In the 1950s, Stewart Warner sold a family of 6 and 12 volt tachometers and senders called the 760 series. Accutach Co. has reverse engineered the tachometer operation and now offers a sender for those tachometers that will work with virtually any modern engine. See below for more information about this product.
About the 760 Series Tachometers
The 760 Series tachometers came with 3 basic models, "The Standard", "The Recorder" and "The Vac-Tach". The Standard came as a simple tachometer. The Recorder added a rev counter (tachometer equivalent of an odometer). The Vac-Tach added a vacuum gauge to The Recorder functionality.
They also had sub models that came with different RPM scales: 0-2500, 0-4500 and 0-8000 RPM. Finally, they came in 6 and 12 volt versions.
Here is a list of the part numbers of the different variants (printed on the back of each tachometer)
760-G: 0-4500 RPM, 6V
760-M: 0-2500 RPM, 6V (for diesel)
760-N: 0-2500 RPM, 12V (for diesel)
760-S: 0-8000 RPM, 6V
760-X: 0-4500 RPM, 12V
760-AT: 0-8000 RPM, 12V
760-AS: 0-8000 RPM, 12V
760-E: 0-4500 RPM, 6V
760-F: 0-4500 RPM, 12V
760-J: 0-2500 RPM, 6V (for diesel)
760-L: 0-2500 RPM, 12V (for diesel)
760-P: 0-8000 RPM, 6V
760-C: 0-4500 RPM, 6V
760-Y: 0-4500 RPM, 12V
The senders for these tachometers consist of a puck-like unit that sandwiched between the engine's distributor and the cap/rotor. These senders, called "switches" were specific to each supported distributor. Here is a list of the switches.
762-AA: Auto-Lite 6 cyl, prior to 1951
762-CC: Ford 6 & 8 cyl.
762-DD: Delco-Remy 6 cyl. (except High Tower)
762-H: Auto-Lite 6 cyl, 1951-1953 (and maybe newer)
762-J: Auto-Lite 8 cyl.
762-K: Delco-Remy 8 cyl. (Except High Tower)
762-M: Delco-Remy 8 cyl. (High Tower)
762-P: Delco-Remy 6 cyl. (High Tower)
762-S: Mallory 4, 6 & 8 cyl.
They also sold a special sender called a 762-B for diesel engines which was driven by a standard 1/2 engine speed mechanical take-off.
Theory of Operation
These tachometers are actually mechanical speedometers calibrated for RPMs. The case looks like two cans bolted together. The front can is the speedometer and the rear can contains what we now call a stepper motor. (I had no idea that stepper motors had been invented that long ago.)
The stepper motor is a squirrel-cage type motor with 6 magnetized bars in the rotor. It has two pairs of windings in the stator. The pairs of windings have a common connection (black wire) to battery voltage and each winding pair has a separate signal wire (white wire and green wire). The signal wires are grounded to activate the winding pairs.
The senders all work the same way, only the form factors are different to fit each specific distributor. Inside of the sender, there is a brush and a slip ring that covers 360 degrees of the rotor. That brush is connected to the black wire to ground. 90 degrees either side of that brush and on a different plane are two brushes, one on either side of the black wire brush. One brush is connected to the white wire and the other is connected to the green wire. in that plane, there is a 270 degree slip ring.
This configuration energizes one winding pair, both winding pairs, the other winding pair and both winding pairs, in that order. Those signals are what drive the stepper motor which then keeps pace with the running engine.
Many people would like to use one of these tachometers in their vintage hot rods but are no longer using a compatible sender or unable to find one. Accutach Co. has designed a modern sender that can accept the ignition signal from a modern or a vintage ignition system and drive any of the 760 series tachometers. Note: it will not drive other series Stewart Warner tachometers such as the 765 Series or the 766 Series.
The Accutach Co. SW760-Sender is designed to read a 4, 6, 8 or 12 cylinder tachometer signal from a distributor, an electronic ignition system or an engine control computer. If your modern EFI system does not have a tachometer signal, you may need to buy an additional tach adapter such as an Autometer 9117 Tach Adapter.
The Accutach Co. SW760-Sender can drive a 12V 760 Series tachometer with no additional hardware, but a 12V-6V DC-DC converter is required to drive a 6V 760 Series tachometer.
Accutach Co. has two prototypes of these senders and is actively looking for someone to be a beta tester of this product. If you have a currently running vehicle and are willing to test this unit in your car with your 760 Series tachometer, please contact us.
Here is a video of a prototype sender driving a 760-S 8000 RPM, 6V tachometer in the Accutach Co. lab: