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Re: battery shut off

To: mgs@Autox.Team.Net
Subject: Re: battery shut off
From: "Paul Hunt" <paul.hunt1@blueyonder.co.uk>
Date: Thu, 23 Dec 2004 09:31:33 -0000
I have BT, DT.  The 'heelboard' is the term you are looking for, right
behind the drivers seat for me :o)  The original reason for fitting mine is
that since I stopped using the V8 every day the battery needed replacing
about every 12 to 18 months as the alarm, even when not set, would flatten
the battery in about 2 weeks.  Since fitting the switch the cranking speed
is really good even after two or three weeks.  It also makes a useful
immobiliser, and the final back-stop safety switch in the event of
electrical problems.

I cut the 12v cable as you say, mid-way across where the switch would be
mounted, and soldered on some bolt-through terminals.  After cutting the
hole I painted the edge and before fitting the switch liberally daubed the
area with Waxoyl, with copper grease on the terminals.  After fitting I
disconnected the coil and measured the volt-drop across the switch while
cranking just to check the switch was OK.  It drops about 0.1v, which is
higher than I expected, but causes no real problem as long as all the other
connections in the cranking circuit are OK.  FWIW you should be able to get
down to a couple of tenths of a volt lost in the total 12v path (measured
from the 12v battery post to the solenoid post) and similarly in the total
ground path (measured from the battery ground terminal to the starter body),
I have seen up to 3v lost in bad connections which makes a huge difference
to cranking speed and ease of starting.  While fitting the switch it is a
good opportunity to check/clean up all the other connections.

It's certainly easier to fit it in the 12v cable than the ground, although I
have seen recommendations to fit it in the ground before.  As a cut-off
switch it makes no difference.  When installing it saves having to remove
the ground cable (for safety) whilst fitting it in the 12v cable, but that
is a once-off operation so neither here nor there.  The only other advantage
I can see is that if you want to work on the 12v cable in the future and the
cut-off is in the ground, you only have to turn the switch off instead of
removing the ground cable.  But as most times you would need to do that is
when you will be removing the battery altogether again it is neither here
nor there.  Yes it's true that if the battery side shorted out it would be
ineffective, but that is about 4" of stiff cable which has less chance of
shorting out than it did before.  If it were on the ground side it could
still short out, but you wouldn't know about it until you really needed it,
when again it would be ineffective.

I got fed up having to reset the clock everytime I used the car, as I
deliberately didn't buy a switch with a bypass fuse as I knew I would forget
and blow it on a regular basis!  So I ran a purple wire from an in-line fuse
at the battery post all the way to the clock, disconnecting its original
purple feed.  Then I realised I could have made a much shorter connection
between the in-line fuse at the battery post and the purple wire at the
boot/trunk/hatch light, removed the existing purple fuse, and achieved much
the same effect with much less wiring.

It's been fine since except a couple of times some months ago when I turned
the switch on but the power only came on about a second later, and shortly
after that I had to turn the switch off and on again before the power came
on.  It's been fine since then, and has not been dropping extra voltage when
it was closed, so I don't (yet) know what that was all about.

Merry Christmas
PaulH.




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