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tach troubles and the crane xr-700

To: spitfires@autox.team.net
Subject: tach troubles and the crane xr-700
From: dean r martz <sfgator2@juno.com>
Date: Mon, 6 Aug 2001 10:13:24 -0700
List,
            I  recently adopted a '77 spit  with the aforementioned elec.
ign. system, as a "parts" car from a guy who really put the "D" in DPO
(the car was only $400, even though it's in nice condition, with only
50,000 miles  on it...because the guy couldn't get it running for about 4
years and finally just gave it up. Took  me about 2 hours to revive
it...). The car runs beautifully, but, the tach is not functioning. I
have found references to problems with Smith's elec. tachs on some
"older" British cars and this ign. system in various places, but they
deal with if the tach reads "erratically" not if it's totally dead. What
really creates a problem is actually hooking the slate/white wire to the
coil, it kills the engine. Checking the slate/white wire showed it to be
live (with about 3V or so... it shouldn't have any V, should it ??) and
checking the Haynes manual shows that the tach draws power from either
the hazard switch, or a fuse (I think so , since I don't know what the
little "half-moon" shapes represent in the wiring diagram.) So, aside
from the obvious question of why is this happening, I guess I could use a
 quick primer on how an elec. tach functions... like, why does it need to
draw power from a source other than the coil (I had assumed the tach read
impulses from the coil and converted them to an RPM reading, like my
multimeter can do, when connected to the coil.) ? Any suggestions are
welcome and appreciated !!

                                                                     
TIA,      dean  ('63, '77, and '79 Spits and a  '70 GT-6+, like I'm not
busy enough already...)

p.s. oops, forgot, I tried swapping out the tach with a known good one
and it didn't help either : (
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