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Re: Smiths Tach

To: "Malcolm Jeffcock" <>,
Subject: Re: Smiths Tach
From: "Paul Hunt" <>
Date: Wed, 18 May 2005 09:40:10 +0100
Does it have RVI or RVC on the face plate?  If RVI it is the current
operated tach and so the two white wires will probably be the signal wires.
On these the 12v supply from the ignition switch must feed the ignition coil
(and nothing else) via these wires on the tach.  On early tachs there was an
external pick-up on the back of the tach and a loop of white wire in the
wiring loom went through it.  On later tachs the pick-up was internal and
two white tails were brought out, one with a male bullet connector and one
with a female.  The tach also needs 12v and ground connections, usually to
an insulated spade and an uninsulated spade respectively on the back of the
tach, although the ground could also be via a black wire on a tag under one
of the knurled fixings that hold it in the dash.  Early MGBs used a white
(unfused ignition) 12v supply, later ones a green (fused ignition) supply.
Another potential complication is that early tachs were positive ground, and
later ones negative.  Reversing the polarity of the supply may damage the
electronics, reversing the signal wires will not cause any damage but only
one direction will work.

RVC tachs were used (on MGBs) from 1973 and were voltage pulse operated.
These had the green and black 12v and ground supplies, plus another
connection for the signal wire which came from the coil -ve i.e. the
terminal connected to the points.  These were all negative ground.


----- Original Message ----- 
>                 The Smith's Tach I have has two green and two white wires
coming out of the back. To connect it what are these connected to? I am
putting it on my Mini.

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