Could you post a full datasheet on the meter you have. And pictures of it's conections ( how many?) and any instructions on a sticker.... it may well not be a straight analog meter, but have some internal electronics
I found the linked PDF which is about repairing this company's gauges, page 20 talks about tachs. I'll have to digest this. The back looks like the other attachment.
http://faria-instruments.com/site_manuals/IS0100_L_Warranty_Repair_guide.pdf
Two long screw terminals, which are also used for mounting. When I removed these from the boat panel these terminals each had a yellow wire attached.
The circle is the lamp which has a clamp terminal to the casing of the gauge and one into the lamp. These both had white wires attached. I have tested that 12v to the lamp leads does turn the lamp on.
Operation
Electronic tachometers work by counting pulses generated by the ignition system, alternator, tachometer signal generator, or magnetic pickup sender. The tachometer is hooked up to +12 VDC, Ground, and one of the signal sources listed above.
By selecting the right tachometer and setting the switch on the back to the correct position, you let the tachometer know how many pulses are sent per each engine revolution. From this information, the tachometer displays the correct engine speed. See Appendix I for tachometer dimensions. Instrument part numbers are located on a label attached to the outside of the case (i.e. TC000A).
Application
4 cycle engines: The tachometer signal terminal is connected to the negative terminal on the ignition coil or to a transistorized tachometer driver circuit connected to the ignition system. This circuit will have a wire (usually gray) for connection to the tachometer. The correct tachometer will have a white label on the side indicating which switch position is for each engine type. This label will include 4, 6, and 8 cylinder engines for positions 1, 2 and 3.
Outboard engines: The tachometer signal terminal is usually contacted to the unrectified AC output of the alternator/lighting coil. Sometimes it is hooked directly to the stator output wire (usually yellow) other times a gray tachometer output lead is provided. The correct tachometer for this application will have a white label on the side with switch positions 4, 6, 8, 10 or 12 pole alternators. The number of poles on the alternator can be determined by checking the Faria® Outboard Tachometer Application table. (See Appendix V)
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, or c) a magnetic pickup sensor which counts gear teeth.
a) The alternator tachometer: which is also called a variable 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.
b) 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.
c) 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.
Calibration
Set up a calibrated “shop tachometer” or “strobe tachometer” to monitor the engine’s true RPM. Start the engine and (after an appropriate warm-up period and with the shift in neutral) increase the engine speed to the boat’s normal cruising RPM read on the shop tachometer. Set the coarse adjustment switch to the proper position described on the label. Remove the stop-plug or paper label corner (at the 8 o’clock position on the rear of the case for most) and insert 5/64” Allen wrench into the “Fine adjustment” trim pot, rotating it CW or CCW as necessary to indicate the true RPM.
