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Re: [T3] DBB alternator :Reply to jim..long


On 24 Mar 2005 at 23:41, joshua brooks wrote:

> I'm sure my "shadetree rebuilds" are more precise than that.
> 
> 1.remove brushes
> 2.run engine, sand commutator with very fine sand paper.
> 3.shut off engine
> 4.remove generator
> 5.inspect internals, replace bearings, install new brushes
> 6.reinstall, polarize, hook up, test
> 
> If the commutator is in spec and the internals are not damaged this metod 
> seems to work well for me.
> 
> Total investment in parts from CIP $11.85 +shipping.
> 
> I figure if the rebuilds are that bad; If you have time to spend on this(one 
> afternoon) and monetary investment is about $12; What have you got to loose?

I love to see people get into doing their own work, if only because no one else 
cares as much about your car as you do, but there are a couple of exceptions 
that I would take to the steps above.

I don't recommend that you ever use any kind of sandpaper on your commutator. I 
know that it CAN work fine, but there is always the chance that you will get 
your hands on the wrong kind of abrasive paper and it will leave tiny bits of 
grit embedded in the extremely soft copper. If this happens, the grit will eat 
away the brushes rapidly and you'll have short brush lifetimes.

The appearance of the commutator is more of a cosmetic issue than a functional 
one. Commutators that are working perfectly will be somewhat dark and grey, 
although they should be uniform all around. If they have discrete darkened 
segments, then this is an indication of deeper problems.

Commutators should always be turned on a lathe. This assures that they are 
round and it carries no risk of embedded grit.

As for repolarizing, I suspect that far more systems are damaged by improper 
polarizing than are cured by correct polarizing.  A functioning generator 
repolarizes itself every time it is used, so there is no advantage to doing 
this as an occasional maintenance operation. There are only 2 occasions that I 
know of when repolarizing might be necessary: After the generator pole pieces 
have been removed and mixed up, and after a battery has been installed 
backwards.

It's also possible that a generator might arrive from a rebuilder in a reversed 
polarity condition. I don't know if this ever happens, and I would consider it 
a sign of shoddy or incomplete workmanship, but it's possible. If the generator 
is tested by running it in it's correct direction and correct polarity, then it 
is automatically correctly polarized in the course of the test.

Nevertheless, I don't even bring up repolarization in my generator FAQ, just 
because I'm afraid that it would do more harm than good, and because it is 
almost never necessary. In 35 years, I've seen exactly one generator that 
needed to be repolarized, and that one had been run with the battery reversed. 
The shade tree rebuilds that I do never need to be repolarized, but I never 
remove the field pole pieces. I don't even know anyone who has the tools to do 
that.

Replacing bearings is something that I only do when they go bad. Frankly, I'm 
amazed at how long they tend to last. Most of mine are 30+ years old. Bad 
bearings are easy to identify by listening at each end with a long screwdriver 
with the engine running.

But, above all, I want to heartily agree with Josh's intent here. It's almost 
always worthwhile to spend some time trying to learn about and come to 
understand your problem. It's not just that you might save yourself some time 
and money in the short term, it's also that your understanding will come back 
to repay your effort time after time in the future.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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