mikem writes:

List message posted by: Scott Bilinski

Is there any way to tell if your alternator is on its last leg?
What are the signs?

This is a good post to file away. I agree with all of it,
with minor qualifiers..

Stator: Failure in Stator rectifiers. I have had diodes in the
......

You can go a long time minus one diode and never know it..

There is also a marked increase in bus ripple.

WhatHeSaid. The best test is a portable oscilloscope on the alternator
output. You can SEE the hole in the waveform.

Most stator failures are caused when the soldered electrical
connection between the stator wire and the diode stack comes
apart due to heat/vibration.

Here I'll mildly defer. The {auto} ones I've seen are always dead
diodes, not bad solder joints. But it never hurts to try. Note some
diodes do fail only when hot, and "repair" themselves when cool.

Rotor: The brushes can wear down to where they no longer "ride"
on the rotor slip rings.

GM alternators used to have a "full field test tab" to bypass
the regulator. Ground it & the output voltage SHOULD go up
to 15++ volts and the engine/belt alternator all GROAN. If you
are doing a full-field test in-situ, be SURE all the avionics
are off-line.

Some brushes can be replaced without major work. Even if the
rings are dirty, with more brush pressure they often clean
up.

At annual, remove the belt, and see how much play is in the bearings.

And SPIN the pulley! It should be smooth and sweet.


BTW, there is an alternator parts supplier in most major cities.
These are the folks who sell parts to other rebuilders and work on
marines, 300A 28VDC truck units etc. (NOT an auto-parts retailer in
a strip mall..) If you don't require FCC blessed parts [1], you
might seek such out. They stock and/or can order brushes, diode
arrays, etc.

One last rant! Grounds, grounds, grounds!!! Inspect/clean the ground
bonds, be they engine to alternator, alternator to firewall, whatever.
Grounds are Murphy's way of making troubleshooting electrical issues
an adventure.


1] Your '540 is used in an airboat...
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Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433