Plymouth with the 318 V8 (see earlier blog post about its electrical problems).
Vehicles of that era certainly did have spark control computers! Chrysler offered the electronic spark advance, and was working very hard on electronic fuel metering. The silicone and sand potting mixture was fairly effective, and the easy way to remove it was to soak the assembly in gasoline for a week or two and then carefully peel off the silicone. Note that it did not work with the uerethane based system that they changed to as a cost reduction. I had an old Dodge van with the 318 V8, and after 27 years it also developed an ignition failure mode. The source of the problem was different,however. IT had a replacement ignition module, one of those with the lone transistor on the gold heatsink. But on the replacement module that transistor was a dummy, theswitching was done by a power-tab TO-220 transistor inside the case. Unfortunately the poor encapsulating job had allowed water inside, and the steel case had rusted below the transistor leads. The rust crystals had formed a varistor that was effectively snubbing the ignition pulse so that there was not enough sperk energy to jump both the gap in the distributor and the gap in the plugs. By the time I discovered that, I had already replaced the module with another of the original style ones, which had the engine running again. But I saved the aftermarket module as an example of poor quality.
It was definintely NOT a computer! Just a simple CD ignition system, very popular in that time frame. The idea was to dump a capacitor charged up to a couple hundred volts into the ignition coil; the greatly-reduced rise time and increased peak voltage made it more likely to properly fire even a moderately fouled plug, increasing both performance and fuel mileage. Delta Electronics sold a very popular aftermarket unit you could buy as a kit or fully assembled and tested. I bought the kit, and built it for use in my 1968 Rover 2000TC sports sedan, because the factory Lucas electricals were notorious for unreliability, especially in wet weather. It worked great for quite a while; then the law of unintended consequences bit me! The Rover used carbon ignition wiring for EMI reduction, and I still had the Lucas distributor cap with its tendency to collect condensation inside. Because of the former, and the increase in voltage and faster rise time, I started getting misfiring that I finally traced to a breakdown in the spark cable insulation due to arcing to places where the cables touched grounded metal (valve cover in this case, leading to degradation of the carbon conductors inside the wires. I decided to get rid of thoase ignition issues forever! I bought a new distributor cap, about 10 feet of HP-440 copper ignition wire, four screw-on spark suppressor resistors, and a bunch of inner-wall sealing heat shrink tubing. I took the distributor cap into the shop at work and milled a groove in the bottom edge where the cap seated against the distributor body, and made an O-ring gasket using (then brand-new) industrial-grade RTV silicone rubber. Then I assembled a new harness using the copper wire, suppressors, and shrink tubing. (The Rover already had full shroud gasketed spark plug covers, so that part of the system was already waterproof). I installed the new harness and cap, and drove that beast trouble-free (at least from the ignition standpoint) for 7 more years! Only time it ever stalled was one time when I got caught in a flash flood from a tornado. The engine ran fine until the water reached the air intakes!
My brother had one of these with a Slant 4 (half of a V8) engine in it. He used to call it his "Hickup truck". He drove it in the mountains of Colorado and used to have to get out a lot to adjust the mixture in the carburetor as he went up and down in altitude.
I didn't spend much time looking at it under the hood, but I seem to remember that it was a stone simple and stupid system. My best guess with the "computer" module is that it probably had a few operational amplifiers for adjusting the timing. That would have been a typical thing to call a "computer" in those days.
Very cool looking truck. One thing strikes me though--I'm surprised there was any kind of computer module in a truck that looks like an antique. I know it was the 70s and computers did exist, but I'm shocked that there was any kind of electronics on board at that time to even trouble shoot.
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