Stewart Warner Tachometers

In the 1950s, Stewart Warner sold a family of 6, 12 and 24 volt tachometer monitors and transmitters 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 Tachometer systems

The system consists of an RPM display, called a "monitor", and a signal generator, called a "transmitter".

The 760 Series tachometer monitors came as one of 3 basic models, "The Standard", "The Recorder" and "The Vac-Tach".  The Standard came as a simple tachometer monitor.  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, 12 and 24 volt versions.

Here is a list of the part numbers of some of the different variants (printed on the back of each tachometer)

The Standard

760-AT: 0-8000 RPM, 12V 

760-AY: 0-6000 RPM, 12V

760-CC: 0-3500 RPM, 12V

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-V: 0-3500 RPM, 6V

760-X: 0-4500 RPM, 12V

The Recorder

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-W2: 0-4500 RPM, 12V

The Vac-Tach

760-C: 0-4500 RPM, 6V

760-Y: 0-4500 RPM, 12V

I have also worked on a 760-BM : 0-3000 RPM, 24V Standard tachometer

The transmitters for these tachometers consist of a puck-like unit that sandwiched between the engine's distributor and the cap/rotor.  These transmitters, also 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 transmitter called a 762-B for diesel engines which was driven by a standard 1/2 engine speed mechanical take-off.

If the model number of your 760 Series tachometer monitor is gone, you can identify a 6V vs 12V vs 24V model using an ohmmeter.  Measure the resistance between the black wire (the pin farthest from the other two pins) and each of the two pins that are close to each other (green and white wires).  

6V models:  About 4 ohms

12V models:  about 15 ohms

  24V models: about 100 ohms

The resistance will be twice that if you measure the resistance between the two pins that are close to each other (green to white).  The resistance will be close to 0 ohms if the insulation inside the case has crumbled, a frequent condition.  Always check inside of the tachometer for crumbling insulation on the wires prior to attempting to use the tachometer.

Theory of Operation

These tachometer monitors 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 transmitters 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.

Modern Transmitter

Many people would like to use one of these tachometer systems in their vintage hot rods but are no longer able to use the original transmitter or are unable to find one.  AccuTach Co. has designed a modern transmitter that can accept the ignition signal from many modern or 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 V2 Transmitter 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.  Any 12V or higher square wave tachometer signal can drive the SW760 V2.

The AccuTach Co. SW760 V2 Transmitter can drive a 12V 760 Series tachometer with no additional hardware, but a 12V-6V DC-DC converter is required to power a 6V 760 Series tachometer in a 12V vehicle.  A 6V-12V DC-DC converter is required to power the SW760 V2 Transmitter in a 6V vehicle.  It can also power a 12V tachometer monitor in a 6V vehicle.

Be aware that these tachometers are well over 50 years old.  They all should be oiled.  Even with fresh oil, they may not operate smoothly or at all.  My experience is, however, that when they are working, they are actually fairly accurate.  Please download the install instructions to learn how to oil your 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: