Marine tach?

[Ajax]

I stumbled upon an old Stewart Warner tach, p/n 997DS7.
I'm unsure if it's a diesel tach.

It has three taps: Gnd, Ign and "S".
I think Ign is the + side and "S" goes to a tap on the alternator, so maybe a diesel tach.

Does the stock alternator have a signal tap for a tach?

[Boomer]

A picture would help. I suspect it's a diesel tach, too, with a "S" sensor wire. I suspect you meant the coil or distributor wire, the alternator shouldn't come into play. Did you get the sensor with the tach? The tach should have some switches, that set it it for a specific amount of teeth, which are set to specific instructions.

The main difference between a gas tach, and a diesel tach is in the counter interface, and the counter itself. The counter interface for a gas tach will have a high voltage signal conditioning to handle the spikes coming from the coil. Once past the interface, the counter counts the pulses and divides by time. The number of pulses for a 4-stroke gas engine is 4 per revolution, but the sensor for a diesel tach is a pulse on every full revolution.

Note what happens when a gas tach is hooked to a diesel.
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[ndutton]

A picture would help. I suspect it's a diesel tach, too, with a "S" sensor wire. Did you get the sensor with the tach? I suspect you meant the coil or distributor wire, the alternator shouldn't come into play

Actually alternator driven tachometers are quite common on diesels and the input terminal is identified 'S'. Here are pictures of my Datcon tach.

[Boomer]

I edited that phrase out, knowing that would come up. But put it back in after your post. Yes quite right on diesel engines, however the alternator shouldn't come into play on a gas engine, unless one wants to go that route.

On 4 cycle gas engines the tachometer sender terminal is connected to the negative terminal on the ignition coil or to a transistorized tachometer driver circuit connected to the ignition system. The tachometer ranging switch should be set to the 4, 6, or 8 cylinder position, depending on the amount of cylinders in the engine.

On 2 cycle gas engines the tachometer sender terminal is connected to the unrectified AC output of the alternator/lighting coil. Sometimes it is connected directly to the stator output wire. The tachometer-ranging switch should be set according to the amount of poles in the engines alternator. The amount of poles in the alternator can be found by checking the Outboard Tachometer Application page.

On diesel engines tachometer signal is generated by one of 3 things: 1) the alternator, 2) the magnetic sensor, which counts gear teeth, or 3) a tachometer signal generator that is spun by the mechanical take-off.

The diesel alternator tachometer is hooked up the AC tap of the alternator. This connection can be marked as: AC, AUX, S, R, TACH, or nothing at all. Once installed, the tachometer can be calibrated to the engine by using a shop tach, or a known “no load” governed speed. The tachometer can be bench-calibrated by using a frequency generator and the following formula:
Full scale frequency = full scale RPM x ½ poles in the alternator x pulley ratio
60

There are 2 versions of the alternator tachometer the 300-900 Hz model and the 700-2100 Hz model. By using the above formula you can choose the correct tachometer for your application.

The magnetic sensor tachometer is hooked up to a hall-effect sensor that counts gear teeth as they pass by the sensor. Both wires from the sensor are routed to the tachometer as a twisted pair. One wire is connected to the signal input and the other to the ground terminal on the back of the tachometer. The ranging switch code for the amount of teeth on the wheel can be found in your tachometer’s installation instructions.

The generator driven tachometer is connected to a pulse generator, which is spun by the engine’s mechanical take-off. One of the wires is connected to chassis ground and the other is routed to the signal input of the tachometer. The switch for this tachometer is marked as .5:1, 1:1, 1.5:1 and 2:1. Set the selector switch to the position that corresponds to the mechanical take-off ratio of your vehicle.

[ndutton]

Does on mine. An alternator driven tach works equally well on diesel and gas engines, exact same input. Said another way, there's nothing prohibiting an alternator driven tach form working on a gas engine. Ajax needs to find out exactly what input his tachometer is designed for.

Alternator taps are not standard equipment but a good alternator shop can install one in minutes. Be advised, after installation an alt driven tach must be calibrated. Harbor Freight Tools to the rescue again.

[Boomer]

It would be easier to buy a gas engine tack....but Ajax can have at it if he wants. I know he likes recycling old parts and equipment, after helping him mount a recycled bulkhead compass.

[ndutton]

It would be easier to buy a Moyer rebuilt engine with all the bells and whistles too. My Datcon was under $30 on ebay. That it was alt driven didn't give me the slightest concern. In fact, remembering a recent thread where a coil style tach shorted, killed the engine and created a challenging troubleshooting episode, maybe there's merit to the alt style. Here's the Datcon in the latest panel iteration.

[Boomer]

There you go then, not only easier, but less expensive as well. BTW...nice upgrade!

[ndutton]

Thank you. Joe turned me on to the gauges from Chief Aircraft. I really liked the big needle sweeps. What you see fits into the MMI large panel recess.

[Boomer]

I like the big needle sweeps as well.

[Ajax]

This gauge is pretty old and Google isn't coming up with any information on it whatsoever, so the odds of figuring out what kind of input it wants, is pretty slim.

I guess I'm just going to buy a new, ignition-driven tach and be normal like everyone else.

[joe_db]

I got a "deal" on a VDO tach from FleaBay. It was an odd metric hole size, so I had to spend $20 on a special hole saw. Then I went nuts tracking down a manual and getting it calibrated. I would have been better off with the right tach in the first place. YMMV