[Vwdiesel] 89 Jetta na charging system

[Doyt W. Echelberger]

  When my car is running, is there EVER a time that the battery dash light 
should come on red, other than the brief interval when I first switch on 
the ignition ? (If this is a duplicate post, I apologize.)

The red dash light for the battery on this 89 Jetta diesel has glowed dull 
red at times and not at other times. The dim red light on the dash called 
my attention to this by coming on during a drive around town recently. OK, 
red dash lights are put there for a reason by the designers so I popped the 
hood and with the engine running put the volt meter across the battery 
posts. It read a steady 12.8 volts.

Conclusion: The alternator wasn't charging the battery at that time, and 
12.8 volts didn't trigger the voltage regulator to fire up the alternator 
fields.

So, I made an appointment to have the charging system checked. I take the 
car in tomorrow.

This morning, I started the engine and there was NO RED BATTERY LIGHT 
flickering on the dash. I popped the hood and put the voltmeter across the 
battery terminals and read 14.8 volts.  I turned off the engine and the 
same voltmeter read 12.8 volts. Clearly, the alternator has started working.

I then drove the car for half an hour. The battery light stayed off,  off 
at the end of the trip I again measured the voltage across the battery 
terminals while the car was running.  13.7 volts.

I am puzzled by this behavior.  Will my mechanic be able to identify the 
cause of this unreliable feature and fix it?
I have a spare alternator from my old 1985 Jetta TD, and it worked OK a few 
years ago when the TD was running.
I think I'll put it in the trunk when I take the car in, and mention it to 
the mechanic.

But right now, he doesn't have anything to fix. The alternator seems to 
work just fine. Does he have miracle tools that tell him what made my 
battery light come on one day, and then stay off the next?

Doyt