I was getting ready to switch out the alternator on my subie mod to a larger
120 amp one.  

When I put it up on my bench for testing a couple things became obvious.

1.  my bench tester isnt powerful enough.  When it clamped in it locked
tight and shredded the belt I was using.  Changed to a heavier belt and it
will do what I expected if it was good.  It would spin up and lock and then
drop off, spin up and lock.  Showing me I need another few horsepower on my
bench motor which is only a 4hp motor.

2. it showed me I need to redesign a belt system before I put it in the van.
The K4 belt I am currently using will not be strong enough for when that
alternator clamps in with full load.  It will probably shred the same way
that the K3 belt I had on my test setup did.  I am using a setup with no
power steering or AC.  This means that the belt will need a new routing to
allow for a bigger belt.  The smallest K5 belt is 3 inches to long unless a
pulley is used to reroute the belt over a larger path.

Note.. I called an automotive electronics shop on this after I took it to
NAPA to check on their tester and it locked on theirs as well.  Was warned
that many of the testers used at the FLAPS are not able to test the 120 amp
or even the 200 amp alternators seen in some vehicles now.  For exactly the
reason I noted.  Not enough HP in some of the testers to turn them when they
clamp down for max load.

Wait for warm weather again and figure out how to do a belt reroute.

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jimt
Planned insanity is best.
Remember that sanity is optional.
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