Alternator

[sail4dale]

What is happening when the alternator output shows 0 voltage and the Tach also goes to 0?

every once in a while it is working but other times nothing?  could it be the field wiring or ?????

[SteveLyle]

It could mean that your voltage regulator is sensing that the batteries are fully charged, so it has shut off the alternator output (that also happens to feed the tachometer).

Steve

[Footloose]

I had this happen last year.  The field wire was loose on the back of the alternator. My plug wasn't fitting properly.  A little trimming fixed it.  You can check the field coil by putting a jumper wire across it.  If you have an external voltage regulator it is discribed in the trouble shooting section.

Dave

[Ray & Sandy Erps]

It could also be a loose wire or faulty contacts on the key turned switch on the engine instrument panel.  Key off, no voltage reading or RPM reading.

[Ron Hill]

Dale : It could be all of the items that people have mentioned field wire, voltage regular or broken sense wire (if you have one). 
If both the Voltmeter and the tac are out you can bet that the alternator has 0 output.
 
Here's what happened to me - kind of a goofy situation. 
The port side cockpit cushion moved slightly to the rear of the cockpit.  In doing so it caught the key in the switch and moved it slightly, so it was now between OFF and ON!  The engine instrument panel was DEAD.  The engine was running so I wasn't about to shut it off until we were docked in a marina where I could "work" on the problem.  I figured it out fast when I shut off the engine and then went to turn off the key switch.  It was then I realized  that the key switch was in that halfway position, explaining why none of the gages were working!!