Smiths Tachometer

Posted on Feb 5, 2014

Calibrating the Smiths electronic tachometer some time ago when I realized that my tach was completely out of calibration. I used a technique similar to the one described in Tom Ballous 1975 Tech Tip to calibrate my tach at about 1200RPM using my dwell/tach meter. I began to wonder, however, how well calibrated my dwell/tach meter was and how well my Smiths tach was at high RPM.

Smiths Tachometer
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This curiosity led me to analyze how the Smiths tach works and to design a personal computer based tach calibration tool. This paper is intended to explain how the tachometer works, to explain how my tach calibrator works, and to update Tom Ballous Tech Tip with my opinions and advice. Direct connect electronic tachs have a direct electrical connection to the ignition circuit. These tachs use voltage signals in the ignition circuit to drive the tach. The inductively coupled type of tach such as the Smiths electronic tachs of the 1960s uses a one turn loop of the ignition wire as the primary side of a transformer that couples the current pulses caused by the cars ignition circuit into the inside of the tach. The timing of those current pulses are used to drive the tach. These tachometers were used in Sunbeam Tigers and Alpines as well as in Shelby Cobras and other British cars of the period. Since this is the tach Ive had the most experience with, this is the tach I will describe in detail. The Smiths tach must be disassembled in order to calibrate or repair it. Carefully rotate the chrome bezel until the tabs on the bezel line up with the slots on the case. This can be a very difficult job if the seals have aged badly and stuck. Whatever you do, DON`T pry up the tabs on the bezel or youll ruin it. If you need to, you can CAREFULLY pry a tiny bit around the bezel in an attempt to break it loose. Don`t tap the edge as the...

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