[AT] Case VAH: Cut-out relay?

[Dean Vinson]

Just returned from a quick Thanksgiving trip to the farm.  Got to visit
family, play euchre, eat lots of food, and tinker with the old tractor a
little bit.  (In other words, quite the successful trip.)

The Case is still a great-running little tractor, although it needs some
TLC:  http://www.vinsonfarm.net/photos/VAH_Nov06.jpg.

A few months ago Gene Dotson had hooked me up with some parts I'd needed to
get rid of about 90 degrees of free rotation in the steering linkage.  Got
those put on finally, and it works fine.  (Thanks, Gene).

Next is that the battery is in backwards, negative ground rather than
positive.  Everything works fine except the ammeter reads opposite of what
it should.  I thought it'd be simple enough to just turn the battery around,
polarize the generator, and be back in business.  Nope.

I took the battery out, turned it around, and hooked the starter switch
cable to the negative terminal.  When I went to hook the ground strap to the
positive terminal, all sorts of sparks flew.  Immediate, direct short,
ammeter showing full discharge, as soon as I touched that ground strap to
the battery.  I hadn't touched anything else, didn't mess with any other
wiring, all of which is in good shape.  I looked around to be sure I hadn't
accidently pinched some wires together somewhere, but it all looked right.
So I put the battery back the way it had been, negative ground, and
everything was once again fine.  (Except for the direction that the ammeter
reads).  How could there be a short with the polarity one way, but not the
other?

Only thing I can think of is the cut-out relay on the generator.  I assume
that's what it is, although it's not the original one that was on the
tractor before.  The printed label on it is torn and I can't read it all,
but it says "POS GND" at the top and "NEG GND" at the bottom, so I figure it
can be wired for either one but needs to be the correct one.  See picture at
http://www.vinsonfarm.net/photos/VAH_relay.jpg.  

What does that relay do, anyway?  Could it be polarity-sensitive, and cause
a short when hooked up one way but not the other?

If I'd had another day to tinker with it, I'd have tried reversing the leads
on that relay and switching the battery around again.  But I didn't want to
risk messing it up and leaving it non-working and in my dad's way just
before the end of my visit.  Other option would be to just reverse the
wiring on the back of the ammeter and leave it negative ground, I guess, but
I'm still curious about how the relay works and whether it accounts for the
short.

Dean Vinson
Dayton, Ohio
www.vinsonfarm.net