Faria made a number of electronic tachometers in the 60s and 70s that were popular with the muscle car crowd. They also made ’65 & “66 Shelby, Sprint Falcon, Mustang Rally pack, T-Bird and Thunderbolt Rotunda Tachometers, some of which are now quite valuable.
I decided to analyze one of these tachometers, a 4k RPM model acquired from eBay.. I later received a Faria tachometer from a 1966 Shelby GT350 which I also analyzed and repaired.
You can download the reverse engineering document here.
All of the vintage Faria tachometer documentation on the internet indicates that the Faria tachometers are wired between the ignition switch and the coil positive, which means that they are current driven tachometers. There also is no indication that the tachometers that have terminals on the back of the case are polarity sensitive since there are no marks differentiating between the two terminals. I have arbitrarily marked the terminals A and B. However the Shelby tachometer has a red wire and a black wire, which does imply polarity.
There are a few minor differences between the two tachometers. the 4K tachometer, which I presume is older, has two wring terminals on the back of the case while the presumably newer Shelby tachometer has a red and a black wire coming out of strain relief on the back of the case. The schematics of the two circuits are very similar with 3 added components in the Shelby tachometer, presumably to add temperature compensation to the circuit.
Here are the schematics for the 4K Faria tachometer:
