PaulaJay1 wrote:
: I have a voltmeter plugin in the cig lighter socket. It reads about 11.5 when
: I trun on the master. Reads about 12.8 after engine start and after an hour
: flight has worked up to maybe 13.7 or so. I plan on taking a known accurate
: meter out Sat to check these reading.
: My A&P says maybe the alternator needs rebuilt (at about $250). He says that
: the voltage reg is solid state and either works or not - ie no adjustment.
: I've had a couple of times when the bat needed a boost, so something is wrong.
: Any comments before I start throwing money at the problem.

The only way for voltage to drop is if there's current in the measurement
chain. There are also a number of reasons that could cause these symptoms, and having
a bad alternator isn't very high on the list.

Since it's difficult for most people to measure current, it's worth measuring
battery voltage with a number of load conditions to narrow down the search. To reduce
the number of variables, measure the voltage *AT* the battery with master off, on (no
engine), on (above idle some), on (cruise RPM). Do this with and without heavy loads
on (pitot heat, nav, and landing light on/off together make for a good 25-40A
transient). It would also be good to measure the DC and AC output of the alternator
*AT* the alternator post during these tests. Possibly even the voltage at the
regulator input to the alternator.

Not all those measurements would likely be completely necessary, but with all
of them the problem can almost certainly be found. Possible reasons:

- Bad diode(s) in alternator
- Bad connection on field to alternator (it will get a bit hot)
- Bad connection on output of alternator (it will get a lot hot)
- Bad connection with current draw between bus and cig lighter
- Bad regulator (either adjustment or replacement if fixed)
- Bad alternator (unlikely, since if it's turning and the windings aren't faulted,
there's nothing else to be wrong)
- Bad battery (unlikely since if it's dead it won't draw enough current to keep the
voltage down for a long time)

Lots of reasons... not quite enough info to determine the cause.

-Cory
--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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