Land Rover Repair: 1997 Disco tach erratic, battery not holding charge but both have been tested and shown to be ok, tensioner pulley, vibration test

Question
John I've read many of your comments regarding similar situations but none seem to fit
my problem exactly.  Here's my problem: for a couple of months now my tach registers
zero when I start the car after 10 minutes of driving it finally starts to show the rpms.  
Every once in a while when driving the tach will drop to zero but when I step on the gas
it registers again.  My battery finally went dead recently, jumped it, and now everytime I
go to start the car I cross my fingers and get lucky sometimes when it turns over.  I
pulled both battery and alt. and had Kragen Auto test it.  Battery is fine and the alt
passed their test several times.  Suggestions?

Thanks in advance for your time!  

Answer
Hi Bryan,

sounds to me like your alternator is fried.  First clue:  tach is out of wack.  Just in case, check the serpentine belt and the tensioner pulley - just to be sure.  If the belt was not installed properly OR if it shines, it could be slipping and not giving the alt the ability to do its purpose.  I had a tensioner pulley fail and it caused me to accuse the alt - until the darn pulley actually seized and snapped the belt.  All that time, I was thinking it was the alt.

The tachometer is attached to the winding of the alternator and when the alt starts to fail - so does the tach.  You'd have to monitor the tach with a multimeter for some time in order to see when it bottoms out.  If you are lucky, a multimeter will pick up the slight fluctuations of the alternator, another sign of a flaky alt.

As for the bench test of the alt, did they simply apply a multimeter and see the output or did they do both a temperature and vibration test? Its quite possible it could pass a momentary test but once the heat rises or perhaps the engine + road vibrations and the winding short jumps out!!

Best of luck,

JohnMc