Speedo / Tach / Gauges: Making An OEM Tach Compatible With Modern Electronics

[JoeHall]

If anyone has ever lost tach operation in a Stude, after installing an ignition booster (i.e. MSD 6-AL), electronic ignition, or fuel injection, here's how to fix it: Install MSD-8920, "Tach Adapter".

Apparently, the 60s-70s vintage tachs require a stronger signal than the later ones. Most modern electronics provide a much weaker tach signal, that works great with early 80s and newer, but not with the older.

I lost tach operation on the 62GT two years ago upon installation of EFI, with ESC. The tach would work when cranking over, but the motor would not start. If the motor was already running, it would die when the tach was connected. I am one of those guys who likes to "drive off of the tach" so it has been painful living without it.

Recently, I read on the internet of several other folks with 60s-70s vintage vehicles, who had experienced the same problem after installing modern electronics. Most who fixed the problem did so with the MSD-8920. So I installed one on the GT a couple of days ago, and for the first time in two years the tach is working again !

Though some make it sound complicated, I believe the MSD-8920 is simply an amplifier. It draws only a little juice from the negative side of the coil, just as a modern tach does, then amplifies it to a signal strong enough to operate the older tach.

It took me two years to get the tach back in operation, so just thought I'd pass it on, in case someone else is suffering without their tach, as I was.

[64V-K7]

This is a current situation with my 64 GT and the new Mallory Unilite. I used all Mallory components and wired it up with straight 12 volts, using a new Mallory coil with internal resistor and this eliminated the Pink wire. The engine runs fine... However the tach only runs up to 1500 rpm and then hovers...
I was told this was due to a bad tach ground.... Looking under the dash, you'll see the sender (square unit) green wire goes to a junction and is then continued as a pink wire to the coil (+). Well, the pink wire no longer powers the coil, since the Mallory is a straight 12 volts. ...and the pink wire is not the ground either, so apparently the tach sender is not getting the correct signal. Where did I go wrong?? :-(

[JoeHall]

Not sure about 64GT, but the 62GT's tach signal comes from the NEG side of coil. I am unsure if its' power source is 6 or 12 volts. If it comes of the pink resister wire, obviously it is powered by 6 volts. Also, not sure how the tach is grounded, but the fuel and temp gauges ground via the oil pressure line, which grounds to the firewall, out in the engine bay. So to insure ground, you could try a short jumper wire from tach mount bolt to fuel gauge mount bolt.
Of course, its also easy enough to install a resister in the 12V power line, to rule out wrong voltage.

I read where some say the MSD-8920 changes the tach signal "square waves" to those acceptable to the older tachs (whatever that would be). But I believe (uneducated guess) all it does is amplify the tach signal. It only has four wires: black to ground; red to 12V with ignition on; white to coil negative side, and violet to tach signal wire. It does not affect the voltage to the tach, or ground.

Hope this helps, because I know how frustrating it is to lose tach operation due to an "upgrade".