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Date: Wed, 17 Oct 2012 08:43:30 -0400
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From: Paul Miller <pjdmiller@gmail.com>
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Subject: TCM 60A alternator--diodes
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My primary alternator is original equipment TCM 60A alternator with 660 =
hours.  It should have been inspected at 500 hours but I deferred that.  =
This note suggests that the diodes get a check during that 500H =
inspection.  My secondary is a B&C 20A which was just replaced this year =
after 600+ hours after some windings failed.  I have had a lot of radio =
noise but nothing so bad as to take steps to fix it.  I had no idea that =
my primary was only partially functioning.  On this last trip from =
Calgary to Texas and onto Florida, my primary was noted to have Zero =
output while taxiing for takeoff at Midland TX.   I flew using the =
secondary to charge both busses for the two legs into Florida.  After =
some testing I ruled out the primary B&C regulator.  Today, I tested the =
field resistance (15 ohms, acceptable range should be 12-100 ohms) which =
was perfect.  The field is the rotor on this unit.  I decided to pull =
the housing and check brushes which were found to be almost perfect (90% =
remaining after 660 hours).

The stator was uniform in color and not shorted.   It pulls out easily =
for inspection.   The body of the alternator was not removed from the =
engine making this test very fast and easy.  A quick check revealed all =
six diodes (two each for the three phases) had failed.  Everything else =
was in excellent condition.  This unit is similar to the standard Delco =
Remy 10Si alternator and shares many parts.   The diode replacement from =
Aircraft Spruce is almost $200.  The new alternator is about $1400.   A =
repair with new rotor is about $500 including shipping and labor from a =
reputable shop I spoke with.  =20

The diode assembly was sourced at my local shop with a spare set of =
brushes for $20 total.  These are 50A diodes (50A x 6 diodes) which is a =
robust replacement satisfactory for even the 100A alternator.  I had to =
remove the B+ stud from the old diode assembly and press it into =
replacement diode package.  The re-assembly took about 20 minutes and it =
fired up with 10A charge on low engine idle.  This is much higher charge =
at low idle than I can ever recall.

This means that I've been flying for some time with less than 6 diodes.  =
 I probably was flying on one or two phases and getting some radio noise =
from the failed diodes.   This may have been ongoing for years.  Testing =
diodes as part of a 500H inspection is simple to accomplish and =
guarantees some margin for partial failure.   I have photos I can markup =
and add for reference in a follow up email.  Here are the excellent =
contacts who helped today:

1) TCM technical support 888.826.5465 Provided technical guidance and =
emailed me the alternator service manual in PDF form (48p) without =
hesitation.
2) Bill at B&C Specialty, 316-283-8000, always helpful.  Provided =
testing procedure for regulator and the following reference for repair.
3) Aircraft Systems, Rockford IL, 815-399-0225 TCM alternator repair =
specialists
4) Rupp Auto, New Smyrna Beach, bench testing, parts and expert on 10Si =
alternators and repairs

It took about a day to get it done but the cost was right and I kept all =
my original parts and did not need to remove or ship the alternator.

Paul
Legacy, Spruce Creek.=