New Tachometer for Universal M25 with original Motorola 50A alternator

[Kenneth K]

Time to put a new Tach on the engine panel. I'll probably redo the whole suite, and replace the old ammeter with an oil pressure gauge.

I like the Faria Chesapeake gauges in white. Options for Tach are:

1. Diesel - Mechanical Takeoff & Var Ratio Alt, or
2. Diesel - Magnetic Pick-up

​

Current Tach runs through the #10 Grey wire that connects to the "A.C. Tap" output of the 50A Motorola Alternator. I'm guessing this means I should buy the "Mechanical Takeoff & Var Ratio Alt" model.

Anyone know for sure before I order it?

 

[sharonov]

Don't know about Faria. I used Sierra Heavy Duty gauges from West Marine. Tachometer worked fine but had to be adjusted to show correct RPM. As far as I recall it had a switch and a trimming pot. You could figure out pulley ratio and number of poles or use reference handheld tachometer and just tweak the gauge until readings match. I did the latter at idle and max RPM.

 

[Tom Metzger]

I'm guessing this means I should buy the "Mechanical Takeoff & Var Ratio Alt" model.

Anyone know for sure before I order it?

From their website I couldn't find a clue. You are right, what they say indicates that it may work, but I would need more info... which they seem disinclined to supply. Phone call???

 

[Kenneth K]

Yes, a call to Faria cleared it up. The Variable Ratio tach is the correct one to use with an alternator sender:

Faria explains it on p7 of this document: https://fariabeede.com/site_manuals/IS0100q.pdf (excerpt below):

Diesel engines: The tachometer signal terminal is hooked up to 1 of 3 things: a) the Alternator b) a tach signal generator that is spun by the Mechanical Take-off c) a Magnetic Pickup sensor which counts gear teeth

The alternator tachometer: which is also called avariable ratio tachometer is hooked up to the AC output terminal on the alternator. This terminal can be tracked in a variety of different ways: AC. AUX., S, R, TACH or nothing at all. Once installed, the tachometer is then calibrated to that specific engine by using a shop tachometer or a known “no load” governor speed.The white label on this tach gives the formula: [Crankshaft pulley diameter divided by the alternator pulley diameter times the number of Alternator Poles = N]. “N” is used to determine the correct switch setting. Another adjustment on the back allows for fine tuning.

The Switching Diesel Tachometer: is hooked up to a tachometer signal generator which is spun by the engines’mechanical take-off. One of the signal generator’s wires is grounded to the engine and the other is connected to the tachometer’s signal terminal.The white label on this tach is marked: 1/2:1, 1:1, 1.5:1,2:1, which corresponds to the different mechanical take-off ratios.

The Mag Pickup Tachometer: hooks up to a magnetic pickup sensor which counts gear teeth. Here neither of the wires is grounded to the block. They are both routed up to the tachometer as a twisted pair. One hooks to the signal terminal and the other to the ground terminal on back of the tachometer.The switch is set to the approximate number of teeth that the sensor sees on each engine revolution. Another adjustment on the back allows fine tuning to the exact number of teeth. The label is marked in ranges generally from 30 to 160 gear teeth.​

 

[Tom Metzger]

Ken - You will need a portable tach to set up the new tach. They can be had for less than $20 at Amazon, or you can use a 120VAC fluorescent light which is what I have done.

Remember that you want to use the crankshaft pulley, not the alternator for the reference.

 

[Kenneth K]

For $20, I'll probably just pick up a tach from Amazon. I had been looking at Harbor Freight, but I think they want more $.

Hmmm, a fluorescent bulb? I'm guessing maybe at 1800 rpm under a 60Hz AC flicker you'd get two stationary reflections from the crankshaft pulley?

 

[Tom Metzger]

Actually, 4. The tubes flash on both halves of the sine wave.

One piece of whte tape on the pulley is enough.

 

[Kenneth K]

That one deserves another martini.

Drive carefully.