On Nov 27, 10:17 pm, skym wrote:
The alternator on my 180hp Lycoming is putting out a whine in my
headset. My a/p recommends a replacement. It's 7 yrs (about 600 hrs)
old. Is this reasonable time frame for an alternator to last?

Did the alternator whine begin or get much worse recently, or was it
always there? If it got worse suddenly, then you have lost a diode in
the rectifier stack (six diodes) inside the alternator, and yes it
needs to be rebuilt. Frequently, what happens is that a solder
connection between the stator wires and one of the diodes comes apart
due to heat and vibration, but the diodes are actually ok.

If the alternator whine is long standing, then unlikely that o/h the
alternator will cure that. The consequence of a partially failed
alternator is a less-than-fully charged battery, with cranking
problems, especially in cold weather. With worn brushes, or a
partially failed diode stack, the alternator still sort-of works at
light bus loads, but when you turn on most of the big loads (Nav/
Landing/Taxi/Pitot heat), the alternator isn't keeping up with the
load current, so your battery is discharging. Under day VFR (light
load), it may produce just enough current to sort-of work.

My Ford alternators in my Cessnas have lasted more than 600hrs. My
buddies with Mopar alternators in their Pipers aren't so lucky...

This topic has come up lots of times, so Google Groups search in
rec.aviation.owning back about 5 years.