In the course of our T-Bird restoration we converted the cable-driven, 50 year-old tachometer, to a new electronic unit. The original one was driven from the distributor, which had a special geared assembly that attached to a speedometer-like cable. Long ago it had been replaced by a standard distributor and we didn't want to go through the trouble and expense of obtaining a new one.
In addition, we've had a lot of experience with mechanical tachometers and, at best, they tend to quickly develop "needle bounce" and squeaking noises, not to mention their inherent inaccuracy. No thanks, we thought, it's better to convert to an electric tachometer. Such instruments are quite simple, actually. They typically come with four wires attached to the circuitry. One is a ground wire, one is a 12-Volt power lead, one is a signal wire and the fourth is for the internal lighting (it connects to the dash lights.)
Electric tachs are essentially voltmeters. They count the pulses from the ignition and convert the signal to a constant voltage reading, calibrated as revolutions on the face of the gauge. The pulse signals can be obtained in a number of ways, depending upon the type of ignition. Here are a few of the most common applications:
On points-style distributors the signal wires are attached to the coil's negative post (the same one that goes to the points). This is the most common of all attachments, since most collector cars have this type of ignition. The only possible adjustment left after connecting to the coil is to set the proper number of cylinders on the gauge itself. Tachometer manufacturers give instructions for such adjustments.
Later style distributors such as Delco HEI systems have external coils with sealed connectors. These distributors have special terminals for tachometer connections. GM integral coil HEI distributors are found on millions of later designed cars and these too have tachometer connections.
For all other applications the tachometer manufacturers provide inductive pickups for the signal wires. Inductive pickups are simply small wire-wound coils that act as little radio receivers. They pick up an induced (hence, the term "inductive") electromagnetic charge as it travels through a wire and send it as a signal pulse.
Inductive pickups can be attached to the coil's central output wire or to an individual spark plug wire, depending upon accessibility. In each case the electronic circuit must be "told" how to read the number of cylinders vs pulses. This varies by manufacturer, but most tachometers have switches that are either set for cylinder arrangements or are pushed a certain number of times while power is applied.
Okay, Now For The More Challenging Part
If you happen to have an in-dash mechanical tachometer (such as that in our T-Bird, '53-73 Corvette, etc.) and you want to convert it to an electronic one, you have to make the decision to sacrifice the "innards" of the original instrument and fit the new electrical parts to its case. Of course, you will want to use the original tach's face and needle so that the car appears stock.
This isn't really as much a problem as you'd think. The reason is that the mechanical workings of the old tachometers take up more room than the modern electronic parts. In fact, most modern tachs only have a small circuit board inside, so it will fit pretty easily inside the housing of an old mechanical unit.
We recommend simply gluing the electronic tach's circuit board to the backside of the original tach's face plate. Hot glue guns work very well in such cases and the glue itself isn't corrosive or electrically conductive. If the original tachometer's needle hole is larger than the electric tach's shaft you'll need to put a little hot glue on that connection too, but as likely as not you will find the needle mounts without much effort. The only things to consider when mounting the needle are: do so gently, because you don't want to damage the delicate mechanism; and make sure the needle is oriented at the zero position before fixing it in position.
Since electronic tachometers have wires attached you have to remember to drill a sufficiently-sized hole through the rear of the housing to allow them through. A rubber grommet is always the best way to line the edges of the hole.
Anyway, conversion from mechanical to electronic tachometers isn't really a big deal, and you probably had to clean and restore the old tach in the first place. Happy tinkering!