I have a 1978 M20J Mooney that has been doing exactly that for 15 years.
When I first got the plane it started doing that intermitent flashing. I
changed out the voltage regulator twice and the alternator once to no avail.
I was then told by someone who should know that it was probably some kind of
a phase poblem that was not serious, and that when it happens, just check
to be sure the the alternator is charging normally and if it is don't worry
about it. He seemed to feel that it would probably go away eventually or get
bad enough that it could be tracked down. In fifteen years it has done
neither. Sometimes it will go for months without flashing, and then it will
sometimes flash for a while on almost every trip.

It has long since stopped bugging me and I now just consider it to be a
harmless quirk of my bird. Only once in the time that I have had the plane
was the warning real and it that case a check of the ampmeter immediately
indicated that the alternator was not charging. (the alternator field ground
wire had broken.) Incidently, the plane was based at LZU for about a year
when I first bought it. However, I am no longer in the Atlanta area.

Don

I don't know whether this will make you feel better or not.
wrote in message
oups.com...
Can anybody help me diagnose this problem? Just bought my new (to me)
1978 Mooney 201, flew it back from Kansas City to Atlanta, no problem.
Flew to Richmond, VA - again no problem. Leaving RIC for Atlanta, the
low voltage light started flashing. Didn't want to carry on IFR so
stopped into Danville. Left the plane there, and the mechanic removed
the altinator and tested it (all ok) and replaced the voltage regulator
with a new one. All ok (charging 13.9 volts). Started the plane, the
altinator gauge was showing a charge, and took off. About 30 mins into
the flight again the low voltage started flashing. By the time I had
checked all the CB's it stopped. Then about an hour later it started
again. I tried pulling the Alt Field CB and resetting, no joy. It
continued to flash untill I was in the pattern at LZU.
So, now - what next? Bad Diodes on the Altinator breaking down at
altitude?(I was at 8000'), loose altinator belt? (unlikely - it had
just been reinstalled). Over voltage on the Altinator causing the
Voltage reg to drop off line, and I didn't wait long enough when I
reset the Alt-Field circuit breaker? Bad ground straps??
Any and all thoughts welcomed at this point - this is the worst kind of
problem to hunt down.
Thanks
Ian