Tachometer woes

[TWINSCREW]

Can anyone point me in the right direction regarding my Tacho's, the port side failed some time ago and we have been using the starboard one to indicate rpm and lining up the throttle levers to match the revs, now the good one is becoming erratic and I would like to know where the wires go to from the back of the gauge. They are Smiths instruments age unknown but the boat is 1982 vintage, there are three terminals on the back labelled (NEG + GEN) Z (GEN) X and one labelled (POS) the number on the gauge face is RGP 1152/OOF 12V, the case is 80mm diameter, engines are 6cyl Fords. is there a way to test the gauges independently, is the signal from the alternator,

 

[AndyJ76]

What alternators do you have? Are you using an external regulator? (this can cause the field current to drop and the tacho signal to become weak)
You can test the alternator signal with an oscilloscope (I bought a reasonably nice digital one off ebay for about £100)
Are the alternator warning lights working? If the bulb has blown on the port one, that might lead to lack of excitation current to the alternator

 

[TWINSCREW]

Thanks Andy, I don't know what the alternator make is or whether we have a regulator, and you completely lost me with oscilloscope. Yes the lights still go out (both engines) and we seem to be charging ok, if the signal comes from the alternator what should I be looking at there.

What alternators do you have? Are you using an external regulator? (this can cause the field current to drop and the tacho signal to become weak)
You can test the alternator signal with an oscilloscope (I bought a reasonably nice digital one off ebay for about £100)
Are the alternator warning lights working? If the bulb has blown on the port one, that might lead to lack of excitation current to the alternator

 

[AndyJ76]

Before you start the engine, are both battery lights definitely lit?
An oscilloscope is the only way to tell if the signal from the alternator is good. I bought this one http://www.ebay.co.uk/itm/140869674970?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Of course, wiggling the connections at both ends might fix it without the need to tell if the signal is ok

This is the alternator that I have on my engines http://www.fleetalternatorstarter.com/alternator_10208.aspx
The tacho terminal is the one that is centre-top in the photo (It's marked 6/7V on their diagram, but it's usually labelled W or Tacho)

Photos of the front and back of your tacho and connections on the alternator would help

 

[Billjratt]

A pic of the back of the tacho heads would be of assistance.
If they are alternator-fed, the signal is taken from the raw AC 15volts before it is rectified.(The terminal could be labelled "W") set a meter to AC and you will be able to measure it without a 'scope.
If the alternators are charging OK (do you HAVE an ammeter fitted?) then the AC side is obviously OK.
The fact that they have failed in sequence makes me wonder if everything else is working OK? I would look at a common ground fault first.